Electric bikes, and other "off label" uses for EGO battery packs

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This conversation has been merged. Please reference the main conversation: I would like EGO to build e-bikes.

I highly doubt EGO wants to get into the bicycle business, but their battery packs could easily make some waves in that space....

Some quick background:  Electric bicycles have been a "thing" for a while now.  They're basically normal bikes, but with a battery and motor grafted on.  You still pedal, but the motor makes it easier to climb hills, accelerate from stops, or just cruise effortlessly at high-ish speeds.

You can buy a "cheap" electric bike for around US$1,000 (crappy motor, crappy battery), while the fancier e-bikes can cost $5,000 or more.  There are also conversion kits that let you add the electric bits onto most any bike you already own.  A nice conversion kit also costs about $1,000. About $500 for a nice mid-drive motor, and about $500 for a ~50V, ~8AH lithium ion battery pack.

That last part should sound familiar!  EGO is now selling battery packs in that power range, and they are selling them for a very good price.  EGO's packs also include battery management, and advanced cooling technology that is comparable or better than what even the fancy electric bikes are using these days.

I'll finally get to the point:  It would be fantastic if EGO could sell some sort of "universal battery connector" that would let hobbyists of all persuasions easily connect any of the existing EGO packs to whatever they want.  In other words, a clickie battery socket, just like what the mower uses, except without the mower part!  I'm guessing that this would result in lots of people buying "bare" EGO batteries for their various crazy projects, but once they own an EGO battery pack, they'd probably also start buying the various EGO devices that "officially" use those batteries.  Everybody's happy!

(I'm guessing that EGO's lawyers wouldn't be thrilled at the prospect of people building various EGO-powered devices, and then setting themselves on fire, or wrapping themselves around a tree somewhere.  But hey, that's what disclaimers are for!)

To put it another way, I'm tempted to crack open my mower battery, just to see if I could bodge up a connector so I could also use it to power an e-bike.  But it would be awesome if I didn't have to "hack" it, and could instead just buy an official connector/holder that I could permanently attach to my bike.

Disclaimer:  I don't actually own an e-bike at this time.  But I'd like to, especially if I didn't have to buy that overpriced $500 battery for it.
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Dave Nagy

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Posted 4 years ago

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Stan

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Hi Dave,

Love your idea!  I have not seen any other lithium-ion batteries and chargers like EGO's anywhere.  To me, it was their battery technology that swayed me into buying the lawn mower (even though it's not self-propelled) and the 12" string trimmer. 

And while they are making that universal battery connector, make it so you could hook it up to a solar panel to charge it.  If your going green with electric power why not go all the way!!!!

Stan Fayfar
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Blue Angel, Champion

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Dave, I love the idea! So much so, in fact, that I almost built an e-bike based on Ego parts.

My plan was to use the 15" string trimmer as a donor, since it has an extremely powerful and efficient brushless motor as well as a variable speed control. Just think... for the price of a trimmer kit ($179 at Home Depot USA) you get everything you need to Macgiver together a pretty cool e-bike (donor bike not included).

Once the bike is up and running you would just need to figure out what your riding required as far as battery packs go. A 2Ah pack likely wouldn't get you too far, but I bet the 7.5Ah pack would!

Going one step further, a backpack with battery connector as well as storage for additional batteries would keep the bike from looking much like an e-bike (no batteries attached), and could connect to the bike via electrical cord. This way you're not leaving the expensive and easy to remove packs with the bike.

In the end, I really have no need for an e-bike nor do I really have a place to store it. I ended up giving the spare 15" trimmer I bought to a friend as a housewarming gift. He's blown away by its power, though he already had the Ego mower so his standards were pretty high. :-)
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Blue Angel, Champion

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That's what I'm not sure of. I don't know if the packs could be used off of the Ego tools without upsetting the control circuitry. The circuitry monitors the individual cell voltages (cell balancing), and might not be happy if the pack is drained without it knowing what's going on.

