Ego Battery Review from a Battery Engineer

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  • Updated 2 years ago
So I thought others might be curious about what I've learned about the EGO battery since I've been messing around with it for the last month or so.  My trade and expertise is in li-ion battery management systems (BMS).  I am definitely not an expert in the EGO battery, but I can share what I've learned so far.

I believe the 7.5 Ahr battery is a 14S3P battery, meaning it has 3 parallel sets of 14 series cells (for 42 total battery cells).  The battery cells are the Samsung INR18650-25R.  The Samsung cell is a high-power, somewhat high capacity cell with a maximum rated voltage of 4.2 V, min rated voltage of 2.5 V and a max discharge current of 20A continuous.  This means the battery pack could handle up to 60 A continuous during discharge if heat were not an issue (that's a lot).  Where Samsung shines in my experience is in their high charge currents with minimal increased degradation.  This cell is capable of being charged at 4 A, which is a 1.6 C rate (1.6 times the capacity of the cell), which is pretty high considering a 1C charge rate is usually considered a fast rate of charge and many cells are only capable of C/2 maximum without significant degradation.  The 42 cells in the 7.5 Ahr battery make a 50.4 V battery pack with 7.5 Ahr of capacity.  If EGO charged the cells up to 4.2 V (their highest rated cell voltage), this would equate to a battery pack capacity of 378 Whrs.  But I don't believe that they charge the battery to 100% (4.2 V per cell) because I measured my battery pack voltage at 57.5V right after a charge (but not while on the charge as I probably should have to get a more accurate number).  This equates to 4.107 V per cell.  There are some trade-offs to not charging a battery cell to 100%.  One benefit is that the cell lasts longer because higher voltage degrades the battery cell faster and another benefit is that the charge time can be significantly reduced because you can skip the time-consuming constant voltage portion at the end of the charge.  One downside is that you don't get as much usable capacity with each charge.

So that means that the average voltage is probably a little less than 50.4V, therefore the capacity of the battery is a little less than 378 Whrs (sorry EGO, your 420 Whr sticker rating is not correct because Whrs are calculated from nominal voltage, not max voltage).  I haven't yet performed any cycle testing where I measure the usable capacity of the 7.5 Ahr pack, but my guess is that it would be somewhere between 360 to 370 Whrs at a fairly low current discharge.  I haven't measured the EGO cut-off voltage yet, but if it's higher than the 2.5 V minimum that Samsung spec's it at (which it most-likely is due to the high amount of current that is required from their tools and the large associated voltage drop that it would create out of the battery cells), then that would further reduce the usable capacity.

I noticed that the small 2.5 Ahr or 2.0 Ahr (not sure which ones or both since I haven't taken mine apart) have a PCM (phase change material).  I don't have much experience with PCMs, but I believe the concept is that they are solid at room temperature, but change to a liquid (hence phase change) at higher temperature and absorb a ton of thermal energy in the process.  So these things wrap each individual cell to act as a little rechargeable air-conditioner-type system to help keep the cells cooler longer during use.  The 7.5 Ahr battery doesn't have these, probably because the 3 parallel sets allow the battery to deliver current without heating as much as the small packs do at the same current (because the current per cell of the 7.5 Ahr pack is 1/3 that of the smaller packs at the same current).

The 56 V max rating would be 4.0 V per cell, but in actuality the charger goes above that, so they either have chargers with a high tolerance or they just decided to choose 56 V for some unknown reason.  I kinda think the chargers just have a high tolerance because someone else on these forums measured 58.5 V when their pack was fully charged and that's one volt higher than I measured mine at.  That's 4.179 V per cell and is getting pretty close to the 4.2V rating, so that makes me think maybe the failure to charge to 100% is not intentional, but a result of wide tolerance electronics in the charger.

I can't really inspect the BMS board because it's encased in a bunch of conformal coating-type material and I don't want to destroy one of my battery packs just to look at the BMS.  I'm guessing the "T" on the battery electrical connector is for the charger and devices to monitor battery cell temperature and the "D" is probably for data.  Since there is only 1 data pin, it appears to be a one wire interface rather than a typical battery SMBus protocol, or maybe there is a translator chip inside that they use so that they don't require an additional 2 pins externally for communication.  I also don't see any large power wires going into the BMS board, so that either means that the battery pack doesn't have any internal protection, or that the protection devices are located on the PCBs that are sandwiched in-between the cell stacks themselves (and I don't want to damage mine by separating them).  Typical protections include over-current/short-circuit, over-temperature and over-voltage (also sometimes called over-charge).  If the battery doesn't have internal protection, then maybe the charger handles the protection and prevents charge when the battery measures something wrong.  Hopefully the charger is not the only place that there is protection because li-ion battery packs should be able to take anything that a rogue user can deliver and still shutdown safely, even if the charger isn't present.  Also, it would be difficult to pass the UN38.3 transportation testing without internal protections for over current and voltage.

I love the mechanical design of the charger contacts where they slide back and fourth to self-align during battery insertion.  This is a simple, yet genius mechanical design and doesn't require expensive tight mechanical tolerances.  I can see that connector alignment was a problem in the design phase and this is an excellent solution to prevent damaged or excessively worn contacts.  I also love the idea of a fan in the charger to pull cool air through the battery cells.  I can see that you've determined that cool-down time was a significant barrier to your fast charge time and that's an excellent solution to that problem as well.

Overall I think this is a great battery pack and I'm excited to own 7 of them (also, the lawn tools are great).

That's all I have for now.  Maybe this will help some people out there.

-Adrian
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Adrian Ramirez

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Posted 4 years ago

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Larry

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Wow I am impressed. Thanks for all the info.
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DavidH

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Adrian, thank you for the taking the time to give us a very thorough technical explanation of the Ego Battery. It is much appreciated.

DavidH
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Blue Angel, Champion

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Adrian, thanks a TON for your detailed look at the Ego battery! I'm a bit of a battery geek, but only based on what I've seen and read on the Internet... definitely no professional experience with them.

Regarding operating voltage, I measured my new 2.5Ah pack both fully charged and fully depleted after running it down in my string trimmer. Fully charged it measured 57.8V (4.13V/cell) and fully depleted it measured 45.3V (3.24V/cell). I also measured my 4Ah pack after depleting it with the lawn mower, and it measured the same 45.3V.

