Ego Battery Review from a Battery Engineer

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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 3 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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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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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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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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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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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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Blue Angel, Champion

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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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Blue Angel, Champion

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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.