Also, I would think the packs would no longer have warranty coverage if used in a non-Ego application? There are protection circuits that allow the battery and tool to be shut down in the case of high temp, over drain etc. If the battery terminals are live all the time you could do serious damage to the battery without the circuitry being able to intervene.

For those reasons I thought it might be best to use an Ego tool for everything.
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Dave Nagy

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My guess would be that all the battery protection, balancing, and monitoring circuitry would reside somewhere in the battery pack itself.  So if (big if) one could convince the pack to "dispense" power via the pack's normal output terminals, I would again suspect that you'd get the benefit of all that circuitry, even when an EGO tool was not connected.  I'd guess that the circuitry present in the tools themselves is devoted to protecting the tool itself. (Over-temp sensors on the motor, etc.)

Yes, this whole endeavor would certainly void your warranty coverage, even if you never cracked open the battery case.  You could of course lie about your off-label use, but that would be pretty dishonest.

Another fly in the ointment could be the max discharge rate that these packs can safely support.  Purpose-built bike battery packs often use somewhat high discharge cells, so that they can supply 1000 watts or more if you crank the throttle wide open.  Most of the time a bike is only drawing a couple hundred watts or less, but it's desirable to have the ability to power up the occasional steep hill.

If I remember right, the 4AH pack will last about 30 minutes in the mower, if you really work it hard. (Not hard enough to trigger the overload cutoff, but close to that.)  If that's right, the mower must be pulling about 8A.  8A times 56V is about 450 watts. Two thirds of a horsepower.  That's a bit on the skimpy side if you want a bike that can climb hills.  750W would be better.

Perhaps if you used two 4AH packs in parallel, rather than a single 7.5AH pack, you could double the peak wattage available to you, without either of the packs ever feeling overtaxed.  Unfortunately, that brings you up to the $500 price of a "real" bike battery.  But again real bike batteries can't easily power your weed wacker!

It's a quandry.
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Blue Angel, Champion

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If I remember correctly, the mower is designed to draw a maximum of 600W. That is a peak load, however, not a sustained load, so your 450W estimate is likely a more realistic target for sustained output.

Yeah, two packs running at the same time might be the answer and might supply a reliable 1000W or so, with your choice of high voltage (series) or high current (parallel), depending g on the needs of the motor and controller.

Interesting project, for sure. Be sure to keep us posted if you embark on it!
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Dave Nagy

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Yeah, I'm not at all sure I'm going to be the pioneer on this!  Not enough discretionary income for science projects.  I mostly just wanted to get this on EGO's radar.  (The idea that beefy battery packs could potentially be sold as more of a commodity item, for general use, rather than just as an accessory for a particular "appliance".)

I'm sure somebody like Energizer, or Eveready, or maybe Tesla will eventually start selling a line of "generic" battery packs the same way that AA batteries are sold today.  Since EGO seems to have nailed the price/design of a consumer-friendly pack, they are well positioned to be a leader in what will probably be a big industry in a few(?) years.

(It does seem like having an industry standard pack form-factor would be super helpful, but those things always take time.  Perhaps EGO could start that ball rolling as well.)
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Blue Angel, Champion

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There has been a demand for a 120V power supply from Ego. I suppose if they offered one, the option to power just about anything would then be available. In a pinch, even garden tools they don't yet offer... bladed edger comes to mind. :-)
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Dano

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Allow me to resurrect this thread.  I was thinking the same thing!  Has anyone else done this yet?  I have never owned an e-bike yet but plan to build one (low cost, for fun) and want to try to integrate the Ego batteries.

So far what I know is that there are 4 terminals on the batteries.  + and - for your DC voltage source.  T = temperature, used for the charger.  D = data or discharge, used to communicate for overload and battery protection.  I don't know that it's a wise idea but I plan to just start with hooking up to the positive and negative and seeing how it goes.  Not sure if the controllers the e-bikes have would be able to support the "D" terminal and possible shut of power if things got to hot or a safe discharge level.