Measurements were taken with both my new Fluke multimeter and Milwaukee clamp meter since I was curious if the two would measure the same. They did, and they are both decent quality instruments, however neither have been calibrated or assessed... something I hope to do soon since I work in a calibration facility.

Assuming my meters are reading accurately, Ego would appear to be doing as you say; keeping things conservative up top (cell life and charge time benefits) as well as keeping a reasonable buffer down low (cell life and maintaining power output). As you say, it would be interesting to monitor real time voltage information in both the tool and charger to see what's going on.

Regarding battery protection, it would appear that the cells are all tab welded together with high current direct connections to the battery terminals, and that all the protection smarts are built into the charger and the tools. The light gauge wires from the BMS would appear to be for cell level voltage checks and/or cell balancing perhaps? You would know more about that than I would.

The fan cooled charger and PCM cell wraps are what sold me on Ego's stuff. From what I've seen, nobody else in the business has come close to what Ego has done with their battery and charger system Engineering. It really is some well thought out design. The tools are nice too, but the heart and brains are all with the battery system, for me anyway. I got into Makita's 18V tools back in 2008 for much the same reason, the fan cooled batteries and quick charge times.

If you're interested, do a search on YouTube for Ego vs Echo. Superspeeder does a neat job of tearing the Ego and Echo packs apart, as well as putting them through their paces both charging and in use on the tools. As a battery guy you might find it entertaining.

Welcome to the Ego community and thanks for a fantastic first post! :-)
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Jacob

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Awesome post...
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Jennifer VandeWater, Community Manager

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Adrian, thanks for the excellent review and research you've put into this post.  We hope this is helpful for everyone here!
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Adrian Ramirez

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Jennifer, would you be willing to provide me with information about what the chargers look for before charging an EGO battery?  Or a brief list of things that will prevent a battery from charging?
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Flow Ir In

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i'm after same information.
Ego doesn't make a high Ah backpack battery - i'd like to have 20Ah at least, 50Ah would be ideal, that i can plug into the brush cutter tool. A high capacity cell is all that is missing before the tool is useful outside of a small garden. If you guys don't hurry up and make one, i can see myself needing to build one.
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Blue Angel, Champion

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Check Ego's UK site, they already have one. It's just not sold in North America yet.
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Jacob

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Yay a fellow builder.

I took 2 batteries and used a schotty diode to do the combining. It works actually quite well but not as efficient as an op amp. But quick and dirty is sometimes good enough. My home built mower has never been so powerful.
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szwoopp, Champion

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Sounds like we need an updated video highlighting this and any other enhancements since your initial build and use.
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Jacob

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I guess I could do that. This weekend
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Jacob

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https://youtu.be/HzN4YiwsM58
Not the best video.
(Edited)
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szwoopp, Champion

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That is one monster grass eating machine.  Very impressive build, dual battery, self propelled - very nice. 
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Jacob

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Im going lead acid next year I think with ego battery backup. Ego batteries just aren't cost effective for this application. I couldn't care less about weight and charge time.
(Edited)
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szwoopp, Champion

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Interesting - I think that matches the battery riding mowers currently on the market.  Likely for the same reasons.
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Adrian Ramirez

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Blue Angel, the information you've provided is interesting.  Based on your measurements, I would guess that they have a cut-off voltage of 3.0V per cell and the battery voltage recovered a little before you were able to take the measurement.  When my 2.5 Ahr battery died in my blower, I noticed that it cut power and then a few seconds later, it was usable again.  This repeated over and over, so it appears that the voltage recovery of the battery pack tends to be greater than the amount of hysteresis that they provide in the electronics between the cut-off and recovery voltage thresholds.

You are probably correct about the small wires being for cell voltage measurements and cell balancing.  I would expect a battery with 14 series cells to have cell balancing.
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Blue Angel, Champion

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The funny thing is, the 4Ah battery depleted in the mower measured the same as the 2.5Ah battery depleted in the 15" string trimmer. With the 2.5Ah battery I did the same thing; it died, and I was able to fire it up a few times, just briefly, before it refused to run at all.

Since you have 7 (!) Ego batteries, maybe you can check a few of yours after depletion in different tools and see if your experience reflects mine?
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John

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This may sound silly, but my hearing aid batteries act in the same way. When they stop working in my hearing aids, I have found that if I open the battery doors so the hearing aids don't work, after a few minutes, I can close them and they will work for a few more minutes. If I leave them out over night, the hearing aids will work for more than an hour before they signal they are dead again.
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Adrian Ramirez

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Jennifer, would you be willing to provide me with information about what the chargers look for before charging an EGO battery?  Or a brief list of things that will prevent a battery from charging?
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Jennifer VandeWater, Community Manager

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Hi Adrian, sorry for the late reply.  I also saw this question on the other thread.  Unfortunately, this is not something we share publicly.  Best of luck on your ventures and we're sorry we can't help you here.
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Jacob

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Awe. Thats expected I guess.
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larryh

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Thank you for the excellent info on the battery.  I just purchased the Ego lawn mower (LM2100SP)today.  I have not had a chance to use it yet.  I haven't had a chance to read over the posts in the forum so I may be duplicating existing information. 

I have a PHEV (Ford Fusion Energi) and have experience with the battery in the car monitoring the BMS via the onboard OBD II connector for the past three years.  However, I am not an expert on Li-Ion batteries.  You can see posts I made regarding degradation of the car's battery here:  http://www.fordfusionenergiforum.com/topic/4121-hvb-degradation/.  After three years, degradation is approximately 5% of the original capacity.  But others, in warmer climates, have experienced far greater degradation (20% or more). 

I just did a quick test to see the energy consumed by the Rapid Charger charging the battery using a Kill-A-Watt meter.  I see it consumes 486 watts of power (117 volts at 6.3 amps with a power factor of 0.66).  I'm not sure why the power factor is not 1.0.  When I charge the car it is 1.0.  I don't know how efficient the charger is and how much power the electronics and fan consume.  If I make a rough guess based on what I observe with the car, the fans and electronics consume about 60 watts.   The charger in the car is 80%-90% efficient depending on whether I charge at 120 volts or 240 volts.  So I will just guess it is 85% efficient.  If it takes an hour to charge the battery, then the capacity of the battery is roughly (486-60)*0.85*1 = 362 watt hours.  This matches your estimate.  That is again not very close to the rated 420 watt hours.  I will have to measure how much energy the charger actually consumes charging a fully depleted battery.  I would expect that they do not fully charge the batteries to their rated capacity--that would significantly reduce expected lifetime of the batteries.  For the car, the rated capacity is 7.6 kWh, but the BMS only charges it to a maximum of 7.2 kWh. 

