Help me diagnose new lithium battery problem please

I ride an E-Moto Ridge 2.0 which is now 2 years old and I have two extra manufacturer's battery packs to take with me for extra range which I swap out when each pack is exhausted. The vendor replaced one of them that lost about 33% capacity within the warranty period, but that's now expired and the other two batteries are starting to lose capacity too, so I decided that the next replacement would be a 20Ah lifepo4 pack that I could mount in my rack and not hassle with changing during a ride.

I ordered one from China where it was built to order. I asked for something suitable for a 250W brushless hub motor. They were never very forthcoming with details but this is what I bought:

Item: 12S1P-- 36V20ah lifepo4 battery pack
Unit price: US$203/pack.
The cost of 12S 4A plastic charger: US$42
BMS Specification:
12S working current: 7A Peak current: 14A
The cost of BMS: US$26/pcs
The shipping fee: US$160
Total value: US$431.

I wired up suitable plugs so that I could power the E-Moto from it, and disconnect from the bike to charge it, then I tested it...

If I start pedaling from a stop, the battery cuts out immediately, and won't re-engage until I physically unplug and reinsert it. Just using the on/off button on the handlebar doesn't reinitialize power to the bike, which suggests to me that the battery pack has shut down rather than the bike.

However if I use the throttle alone to pull away from a stop (not a good practice) and then start pedaling once the engine engages, it works fine and runs indefinitely (well, at least until the next stop at a junction)

(Here's a potentially related issue: if I feed the battery through a 37V to 12V converter into a 12 - 110Vac inverter, the inverter powers itself off immediately I try to power it up. Even with no load attached. I've done the same thing with the EMoto vendor's battery packs and they power the inverter just fine.)

So I can work around the problem at a pinch but really am not happy with that solution. Does anyone have any ideas what's going on here? Some parameter set wrongly on the BMS that's presumably inside the battery pack? Is it fixable? Shipping the battery back to China is not an option. I don't know if removing the tape to examine the BMS is an option - they told me it was compression tape and not to remove it.

Thanks,

Graham
PS I have some stories to tell about dealing with the Chinese company but not relevant at this point, and I'll withhold their name until I'm sure of the facts here.

First mistake was asking for a pack for a 250W motor. The motor has nothing to do with the amount of current drawn from the battery pack. That's all controlled by the controller. The motor size is irrelevant. The bms has a 14A peak, which is pretty darn weak. I'm assuming your bike has a pas system, and it draws more power than the throttle initially, thus exceeding the 14A peak of the bms. Or you're pushing the throttle to full power when you start pedaling first and go easy on it when you use it alone. Without knowing anything about your controller, all this is just an educated guess as to the problem.
As a workaround, you could bypass the bms output and take the output right off the battery pack. If the pack is even halfway decent, you should be able to draw at least 30A from a 20ah pack without overloading the battery.

do you have a voltmeter? who did you buy the battery from since it looks like a ping pack? how did you arrive at the opinion that the battery had lost so much capacity? do you still have those older batteries and their chargers? can you post up pictures of those batteries and all of the BMSs?

I think Wes pretty much nailed it. The bms is too weak.

The pack should be able to handle 1c for sure, and likely 2c is not a problem for it for occasional bursts when starting.

I'd look for a decent bms with 40 amps max cut off. So that above 2c, 20 amps x 2, it would shut off. Right now it's shutting off at less than 20 amps.

If reallllllllly confident you will never run the pack down 100%, you could discharge it bypassing the bms, and use the bms only for balancing while charging.

DONT, try that unless you have a voltmeter on the bike to watch for overdischarge.

dnmun said:

do you have a voltmeter? who did you buy the battery from since it looks like a ping pack? how did you arrive at the opinion that the battery had lost so much capacity? do you still have those older batteries and their chargers? can you post up pictures of those batteries and all of the BMSs?

Click to expand...

It was a company called Victpower in Shenzhen.

As for the original E-Moto batteries, when I first got the bike I did a test on a long flat road on a calm day to set a baseline on how far I could go on throttle alone, and when I started to get a lot less than that I sent the battery back to the US E-Moto office where their engineer did a proper capacity test and reported that it was only holding 66% capacity.

I do have a voltmeter (and know how to use it ) but most of my electronics experience is low power 3V and 5V stuff and I'm the first to admit that I'm woefully ignorant of battery issues, although much less so in the last year since I've started reading this forum.

I'll post photos of the old batteries this weekend. I have 3 chargers and I'm still using the packs, I just have to be careful on estimating how much distance I have left. They're fine for around town and 10 - 20 mile rides but they guys I hang out with want to do 30 - 50 mile rides and I'm not confident I can do that safely. (I'm 280 lbs, have a heart condition with a pacemaker, and it's a heavy bike, especially with three battery packs. This is not a bike for pedaling far if the batteries die!)

Thanks,

Graham

dogman said:

I think Wes pretty much nailed it. The bms is too weak.

The pack should be able to handle 1c for sure, and likely 2c is not a problem for it for occasional bursts when starting.

I'd look for a decent bms with 40 amps max cut off. So that above 2c, 20 amps x 2, it would shut off. Right now it's shutting off at less than 20 amps.

If reallllllllly confident you will never run the pack down 100%, you could discharge it bypassing the bms, and use the bms only for balancing while charging.

DONT, try that unless you have a voltmeter on the bike to watch for overdischarge.

Click to expand...

I'm not confident enough to bypass it altogether, ... can you recommend a BMS? Also any tips on how to remove the old one? Is it safe to just remove the external blue tape?