Good to know from reading above that the max wattage capable from these batteries is around 600 watt.  I was debating between a 250w up to a 1000w.  I should probably stay clear of the 1000w range.  Does anyone know if there are different maximum instantaneous wattage outputs for the different size batteries?  I have down at a 2.0Ah battery that I plan to start messing around with and also have a 5.0Ah if this thing actually works like I'm hoping.

Any opinions on where to buy the controller and motor?  I'm just browsing around on eBay and some random websites like elifebike.com  I'm just going to buy a cheapy one to start and see if i can get it going.  Possible move up to more.

I was also thinking of buying a used/spare parts trimmer on eBay to disassemble and take the battery harness from it.  I would love to use the motor too but I don't think it would be very good for an e-bike.  They go pretty cheap it looks like, possible around 50 USD.

I'll post some pictures as I progress.  If anyone has any opinions on this please let me know!
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Jacob

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The batteries can put out more than 600w. The 21 sp mower is a 1000 watt motor, and sp feature is even more. Id say your goood for 1250 to 1500 watts of battery discharge rate
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Adrian Ramirez

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That's a nice meter Blue. So I've been letting my church use my Ego inverter system the last couple of days for their fireworks booth and I gave them 3 of the 2 AHr batteries because I didn't want to risk having problems with the larger ones. The 2 AHr packs aren't showing the same issue that one of my 7.5 AHr packs are showing. They discharge down to 42V and Ego chargers have no issues taking them back up to 58V or so after that. So maybe something unrelated went wrong with my 7.5 AHr pack and it's only coincidence that it exhibited itself after my full discharge in the inverter.
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Jacob

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I would assume at this point, with technology, D Would be for data, not voltage drop over a shunt. The system seems to communicate too much between the battery and the tool. I cant find my security torx bit to disassemble. Ill pick one up tomorrow. Also, the chainsaw knows what battery is installed in it. Perhaps the tools do the decision making.
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Blue Angel, Champion

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Dano, correct. I measured the same 45.3V on a 4Ah pack depleted in the 20" mower and on a 2.5Ah pack depleted in the 15"'string trimmer.

I would assume that the charger and/or BMS would look at the condition of the battery before charging would begin, and if anything wasn't within expected limits charging would be refused? Just guessing. Adrian's experience with the 2Ah packs would suggest otherwise.
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Blue Angel, Champion

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Adrian, that's quite possible. I would be nervous discharging the batteries that far knowing the Ego tools are set up to shut down well above that! Who knows though, the tools could be using the voltage under load to determine a shut down point (rather than a static voltage), and it could be pure coincidence that the mower and trimmer stopped at the same level in two different packs.

Maybe I'll deplete a pack on the blower running on low speed, about the lowest power draw application I can think of, and check the pack voltage again to get a better idea what the real minimum voltage is. The mower and trimmer are relatively high draw tools.
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Blue Angel, Champion

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Jacob, my gut tells me that the tools and chargers do the decision making. My thoughts are based simply on the fact the battery cells are hard wired to the terminals with no method of interrupting power, so the tools must decide when to shut down based on what they're seeing, or perhaps from data supplied by the BMS? Yeah, I have no idea! :-)
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Blue Angel, Champion

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Dano, I would be very surprised if the BMS system in the Ego packs could talk to controllers made by other companies. I think you would be in for monitoring the packs yourself in order not to over discharge or overheat them.
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Dave Nagy

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Does anyone know if there are different maximum instantaneous wattage outputs for the different size batteries?
Yes, generally speaking, you'd expect a "big" 56V battery pack to be able to output more amps than a "small" 56V pack, if both are using the same cells.  In any pack, you need to wire some number of cells in series with each other to achieve the desired output voltage.  Each cell is good for about 3.8V, so to get 56V Ego must be using about 14 cells in series.  The 2AH Ego pack probably uses 14 cells. (Or maybe 13, if they're fudging on the nominal voltage a bit.)