If the battery voltage is 56 V and 360 watts is applied, that means 6.4 amps is being applied to the battery.  The charge rate is then roughly 0.85 C which is considered fast, but reasonable. 

If these Li-Ion batteries work the same way as the ones in the car, then the analysis in the post above applies on how to maximize their lifetime, which is of special concern to me, and most likely to  anyone else given their cost.  It would be nice if EGO provided guidelines on maximizing their lifetime. Based on what I do with the car, I have come up with the following..  But I am not sure how practical these guidelines are with the EGO batteries since I have no experience yet and how appropriate they are for the EGO batteries:

1.  Leave the batteries discharged until needed.  Store them in a cool place (inside the house rather than a hot garage).  You want to charge the batteries as quickly as possible and then use up the charge in the battery right away.  Its the combination of high state of charge (SoC) and high temperatures that cause rapid degradation of the battery. 

2.  If you do not need to fully charge the batteries to mow the lawn, then only charge them enough to finish the task. 

3. When Depth of Discharge (DoD) exceeds 70%, battery lifetime will be impacted.  EVs attempt to keep the SoC between 80% and 25%.  It might be better to charge the battery only to 80% and then let it deplete to 25%.  This might be easier with multiple batteries since then you don't have to wait for the battery to recharge if you don't complete mowing the lawn before SoC falls to 25%. 

3. Degradation increases when the SoC is below about 25%.  I would try not to deplete the batteries much below that on a regular basis.  Do the parts of the lawn that require the most power first (when the SoC is high) and reserve the parts that require the least power for last (when SoC is low).

I think in terms of priority, item 1 is the most important.












(Edited)
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Adrian Ramirez

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Great explanations Larry!  I would add that you could also store the batteries in the fridge to further extend the lifetime if you choose.  But then you would want to wait for the battery to warm up before charging it, which is kind of a hassle.  I personally don't do that, I just keep mine in the kitchen (next to the garage) and I store it nearly fully charged because I prefer to accept the increased degradation as a trade-off for convenience.  I like to be able to grab my mower whenever I want without planning ahead and it's acceptable to me that I'll have to replace the battery more often with this use profile.

One thing I would add to the great information you've provided already is that charge power is most-likely not constant throughout the entire charge.  When charging li-ion batteries, it's common to first current limit (called CC or constant current mode) for the majority of the charge and then toward the end (once the charger voltage reaches the maximum cell voltage), transition into a voltage limiting mode (called CV or constant voltage mode).  Assuming the charge begins with a dead battery, during the constant current mode, the cell voltage is increasing at a constant current, so power output from the charger is increasing also.  The highest power point is right at the transition from CC to CV mode because that's where both the current and voltage are at the maximum.  Once the CV mode is entered, power begins to fall because at a constant voltage, as the cell continues to increase in voltage internally, the current naturally falls, approaching 0.  It never actually gets to 0 because the charger usually stops charging once the current is low enough that the cell is considered fully charged.  I normally use the cell data sheet taper current value to set the charge termination point.  This is usually between 30 and 100 mA.

The Ego battery might be a little different than this though.  In order to optimize their charge time, they may have removed the CV portion since that's a very time consuming portion of the charge and accounts for a very small percentage of total capacity that is put into the cells.  If that's what they've done, then they could still charge up to 4.2V per cell, but then stop charge once 4.2V is reached, and the cell would then relax to some lower voltage, maybe around the 4.1V that people have measured after charge.  The other thing they might do is to actually charge to 4.1V, which would prevent 4.2V from being on the cell (decreasing rate of degredation) and keep the CV mode in there, but at 4.1V instead of the typical 4.2V.  But this would extend the charge time quite a bit and since Ego has put great effort into short charge times, I would think they probably charge to 4.2V, but skip the CV mode.  This would be an interesting thing to test.  Unfortunately, I don't have a data logger or battery life tester at my house, but maybe I can cobble something together with a volt meter, current probe and a video recorder.
(Edited)
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David Cline

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Don't store in the fridge, per Ego.

The condensation that forms inside when you take it out can do more damage than the fridge does good. The internal electronics are sealed to be moisture resistant, but may not moisture proof against condensation from rapid cooling and warming.
(Edited)
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Adrian Ramirez

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Ah, I didn't think about that.  That makes sense
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larryh

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The EGO charger works similarly to how the charger for my car works. It tapers charging at the end. The graph shows power consumed by the charger. I don't know why power is increasing with time until 6:21 pm. I would have expected it to be steady.

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Blue Angel, Champion

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Awesome graph!  What hardware are you using to capture the data?

I think the power increases slightly with time because the battery's voltage is going up, so increasing voltage with constant current means more power.
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larryh

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I have a Current Cost EnviR Home Energy Usage Monitor.  You connect it to your electrical service panel to monitor electricity usage.  The data is downloaded to a server.  You can access your energy usage from a web browser.  You can also download the data to your computer.  I use it to identify wasted electricity to reduce my electric bill.  I generally use less than 400 kWh of electricity a month even with a PHEV. 

I have an individual appliance monitor (IAM) to monitor electricity consumed by an appliance.  It plugs into a wall outlet and then you plug the device into the IAM.  It sends the data to the EnviR device which sends the data to the server or you can download it to your computer. 

It looks like the charger spends a lot of time charging the battery from 90% to 100% SoC.  I think where the power begins to taper off is at 90% SoC. 

The charger for my car reduces current as the voltage goes up to keep the power constant for charging until SoC reaches 92%.    It then reduces current to the battery in steps similar to what appears to be happening with the EGO charger.  The car spends much less time than the EGO charger charging the car from 92% to 100%. 

(Edited)
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Blue Angel, Champion

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Could that be because the car's 100% is actually the cell's 80%? It would make sense since the CV section of the charging curve would be relatively minimal.