Thanks,

Graham

expose the BMS and take a picture. if it is capable, then you could add some more mosfets and solder up the shunt to raise the current carrying capacity of the BMS so it would not cut out. but that assumes it is the over current causing the shut down and not a low cell going to LVC. but since it is only rated for a few amps it is most likely over current causing the cut off.

you can buy another BMS too if you wanna. unless you are gonna use a balancing charger and cellogs like everyone else.

Ok, no bypassing the bms.

Very carefully, expose the bms from the shrink. Then you can see how the bms hooks up to the battery. There will be big wires that charge and discharge, and lots of little wires to some plugs.

Ideally you'd find a bms with exactly the same small plugs making swapping bms easy. You will also now be able to look at your batterys voltage one cell at a time, and find out if just one cell is the real problem.

Same place you got the battery might be a good place to try to get a higher amps bms with plugs that match.

if he now has three batteries with the same chemistry and serial count he could build all of them into one battery and use a real big BMS too. i suspect his first batteries just got way outa balance so they would never fully charge the low cell. he could solve that too.

dogman said:

Ok, no bypassing the bms.

Very carefully, expose the bms from the shrink. Then you can see how the bms hooks up to the battery. There will be big wires that charge and discharge, and lots of little wires to some plugs.

Ideally you'd find a bms with exactly the same small plugs making swapping bms easy. You will also now be able to look at your batterys voltage one cell at a time, and find out if just one cell is the real problem.

Same place you got the battery might be a good place to try to get a higher amps bms with plugs that match.

Click to expand...

I've mailed the vendor to see if they have an alternative BMS or to get the specs of the one that is in it; we'll see what they say.

dnmun said:

if he now has three batteries with the same chemistry and serial count he could build all of them into one battery and use a real big BMS too. i suspect his first batteries just got way outa balance so they would never fully charge the low cell. he could solve that too.

Click to expand...

That would be great. I did do some of the things recommended here for recovering from that situation (like leaving it on charge for a week) but no joy.

By the way I retired last month so I now have a lot more free time to work on this

Graham

you can just take a picture of the BMS and we can tell you. i doubt if you wanna buy another BMS from them if they did this to you.

Once you get it resolved, stop running your batteries all the way down to cutoff, because that is the root cause of all your battery issues. Like Dnmun said it's quite likely you can build a big honking pack from the lot of them, and you'll learn plenty in the process.

John in CR said:

Once you get it resolved, stop running your batteries all the way down to cutoff, because that is the root cause of all your battery issues. Like Dnmun said it's quite likely you can build a big honking pack from the lot of them, and you'll learn plenty in the process.

Click to expand...

You're undoubtedly right - when I first got the bike and knew even less than I do now, I was under the impression that the right way to treat these batteries *was* to drain them completely :-( Now I know better.

On the other hand, I've never bypassed the BMS, and I thought that one of the jobs of the BMS was to stop you from doing that. Aren't they supposed to cut the battery off before it gets down to that level? Or is the cutoff point on these things set lower than you guys would recommend for good battery health?

G

the only way i could afford to build the big battery i put in the car was to use the dead batteries for pouches to build a new battery from. takes a long time but you can develop techniques to test many pouches at the same time fairly accurately. just takes time to do the work.

The way I like to think of it is like this. The bms is there to prevent a catastrophic over discharge.

But, virtually all types of batteries do better, and last longer, if the cycles are not such deep, or such high rate discharges. What happens is cells are more likely to be driven out of balance when you draw that last 10-15 % out. So a pack is happier, and stays balanced better, and may last more cycles if you don't run them all the way till the bms pops.

So with this new battery, charge when you can conveniently, and try to make trips that leave some more capacity rather than drain it till the bms trips.

But, don't hesitate to drain it all when you have the need. That's what the bms is for, to let you make it home on that last bit safely.

By all means, when you have this new pack working better, DO try to sort the good from the bad cells in your old packs. Great experience, and you might get another very usable battery out of the project.

dnmun said:

you can just take a picture of the BMS and we can tell you. i doubt if you wanna buy another BMS from them if they did this to you.

Click to expand...

This is what they're offering as a replacement: http://ru.aliexpress.com/store/product/16s-51-2v-bms-for-lifepo4-battery-packs-20A-work-current-40A-peak-free-shipping/705561_1097507142.html

(No idea why they gave me the Russian language link, but scroll down and you see the specification in English)

Is this a reasonable device? Is $77 shipped a fair price?

Thanks,

Graham

Hi guys - remember me? Still around, still on the E-Moto Ridge 2. I was wondering, is there anyone in Texas who does the sort of battery pack repair and rebuilding that you guys all do, who I could bring my battery packs to for rebuilding? (in fact I would bring the bike too on the back of the car). I wasn't confident enough to do the things you suggested above beyond stripping down two of the failed packs, which even if the cells are not reusable, the cases are good for housing a full replacement. I still have one working pack and still have the big Chinese battery effectively unused (I keep it in a fire safe). I've topped it up a bit every few years. I've just been using the one pack for puttering around town and haven't been on long rides in a long time. I'ld like to get back out on the bike for proper rides again.
I'm in South Texas/RGV, so I'm looking at a 200-400 mile drive to the big cities in Texas, so it would have to be something I would plan for later in the year rather than the near future. Austin, Houston, Dallas/FW, Corpus, San Antonio - anywhere really is possible as I would arrange for a few days vacation in the area while the work is being done. Happy to work the dates in with the quiet time in anyone's schedule. My email is gtoal@gtoal.com