The 4AH pack likely uses 28 cells.  14 cells would still connected in series, and then two of these 14-cell "strings" are connected in parallel in order to achieve double the amp hours without changing the voltage.  This trick of paralleling 2 strings of cells also doubles the peak amps the pack can output, since in effect, each string of 14 cells is only "seeing" half of the total load.

The 7.5AH pack might be 4 strings of 14 cells, and so on.  In this case each individual cell would only "see" a quarter of the load (amperage) that the pack as a whole was putting out.  That sort of cell arrangement is abbreviated as 14S4P.  14 series, 4 parallel.

All of this ignores any electronics that may be acting as a current limiter.  Ego may well limit the wattage of all their packs to a level that even their smallest pack can sustain.  I dunno.
Any opinions on where to buy the controller and motor?
These two vendors are well thought of, but I don't know if they sell the precise items you are looking for.  Luna Cycle, and EM3ev.  If you really want to go down the e-bike rabbit hole, the Endless Sphere forums are where the hardcore enthusiasts hang out.  I'm not one of them, but I enjoy reading about their antics.

It sounds like Jacob here really knows his stuff, so I'm anxious to hear about what he finds.
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Dano

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Yeah I'm curious to see what he finds when he disassembles his battery!
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Adrian Ramirez

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I can speak about the 7.5 Ahr and the 2.5 Ahr packs.  The combined cells in the 7.5 Ahr pack are good for 60 A continuous as far as their rating goes.  This equates to 3480 Watts when fully charged and 2520 Watts when fully discharged.  Now, all those 18650 cells together would probably get pretty hot with that much power being pulled out, so the pack probably can't sustain those values for long before it would overheat.  Also, I have no idea what the BMS (battery management system) is programmed to allow, so pulling that much could cause it to get mad and prevent future charging.  Also, I don't know what awg wire was used inside the battery pack, but it certainly didn't look large enough to handle 60 A, so the pack construction probably significantly limits the actual current you're able to draw out of it.  The combined cells in the 2.5 Ahr pack are good for 20 A continuous, which equates to 1160 Watts when fully charged and 840 Watts when empty (with the same limitations mentioned above).
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Adrian Ramirez

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Also, you never want to "float" charge a li-ion cell.  Once the battery is fully charged, current needs to stop completely until the battery is discharged and needs to charge again.  This is for safety and reliability reasons.  Float charging is alright in lead acid batteries, but not li-ion.
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Adrian Ramirez

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Sorry, I would have turned the image 90 degrees, but I'm on my cell phone
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Chris Smith

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I want to make one of these is there anyway you can send me the instrudctions of how you did this, or a list of parts I need, because this is seriously the best idea for one of these batteries!!!
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Blue Angel, Champion

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Nice job!
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Dano

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That's a pretty nice setup you have!  What kind of meter is that?  I can't see any beside its heavy blue glow.  You definitely spent time to build a setup.

I plan to do something similar, that is, disassemble and reuse one of my 4 battery chargers.  It's either that or buy a used/broken string trimmer on eBay.  The tools have a much nicer fit to them when they snap in rather then a "gravity" fit for the charger.  With an e-bike I think it would be a much better idea to go with the snap in tool style.

Can you do any more testing to confirm if your 7.5Ah battery was killed by your inverter?  Are you running your batteries until they can't run your inverter anymore or are you stopping at around 42v to attempt to not destroy the batteries.
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Adrian Ramirez

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Thanks, it's not finished yet and the meter is kinda just dangling there because I haven't determined how to mount it just yet.

The inverter sounds a low battery warning at 42V, so I swap out the battery at that time. In order to keep efficiency higher, I also have a buck converter that regulates the Ego voltage down to 12V so that I don't have to invert from battery voltage to 120VAC, then back down to 12VDC to charge my small electronics. The buck converter doesn't have a low battery indication, but I can monitor the battery voltage on the screen and I think the screen also has the ability to set an audible alert, but I haven't played around with it enough yet.

I used the following buck converter:
DROK
https://www.amazon.com/dp/B017SQ2PUG/...

I used the following meter:
Bayite
https://www.amazon.com/dp/B013PKYAV6/...