If your car's BMS behaves the same as a Chevy Volt then charging time may increase as the battery ages. My understanding of the Volt strategy is that, over time and charge cycles, as the battery gradually loses capacity the BMS starts using a greater percentage of the battery's capacity to maintain the electric range as time goes on. For example, when new the battery charges to 80% and depletes to 30%, and years later the BMS will charge to 90% and deplete to 20% to keep the same kWh available to the driver.

If that's the case, going from 20-90% will take longer simply because the battery wills spend longer in CV "top-off" mode, even though the overall energy put into the battery remains the same.

Having said that, that effect on a large PHEV battery would be pretty minimal I would think since the charge rate is much lower on a PHEV than on an Ego battery pack, which on the Rapid Charger is better than what EV's are seeing at DC fast charging stations as far as charge time goes (~80% in 30 min).

Maybe Musk should look into PCM cell wraps and fan cooling his batteries! :-)
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Adrian Ramirez

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Regulating constant power to charge a li-ion battery pack is not typical, at least in small battery packs.  My first thought is that maybe they do it that way because of the limited amount of power that is available in a 15A or 50A circuit that the car is being charged from.  In smaller pack applications, that wouldn't be a concern, so they could charge at the full rate that the cells are capable of, which would be a constant current, resulting in an increase in power until the CC-CV transition point.

In CV mode, current is no longer limited by the charger, only voltage is limited.  So the decreasing current is a natural result of the cell voltage increasing as it approaches 100%.
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larryh

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The BMS of the Volt manages the battery more carefully than the Fusion Energi. The Volt is closer to a BEV with the gasoline engine used primarily to charge the battery in order to extend the range of EV operation. The Fusion Energi is closer to a hybrid with the gasoline engine used to propel the car when the battery is depleted. The state of health of the battery is more critical to the Volt.

The BMS in the Fusion Energi operates similarly to EGO's BMS. As with the EGO:
1. The Fusion Energi has active air cooling to cool the battery.
2. At 100% SoC, the cell voltage is 4.1 volts.
3. The car won't let you discharge the battery below 15% SoC. However, SoC will fall below 15% after the battery cools and reaches equilibrium. The BMS does not always accurately estimate the battery SoC when the battery is being used. It guesses the battery is at 15% SoC at the end of a trip. However, after the battery has rested and the voltage stabilized, SoC can be computed more accurately as a function cell voltage. Many times it ends up being around 9% SoC. At 9% SoC the cell voltage is 3.46 V.

The Fusion Energi is different in the following respects:
1. It is a 25 Ah battery capable of storing 7.6 kWh of energy.
2. It charges using 10 A of current at 240 V at a 0.4 C rate.
3. It charges using 3.33 A of current at 120 V at a 0.13 C rate.
4. It uses NMC rather NCA chemistry.
5. It has 84 cells in series.

As the battery degrades, energy capacity and maximum power output fall.  The range of voltages at which the battery operates does not change.  It will take less energy to charge the battery and charging completes in a shorter amount of time. 


With the Fusion Energi, I am able to interface with the BECM (Battery Energy Control Module), the BMS, to monitor hundreds of parameters of the battery. It would be nice if EGO had a device to talk to its BMS.

(Edited)
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larryh

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The following chart shows the degradation of the Lithium-Ion battery in the car over a three year period.  I would expect the EGO battery to perform similarly.  The BECM provides the estimated energy capacity of the battery.  I have recorded that value along with the temperature of the battery every morning before work for the past three years and plotted the results below.  There is not enough data below 50 F to provide useful information.  The first year, the capacity was near 7.1 kWh at 85F.  This year, it is near 6.8 kWh.  I have lost 0.3 kWh of capacity.  Degradation is much worse at cooler temperatures.  The first year, capacity at 50 F was 7.0 kWh.  This past year it was 6.4 kWh.  I have lost 0.6 kWh of capacity at colder temperatures.   I'm not sure how to compute degradation of the battery since it varies with temperature.  Is there a standard temperature, i.e. 30 C, at which they compute degradation?  Others in warmer climates have experienced much greater degradation, i.e. capacity less than 5.4 kWh in the summer.

(Edited)
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Blue Angel, Champion

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So it looks like the higher the starting temperature of the battery the more capacity the BECM estimates it has (makes sense), and also that with age the battery doesn't tolerate lower temperatures as well as when new.

When I worked for Transport Canada we had two Japanese spec iMiEVs in the fleet. I picked one up from my Director's house one chilly January morning (-25C, -13F), it was plugged in and fully charged and displayed an estimated range half what you'd expect in normal summer weather, 60 km instead of 120 km.

Then I turned the heater and rear defroster on and the estimated range fell to 40 km! That's only about 25 miles... not so hot for a fully charged EV! :-) I'm pretty sure the North American models ended up being equipped with a battery heating system for that very reason. A guy at work has one, I should ask him.
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larryh

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The battery in the car is much more tolerant of cold temperatures than EGO's battery.  The EGO battery cannot be charged at temperatures below 32 F.  I believe that would result in lithium plating at the anode, permanently damaging the battery.  It also attempts to warm itself up when temperatures fall below 32 F before providing power. 

The Fusion Energi's battery can be charged at temperatures well below 0 F, albeit at slower rates when battery temperature falls below -15 F.  Battery capacity plummets when battery temperature falls below 0 F.  At 80 F, capacity is 7.1 kWh.  At 0 F, it is 6.0 kWh.  At -10 F it is 5.0 kWh.  At -20 F it is probably below 4.0 kWh.  Also power output from the battery plummets when battery temperature falls well below 0 F.  If you demand too much power, the battery can't keep up and the car is forced to run the ICE.  Regenerative breaking is disabled at temperatures below -15 F.  The car will use the friction brakes or the ICE instead (DFSO engine braking). 

There is no battery heater in my car to warm the battery.  The battery warms up only via charging/discharging.  However, there is an electric heater in the heater core that draws 5 kW of power to warm the cabin and indirectly warm the battery.  That is not adequate for keeping the car warm when temperatures fall below 0 F.  At those temperatures, if you turn on the heater, the car will start the ICE to warm the car.  In subzero temperatures, I precondition the car.  The car draws power from the wall outlet to run the electric heater.  I have a fully charged battery and a warmed up car before I leave.  That way I don't need to run the heater (nor the ICE) for the short 12 minute commute to work.  When temperatures fall below 10 F, the car will periodically run the ICE regardless of whether you turn on climate control or not. 