I'd like to debug the 7.5 AHr battery, but it's a little difficult without a schematic and basic understanding of the electrical architecture. I'll probably do some basic things like checking for a cell imbalance or disconnected wires, but anything more complicated would require access to the BMS, which is located inside the potting material and would basically require me to destroy the battery. The battery still charges manually, but I'm concerned that the cell balancing might not be an automated process in the battery. It might require the charger to regulate the cell balancing. I would hate to bring the battery through many manual charge cycles, only to find out that the cell balancing isn't working and then one day I end up over-voltaging a battery cell and damaging it.

I could always design my own BMS system for the battery (which would give me a nice 0 to 100% state of charge estimate), but I'm not sure if anyone makes an off-the-shelf 14 series cell BMS system since I've never designed a battery with that many cells. Plus there's quite a bit of time and expense involved in designing a BMS.

So I don't know what I'm going to do yet. One thing that is mote-worthy is that the 7.5 AHr battery that has the issue, is one of a newer design than my 2 AHr battery packs. The new ones apparently have a little clear rubber boot that covers the LED button on the battery. They must have had a problem with water getting into the BMS board through the push-button switch and decided to include a rubber boot in the newer design. It's possible that they also made electrical changes that affect the ability of us to use these batteries in non-Ego products (maybe intentionally or on accident). My problematic 7.5 AHr battery has a date of 2016 while the older 2014 smaller batteries don't exhibit the same failure to charge problem.
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Adrian Ramirez

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Looks like TI makes a couple integrated solutions for fuel gauging, protection and charge control with up to 16 series cells:

http://www.ti.com/product/BQ34Z100-G1/description
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Blue Angel, Champion

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$6 for the chip ain't bad for someone who is comfortable building up the rest of the BMS circuitry. Nice find!
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Adrian Ramirez

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Also, someone does make an electrical adapter for the Ego battery to power a telescope. But it's a little pricey and I'm not sure the awg of wire they use would be enough for a high power need. I'll try to find that adapter later and post it.
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Blue Angel, Champion

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Interesting observations. I wonder if perhaps your 7.5 has a defective cell or whether the BMS caught on to your application? I guess the only way to tell would be to take a meter and measure across each group of three cells and see if one or more are measuring low? I would think a defective cell would drag the whole group down with it.

I would have thought someone would make a scale able BMS circuit that's programmable for the number and type of cells used? Perhaps an Arduino based unit?

Is your inverter low voltage alarm adjustable? If so and knowing the lower voltage limit of the Ego packs, it might be less risky to limit the voltage drop to 46V or so. If you get your 7.5 back on line it might be worth a try. Did it initially charge ok if you manually discharged it but kept the voltage up?
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Dave Nagy

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I'm pretty sure that our Chinese friends manufacture any number of BMSs that can be configured for 14S packs.  That's a fairly common cell configuration these days in the e-bike world.  The trick will be finding a seller that will ship overseas.  Here's some BMS examples <LINK>, although it doesn't look like that particular vendor is set up to sell to American end-users.

If any of guys wanted to start a thread over in the Endless Sphere forums <LINK>, I'm guessing the folks there could point you in the right direction.  There's all sorts of guys pack-building over there.

All that said, I hope Adrian can get his pack back on speaking terms with the stock Ego chargers.  Once you start having to bodge together your own battery management, you've pretty much lost most of the advantage of leveraging the Ego eco-system in the first place.  
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Adrian Ramirez

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The voltage of the low battery alarm on the inverter is not adjustable.  I can probably set the meter to beep at a higher voltage, so I was going to do that, but then I found the problem with my battery and it's not related to the usage inside the inverter.

The T pin on the battery goes to an analog NTC thermistor.  The charger puts out about 3.5V with no battery connected and uses the voltage drop across the thermistor to detect battery insertion.  It appears that the first thing the charger does after detecting a battery is to enable the 4V or so pull-up resistor and attempt to communicate with the battery over the 1-wire communication interface (through the D and - pins).  My faulted 7.5 Ahr battery appears to have a damaged or loose thermistor.  My functioning batteries all measure around 11 kOhms from T to ground (-) at room temp, but my faulted one is measuring around 110 kOhms, an order of magnitude above normal.  It appears that my fast charger has a more strict voltage range on its battery detection circuitry because it wasn't even detecting the insertion, but the slow charger was detecting it, but it was outside of the normal temperature range for the slow charger, so it displayed the defective battery indication.

NTC thermistors for cell temperature readings are pretty typical in battery applications.  An NTC (negative temperature coefficient) thermistor measures lower resistance as the temperature increases, so a very high resistance would look like a very cold battery.  It's interesting that my lawnmower didn't care about the cold temperature reading, only the charger, so it appears that the lawn devices must have an over-temperature threshold, but not an under-temperature threshold or open thermistor protection.  This probably isn't a good thing because if the thermistor wire were to break inside the battery, that would appear as a high resistance and the equipment could continue to operate, even if the battery were overheating.  Probably not the best electrical design in that area, but at least the opportunity for failure is small since the charger won't allow you to continue operating with an open or high impedance thermistor.

In summary, the good news is that you all are free to use your batteries in non-ego equipment without problems, as long as you stay within the normal operating parameters of the battery pack.  I wouldn't discharge lower than 40V.  It's also possible to setup a temperature gauge to operate off the NTC thermistor.  If I can determine a manufacturer and part number, I can probably find a meter that will operate off the voltage curve.  If not, then I'll have to characterize the resistance curve with one of my good battery packs at different temperatures and find an equivalent thermistor that I can swap into my pack and use that curve to find a temperature meter.
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Dano

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This forum has exploded with conversation!  I love it!

I checked all my Ego tools which include the 15" String Trimmer, 16" Chainsaw, 530CFM Blower, and the Hedger.  All have the +, -, and D terminals but the Hedger is the only one that also has the T terminal.
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Blue Angel, Champion

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Dano, my hedge trimmer has the T terminal as well, but I bought it on EBay and the terminal was messed up and I had to take it apart to straighten it out. The T terminal is there for looks, it's actually not even hooked up in the tool.
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Jacob

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All early models of tools all had the T terminal as far as i know. My mower, early trimmer and early first gen blower. LL BOUGHT IN MARCH 2014.
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Jacob

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Also.... this is important, none of those terminals had wires connected to them. They were just there.
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Dano

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Pretty funny....fake out.
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Adrian Ramirez

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Oh, and I forgot to mention a couple things.  I learned that it was an NTC thermistor by measuring one of my good batteries at 11kOhm at room temp, then putting it into the fridge and for about 10 minutes and then measuring it at 16kOhm.  And once I realized my thermistor in the 7.5 Ahr pack was bad, I connected a 12K resistor in parallel with the T and - pins of the charger and the battery charged without any problems.  So if I fix the thermistor, I fix the issue with that battery charging.
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Dave Nagy

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Is the thermistor mounted in such a place where you can "easily" get at it or replace it?
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Adrian Ramirez

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I'm not sure, all of my investigation was from the outside.  Last time I opened this pack, the thermistor wasn't glaringly obvious, but I also didn't look for it.
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Blue Angel, Champion

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I think this must be the thermister:


There are two of them, one per bank on the 4Ah pack, stuck in the V in between two of the center cells:


At roughly 80F one of them measures 9.3kOhm.
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SCDC, Champion

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Now THAT is a thermister.  I have them all over my motherboard.
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Adrian Ramirez

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I'm not sure how that would work with only 2 wires. Cell balancing normally needs a separate signal for the gate of each FET. The idea of passive cell balancing is that you bleed off energy from cells with higher voltages to allow the lower cells to be charged more than the higher ones. For this you would need a FET across each cell and a control signal for each of those FETs. There's also normally a resistor to prevent too much current from being dissipated, but I don't see any resistors either.