Besides decreased battery energy capacity at low temperatures, internal friction in the drivetrain increases dramatically in the cold.  It requires three times more power just to turn the wheels when the car is cold and it is below 0 F.  The viscosity of transmission fluids goes way up.  It requires about 50% more energy for my commute to work when temperatures fall below 0 F.  (Note that increased air density and tire rolling resistance in the cold are also a factor, but not as much as the increased drivetrain friction.)  So with decreased battery capacity, increased friction, and running the electric heater, EV range is going to go way down in the winter. 


(Edited)
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Blue Angel, Champion

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Yes, it wouldn't surprise me to find out that automotive grade lithium cells are a completely different animal than commodity 18650s. It's pretty crazy how many qualifications there are and how much testing is done before something gets into a production car.

Having said that, Tesla reportedly uses a reasonably normal Panasonic 18650. They may make up for performance issues by way of sheer size... their packs are huge.

Don't forget about wheel bearings, whose grease is pretty darn thick at low temps.

When I built my block heater setup I installed a 50W pad heater on my transmission. The fluid I use is slightly thicker than the stock fluid, and with the heater running the car shifts beautifully right from the get go. Without it, shifting for the first minute or so can be a pain on real cold mornings.

Impressive that you've logged so much data from your car over the years. My wife thinks I'm nuts for keeping track of my fuel use with an app! :-)
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larryh

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Another video dissecting the EGO battery can be found here:

http://www.workshopaddict.com/tools/ego-lithium-ion-internal-battery-dissection/

They claim 2000 charge/discharge cycles of battery life in the laboratory. 

The light gauge wires from the BMS mentioned in an earlier post are to activate the PCM to cool the battery. 


(Edited)
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Adrian Ramirez

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I saw that video, I think that guy mis-spoke. I believe PCMs change phase by heat (I don't have much experience with PCMs, though so I could be wrong).  And the 7.5 Ahr cells that don't have PCMs also have those same wires, so that seems to suggest that they don't play a part in the PCM. Blue, did you look to see where the wires on the 7.5 Ahr battery ties into? I suspect one wire into each parallel set of cells. How many wires did you count?

2,000 cycles is awesome, but I bet those are automated back to back cycles with little or no rest. He apparently believes they can get 5 to 8 years out of the battery, so at an estimated 1 mow per week (his comment), maybe 300 to 500 cycles in the real world. When there's a lot of rest time between cycles, calendar degradation is a significant contributor to total battery degradation. Many companies don't test the effects of calendar degradation because to accurately measure it requires years of testing and that's expensive and doesn't usually fit into product schedules. Unfortunately there isn't presently a good way to perform accelerated life testing on calendar degradation, but cycles can be run really quickly, so most people do that. People have created simulation models, but the models need to be proven and you're back to years of testing to prove it.

I've also seen one case (with a Samsung cell in particular) where cycle data looked excellent, with nearly 0 degradation in the first few hundred cycles, but once the batteries were stored at 100%, the calendar degradation dominated so badly that the cell would have been lucky to get 1 year of usable life in the real world (in an application where they were stored that high anyway). eSDI (the Samsung distributor in the US) was very surprised when I showed them my data.
(Edited)
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Jacob

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You cant activate pcm. Think of pcm as water that is frozen (ice). It cannot be activated. It just means Phase Change Material . Uses the "latent heat of fusion" for the material to maintain temperature inside a range.
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I don't have the 7.5, but the 4Ah has a wire tap for every cell set.  I would imagine the other batteries are exactly the same, only differing in the number of cells in each parallel set (1, 2 or 3).  They all use 14 cells in series to get to the same voltage, so they would all have to balance voltage in same number of steps.

I agree with Jacob.  My understanding of PCM is that it reacts to heat input only.  Those lead wires go directly to the cell's + and - connections.  Nothing physically touches the PCM, not even the metal outer jacket of the cell; there's a layer of shrink wrap between the PCM and the cell, with another wrap over the PCM sealing it to the cell.

Not even the elaborate transistor boards contact the PCM in any way, they only contact the end of the cells for what we think may be some form of temperature monitoring (still not 100% sure on that one... Adrian?  :-) )

Barnaby is an entertaining fellow, but he sometimes has his technical details a little fudged up. :-)
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Adrian Ramirez

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That's right, I don't know how many times you've told me it was a 4 Ahr pack, and I still have trouble remembering that.

Awesome, glad I'm not far off on my understanding of PCMs.  My best guess for the transistor board is still that it provides temperature monitoring.  I think David or someone had commented that they may also be part of the discharge circuitry, so that's another great thought about what they might be for.  But the reason I'm thinking they aren't part of a discharge circuit is because they don't have any associated resistors to dissipate the heat and it also wouldn't make sense to have the transistors touching each cell because you want to keep heat away from the cells.

My guess is that the discharge functionality is very low power and takes many many hours (or even days, possibly) to perform, where the heat is dissipated slowly through one or more of the 1206 (or whatever size) resistors that are surface-mounted onto the BMS board from Blue's image of it.  If they are 1206, I think the power dissipation of that package size can be around a watt or so, but for reliability reasons, you wouldn't want to max out a resistor's power rating on a regular basis, so I would guess maybe 50% derating, so maybe 0.5W dissipation over the voltage range from 100% (Ego's 100%, which would be around 4.1V) down to 30% (I'm unsure if 30% that they discharge down to is absolute (based on 4.2V being 100% SOC) or relative to Ego's 4.1V being at their 100% SOC.  But either way, my guess is that those little resistors do the discharging.  Since there are no large AWG wires running into the BMS board, that also means if the discharge happens on the BMS board, it must be very low current out of the cells.
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I've witnessed the self-discharging event. I was moving a few things around in the garage last fall and when I grabbed one of my batteries the end cap was warm... I was alarmed at first, wondering if there was a problem with the battery! When I grabbed the second battery and its end cap was warm too, I knew something was up...

I had charged all of my batteries on the same day before putting them away for the winter, so they all "timed-out" at about the same time and we're going through their programmed self discharge together.