Would it be possible for you to draw up a schematic for this board based on the connections that you can see?
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Adrian Ramirez

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Yeah, that's definitely a thermistor. It's interesting that there are two of them, but only 1 pin out of the battery. Do they both tie into the T pin somehow or does one go to the BMS and the other to the T pin?
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Blue Angel, Champion

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There's a small circuit board under the battery terminals. If I can peel away some of the goop I'll see if I can figure out where the connections are.

As for the transistors, I have a funny feeling two of the connections are shorted together on each unit. I'll remove one to see what the traces look like under them and then draw up a schematic for you. Probably tomorrow morning at this point, heading out shortly.
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Adrian Ramirez

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Sorry, didn't see your most recent post. Sounds like the cells are imbalanced and since some of them are only 1V, I wouldn't recommend trying to charge the pack.
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SCDC, Champion

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Blue... blue blue blue..

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SCDC, Champion

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Blue,

My mother board came with 6 of these Thermisters.  All are two lead, always has been.

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Blue Angel, Champion

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It's covered in the manual as well. There is a resistive circuit that bleeds the pack down for long term storage:
https://cdn.shopify.com/s/files/1/026...
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Dano

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I read this in the instruction manual, I know I know....I guess I'm one of those people who actually opened the manuals, and for the battery of all things!  Mostly skimmed but I did catch on to this on page 12 at the bottom;

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NOTICE: This battery pack is equipped with an advanced self-maintenance function to extend the battery life.  Depending on the battery charge, it will automatically perform a self-discharge operation after one month of storage.  After this self-maintenance, the battery pack will enter sleep mode and maintain 30% of its charge capacity.  If stored for a month or longer, fully recharge the battery before the next use.
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Blue Angel, Champion

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Typing at became time, Dano! :-)
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Adrian Ramirez

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That's a pretty cool feature. Makes sense since most people wouldn't use the battery for at least 6 months out of the year. Thanks for the info, I guess I should read the manual!
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Blue Angel, Champion

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It's definitely one of the more unique features of the platform, along with the PCM cell wraps and fan cooled charging.
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Blue Angel, Champion

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Adrian, since I lack the ability to verbally describe a schematic, here's a quick mockup of the balancing boards:

Just picture Q1-Q14 on one board, Q15-Q28 on a second board, and a connector joining them.

If anyone is interested, I used Scheme-It by DigiKey to do the layout:
http://www.digikey.ca/en/resources/design-tools/design-tools#schemeit
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Adrian Ramirez

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This circuit seems pretty simple, but I'm very confused by it. It doesn't appear to be a balancing network because they all connect together in series and there's only two wires that feed the whole thing. It seems like all it would do is waste energy and create heat.


The location of each transistor is curious. They each make physical (but apparently not electrical) contact with one end of each battery cell, which would suggest that each one either senses something from the cell or does something to the cell. Based on this, I can only think of two reasons that you would want to contact a cell like this, either to sense temperature or to heat the cell if it is too cold. It doesn't really seem like they would need to heat the cell because most people probably won't be doing lawn work in freezing temperatures (except maybe for the snowblower) and this also doesn't seem like a good place to heat the cell. So I would guess that maybe these are used as temperature sensors. I did a little research and it appears that people have used BJTs as a fairly accurate temperature sensor with the collector shorted to the base (as Ego does on these boards). Here's what I found online:

http://electronics.stackexchange.com/...

The difference here is that all of these BJTs are connected in series, so that would mean that they don't know the temperature of each cell independently, but they could tell if one cell were to over heat a lot (but they wouldn't know which one) or if the entire pack were to overheat a little.


So that's my guess, that these boards are to sense for overheating battery cells. This would be a separate sense than the thermistors.


Blue, does the white glue stuff appear to be thermally conductive, like thermal compound? And also is it an electrical insulator?
(Edited)
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David Cline

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Well the battery manual DOES say that if a battery is below its minimum operating temperature (32F/0C) then it will "pulsate" to warm itself up before it can be used. So it does have a self-heating mode.

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