I contacted Ego just to be sure, and they confirmed that the end cap does get warm when discharging takes place and it was nothing to be concerned about. I made the suggestion to add a note in the user manual so people would not be alarmed (like I was), but as far as I can tell there has been no action on that remark.

To be clear, the end cap was not hot but it was noticeably warm to the touch. Definitely not hot enough to do any damage to anything nearby, but warm enough to make me wonder if everything was OK. :-)
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David Cline

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So if the self-discharging produces heat, is this the same circuit that is activated to warm the battery up by making it pulse when it's below freezing?

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That sounds as if the battery uses power drawn from the tool to warm itself and will "pulse" instead of operate with smooth output for the first minute or so.

In my opinion, the small amount of heat generated in the end cap for self-discharge would not do anything significant to heat the cells.
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David Cline

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So is the pulsation just a characteristic of the cells, not a protection feature added by Ego?
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This would likely be something designed into the BMS by Ego, if in fact it functions the way I interpret it. It's possible their studies determined that it's less detrimental to a cold cell to pulse it gently up to temperature than to pull continuous current from it? There I go speculating again. :-)

If that's the case, it might be another unique feature in the Ego battery platform.

Since we now have a resident battery expert who specializes in extreme environment Lithium cell applications, Adrian, do you have any cold temperature cell testing wisdom to share?
(Edited)
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larryh

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Are those wires then used by the BMS to monitor and balance the individual cells? What happens when they are unbalanced? Does the BMS drain a little extra current from cells with higher voltages via these wires? Does it balance the cells continuously while charging and discharging? Is the BMS always on monitoring the battery even when the battery is not connected to anything?

The following is data I have collected from the car showing voltage variation between cells vs SoC (left) and vs. Power output from the battery (right). As expected, with higher levels of power output (positive power), the variance between cells tends to increase. The BMS is having a hard time keeping up balancing the cells. Same for regen (negative power). Also, voltage variation goes up as SoC goes down, especially when SoC falls below 20%. Note that even though the car limits power below 20% SoC, variance still goes up. Does this imply that low SoC and high power outputs are stressing the weaker cells and accelerating degradation of the battery?

What is an acceptable voltage variation between the cells? If I have exhausted the battery, and cells are unbalanced, is it a good idea to charge up the battery a little so that it has an opportunity to rebalance the cells? Or will it do it without recharging?

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Adrian Ramirez

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Sorry for the delay, I was hiking Mt Elbert this last weekend and it didn't leave me with much free time.  That's the tallest mountain in Colorado (but not nearly the most difficult).

Those are a lot of really good questions.  The answers will require a lot of explanation, so I'll provide pieces of information over days or weeks (depending on how much time I can squeeze out here and there).  Let's start with looking at the discharge curve of the Samsung INR18650-25R battery cell.  This is from the Samsung data sheet of the battery cell:



This shows one cycle on their cell at different discharge currents.  As you can see, at higher discharge current, battery cells exhibit less capacity.  This reduced capacity isn't degradation, it's just temporary loss of capacity for that cycle and can be recovered at any point by reducing the load current.  So for example, if you are following the 25A line which starts at 4.2V, and then quickly drops to 3.7V, and then let's say that you lighten the load at 3.3V from 25A to 5A.  Then the voltage of the battery cell would increase from 3.3V to around 3.7V or so.  If you continued to discharge at 5A, you would continue to follow the 2nd curve (pink-ish color) down to cutoff voltage (which Samsung chose as 2.5V here).

To be continued...
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Adrian Ramirez

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The internal impedance of the battery cell is what causes the voltage drop in the Samsung chart.  You can think of the impedance as a resistor in series with the battery cell.  The impedance of the cell is not constant, but varies with (1) temperature, (2) age of the cells and (3) state of charge.  1: At higher temperature, the impedance is lower and at lower temperature the impedance is much higher.  This is one of the reasons that it's a good idea to heat up cold cells before use, because otherwise you won't get nearly the full usable capacity out of them.  But, cold temperature and very high current can also cause increased degradation to the cells, so it's better to heat them up before they experience high current, but if you heat them up too much, the impedance does decrease, but then they also experience increased degradation as well.  Typically you want to be between maybe 10C and 60C (but 15C and 45C is better) during discharge.  The lost capacity due to cold temperature is recoverable once the cell is brought back up in temperature, so there isn't really any permanent degradation here (except for the degradation due to high current at low temperature).  2: As the cell ages, the impedance increases as a function of both the number of cycles and the calendar age of the cell.  At higher voltage, the calendar aging is more severe.  At extremely low voltage, the cell also degrades quickly, so it's important to keep the cell in the middle somewhere during long term storage.  It's common to store li-ion cells between 30 and 50% SOC.  3: As a battery cell is discharged, the impedance increases slightly until the knee of the voltage curve (where the voltage changes from a linear decrease to an exponential decrease, between 2.9V and 3.3V depending on discharge current in the above chart).  After that, the impedance increase is much much greater.

The impedance of the battery cell causes two problems.  (1) It causes energy to be wasted in heat rather than being used to power stuff.  (2) It causes a voltage drop out of the battery cell so that you reach the cut-off voltage or End of Discharge Voltage (EDV) earlier than you otherwise would at lower currents and higher temperature.

In the below screenshot, you can see how the capacity is reduced as the temperature decreases.  Since the cutoff voltage that Ego uses is somewhere around 3V/cell, that creates a very wide range of capacities depending on temperature according to this chart.  In the beginning of the discharge, the voltage drops substantially, but recovers (and voltage increases) due to the cell self-heating because of the 10A load they are putting on the cell.  Notice that at -20C, the voltage decreases nearly all the way down to cutoff voltage before the battery heats and recovers.  It's not typical to discharge down below 2.5V, so I'm not sure why Samsung shows the voltage all the way down to 1.5V.  Usually these kinds of charts stop around 2.75V or 2.5V.



To be continued (on a different day probably)...
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Blue Angel, Champion

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Keep it coming, Adrian, this is good stuff!!!
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Adrian Ramirez

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So the above explains what happens to battery cells at cold temperatures and why, from a usable capacity standpoint, it's important to heat them up before use. I don't know the answer as to why they pulse the cells rather than just a constant load. Maybe it's a type of slow PWM where they pulse at different rates depending on what temperature the cells are and how much they need to heat them. PWM is usually easier and cheaper than implementing a variable analog load, so maybe it's just an easy way to vary the amount of heating.

Continuing with battery theory, the reason it's important to keep cells balanced is because you don't want to over-charge or over-discharge a battery cell. If you have a string of cells in series, and your lower voltage limit is, say 3V/cell, then once the first cell hits 3V, you have to stop the discharge. Any cells that are above 3V have extra capacity that can't be used because it would further discharge the cell that has already reached 3V, so some capacity is wasted if the cells are not perfectly balanced. Remember that in a series circuit, current is equal through all elements, so all cells are theoretically discharged at exactly the same rate. But in the real world, even cells of the same model and lot are not exactly identical. Some cells might have slightly higher internal impedance, which would cause them to burn a little extra energy in heat and also cause them to hit the cut-off voltage threshold earlier.

If one cell remains warmer than the others during use because of uneven cooling or impedance differences, it could also age the cell faster, so it might have slightly lower capacity or slightly higher internal impedance and reach the cut-off voltage threshold faster than the others.

The same problem happens during charge. If there is a voltage imbalance between series battery cells during charge, the cell with the higher voltage will hit the upper limit before the others and if the others are far enough behind, the pack applied charge voltage could cause the higher charged cell to be over-voltaged before lower cells are able to catch up. If the BMS has cell voltage monitoring and cuts charge early because one or more cells have reached their upper limit, then the lower charged cells are not charged as much as they could be and energy storage is wasted. If there is no cell-level voltage monitoring by the BMS, then the cell will likely be over-charged on every charge cycle and will be damaged more and more until it either fails or explodes.

For these reasons it's important to keep series cells balanced. Cell balancing can happen during charge, discharge or rest periods, but it's most common to only do it during charge or rest. Passive cell balancing bleeds off excess charge from a cell through a resistor. It basically wastes energy on higher-charged cells to bring them to a lower voltage so that the lower cells can catch up. The cell balancing is usually PWM'd where each cell is pulsed at a different rate depending on howuch energy needs to be removed from it
Active cell balancing is similar, but moves charge from one cell into another instead of just wasting it. This is usually through a capacitor. Passive cell balancing is more common in my experience.

Since the excess energy on higher charged cells of a series stack is not usable anyway, passive cell balancing can happen during discharge without any effect on the usable capacity.

Even though there are differences between cells, they are usually very small, so cells are usually very balanced. It's common for high quality cells to be within maybe 3 or 5 mV of each other, especially when the BMS provides cell balancing. In my designs, I usually set my cell imbalance protection (the point where the battery is disabled for having imbalanced cells) at 250 mV.

Cell balancing is a very low current thing. Since cells are usuallyostly balanced anyway, it only takes a very small out of current to bring higher charged cells closer to lower charged cells. If a cell has a mild internal short, it could cause a larger imbalance the the balancing circuit can handle and then eventually the pack will be disabled if the BMS is equipped with imbalance protection.

Because of the steep voltage change below about 3V on most cells, it's most for deeply discharged cells to be imbalanced until they are charged back up above 3V or so, so imbalance protection normally doesn't disable packs if the imbalance happens at very low voltages.
(Edited)
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larryh

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So the most likely cell balancing technique for the EGO battery is passive balancing?  The cells are balanced at the top while charging such that they all at the same voltage when charging completes.  That would imply you need to fully charge the batteries to make sure they are balanced?  Is active balancing significantly more expensive than passive balance and hence passive balancing is more common?  It would be difficult to balance while discharging since the cells have voltage differences due to different internal impedances.  The BMS would have to keep track of SoC and impedances of each individual cell. 

Looking at the power consumed by the charger, is there any way to detect cell balancing is occurring.  In the post I made at the bottom, I show the power consumed by the charger when charging a depleted battery.  It looks like the charger uses CC until the SoC is around 85%.  Then it switches to CV.  It looks like it also does a few short intervals of CC at the end.  You can see a few steps in the decreasing section of the curve where power is constant for a short period of time.  If it were pure CV, the power curve would be an exponential decreasing function without the steps. 

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Blue Angel, Champion

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I'll take a wild guess here, and say the balancing would most likely be passive since the BMS already has a bank of resistors to bleed the pack down for long term storage.
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larryh

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If these are  Samsung INR18650-2 cells, then the battery chemistry is NCA.  This is a chart showing storage lifetime for Li-Ion batteries with this chemistry which is derived from http://www.nrel.gov/.../pdfs/53817.pdf.  The contour lines show the expected lifetime of the battery when stored at a  specified temperature.  For example, if the batteries are stored at 50% SoC at 90 F they will last approximately 15 years.  If they are instead stored at 100% SoC, then will last only about 3.7 years.  That is why you want to delay charging until you use the battery.


battery20lifetime_zpsensasrmrpng


(Edited)
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Blue Angel, Champion

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Great chart Larry!

You have pretty much summed up how I use my batteries; charge just before using.  I am guilty of charging them up fully since there's no practical way to limit the charge voltage, but I figure the damage done is pretty low for two reasons:

1. The time they spend fully charged is minimal
2. Ego doesn't really "fully" charge the cells anyway

Assuming my meter is accurate, my packs charge to about 57.8V which is about 4.13V per cell.  Since 4.2V is considered a full charge, Ego seems to be reducing the effect of a "full" charge on the batteries, albeit not quite to the extent the car companies are doing... but these are batteries that will see on average one cycle per week for most people (vs 1 cycle/day for a car).  If an Ego pack has a useful life of 500-1000 cycles, that's somewhere between 10-20 years of use!  :-)
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larryh

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If you search the web, you can find technical data on the batteries.  Search for powerstream INR18650-25R.  It shows the average energy capacity of a cell is 9.38 Wh.  So the maximum capacity of the battery pack would be 42*9.38 = 394 Wh at a 0.2C discharge rate.  They also provide charts showing capacity vs. cycle number at 20A and 30A discharge rates (far more than would be drawn by the lawn mower).  After 250 cycles, capacity loss is 30%.  So the batteries should last much longer than 250 cycles. 
(Edited)
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Thomas Ritz

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"If an Ego pack has a useful life of 500-1000 cycles, that's somewhere between 10-20 years of use!"

If this is the case why was I only able to get one season out of my battery from my mower?  And I only mow once a week.  : )
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Blue Angel, Champion

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Something obviously went wrong with your pack... Ego would be pretty crazy to warranty a product for three years if the expected failure rate was once a season!  :-)
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Blue Angel, Champion

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Larry, the 25R is a pretty solid performer!  Thanks for the link!
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larryh

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Several posts that we made are missing from this thread.  What happened to them?
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Blue Angel, Champion

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The forum hides some posts sometimes, it's kinda confusing. If you scroll up you'll see a bar that says "View previous..." Click that bar and older posts start showing up, one by one on the mobile page anyways:

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larryh

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Thank you.  I found the tiny "view previous" links to expand the hidden posts.  Those links are hard to find--it took a few minutes. 

You posted that the cell voltage for a depleted battery was 3.24 volts.  How do you define a depleted battery?  Was the light on the battery solid green, solid red, or flashing red when you measured the voltage?   The charger doesn't seem to be very accurate when displaying SoC.

The chart below plots power consumed from the wall outlet charging a depleted 7.5 Ah battery (the light was solid red).  The Blue line is the actual power measured.  The Red line is the power predicted under the following assumptions:

1. Charger Electronics/Fan consume 37 watts of power (this is the minimum power observed while the charger was running).

2. Charge rate is 1 C or 2.5 A for the first 46 minutes until voltage reaches 4.02 V.

3. The remaining charge in the depleted battery was 0.23 Ah (SoC is 9% and OCV is 3.28 V).

4.  When charging is complete, the charge in the battery is 2.38 Ah (SoC is 95% and OCV is 4.1 V).

5. The charger efficiency is 88%.

6. The internal resistance of the battery is 0.0225 ohm.

7. The charge/discharge characteristics of the battery is as given in the posts above.

Charge time was 1 hour, 15 minutes.


(Edited)
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JaseSerre

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Hi...i am a new user here. About operating voltage, I measured my new 2.5Ah pack both fully charged and fully depleted after running it down in my string trimmer. Fully charged it measured 57.8V (4.13V/cell) and fully depleted it measured 45.3V (3.24V/cell). I also measured my 4Ah pack after depleting it with the lawn mower, and it measured the same 45.3V.
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Par for the course! :-)

Since voltage meters will all read a little differently, I'd say anyone reading along has little to worry about unless you're seeing a discrepancy of a volt or more... in which case it might be a good idea to have your voltmeter checked before assuming an issue with your battery.

Personally, I have three volt meters and one of them reads very different from the other two, which read identically on Ego batteries.
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Pope

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What a great, informative thread.  I did not realize this, but there is a whole culture centered around adapting power tool packs to electric mopeds... https://www.electricbike.com/dewalt-cordless-battery-cruiser/ is one such example
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This reply was created from a merged topic originally titled EGO Battery Advancement vs Other Batteries.

Have you ever wonder what is inside your "powerful" EGO battery, without taking it apart and voiding the warranty?  Take a look at this YouTube video, another good reason why I invest in this technology - keep up the GREAT JOB EGO !!!!!!

https://www.youtube.com/watch?v=2LP0gzGtsic
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Fred Clarke

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Adrian, if u want a battery to tear apart I have one u can have
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Art Zasadny

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Thanks for the excellent info!!
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(a)Typical Engineer

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Discharged battery voltages measured much lower than the 45.3V (3.24V/cell) previously reported, on multiple packs:

https://community.egopowerplus.com/ego/topics/how-to-add-a-digital-volt-meter-to-the-charger

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Zimms

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 Can you remove the battery from the charger early without hurting the equalizing process between the cells? Since there is no gauge on the battery the only way to tell if the battery is at 50% for storage to get the best life expectancy is with a meter?
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The battery self-discharges to 30% capacity after 30 days without use. This happens automatically, whether the battery is in the tool, charger, or just sitting on a shelf.
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Zimms

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I don't want to fully charge the battery after using it but also don't want to put it on the shelf with less then 30% charge either. I read somewhere that ego recommends not charging it right away and storing the battery between 30 and 50%
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Goodbar

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Any clever folks out there have ideas about modifying a charger to stop around 80% SoC? 

I have two chargers -- one big one that came with the mower, and a smaller one that came with the trimmer. My preference would be to charge to 80% (or maybe less) after using the batteries, then top up to 100% right before use... without needing to babysit. Ideally I could use one charger for a storage charge and the other for full charge, unless it would easy to make the charger switchable. This echoes Zimms's comments.
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szwoopp, Champion

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I throw mine on the rapid charger until the second (50%) light starts blinking.  Then I figure I am somewhere around 30% which is a good storage level.  Usually, only takes a few minutes so it doesn't require extensive baby sitting.
Unfortunately, I am no help in your quest for an automated solution.
Why are you aiming for an 80% charge ?
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Goodbar

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Mostly laziness and lack of planning. I don't want to have to plan ahead to get most of a charge right before mowing. Also, there should be less pack heating right before use.
(Edited)
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Blue Angel, Champion

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You could just charge before using instead of after. That’s what I do and my battery is then sitting at roughly 50% charge until next mow.

Charge time difference between 50-100% and 80-100% is less than you’d think. The last 20% charge takes much longer than the 20% before it. 80% is roughly where the charger switches from Constant Current mode to Constant Voltage mode to top the battery off.

In CV mode the charger reduces current to keep the battery voltage from exceeding the max 4.2V per cell. This is why you see electric vehicle manufacturers claiming that DC fast charging stations can get the car to 80% state of the charge in very little time.
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Goodbar

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I'm familiar with the CV charge stage and am an EV owner. It seems like the Ego low battery warning turns on at about 20%, and I try to swap batteries at that point (I can no longer mow my whole yard on one battery after switching to the high-lift blade). I'm looking for the easiest way to maximize the life (i.e., avoid SoC extremes and heat) of these pricey batteries.

I'm currently charging before use, but home life is a little hectic with young kids, so sometimes I get into a bind.
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szwoopp, Champion

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Red light comes on at 15% so you are correct that it is a good practice to swap batteries at that point and not run the tool to blinking red light or complete shut down.