testing an ebike battery - Page 1
Forum section: electronics / general

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2020, 11:07:37 am
content: hi all,ive just brought a 12ah 13s battery secondhand,it uses groups of 3x 26650 in series,i assume each cell is 4ah,the battery only manages i would say 5 amps output at most,the voltage drops under any load,full charge measures 56v,is there an easy way to test what cells are bad cell groups measure as follows: 4.123v,4.126v,4.127v,4.127v,4.126v,4.126v,2nd string:4.121v4.121v,4.003v,4.044v,4.122v,4.121v,4.125v,any idea how to test these to see if there all bad or just a few?,cheers m3vuv.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2020, 11:47:36 am
content: The pack is likely very dangerous and you shouldn't be playing around with it based on the little experience you have and the internet advice you get.You are likely to burn down your home with that thing. Just stop.

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2020, 02:08:24 pm
content: teach your grandmother to suck eggs!,ive been working with these for 6+ years,had fires etc in the past,for your information,i am a qualified electrician and radio ham,i know the dangers involved ok!!

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2020, 02:11:11 pm
content: pluss the said pack is totaly dissasembled at the moment with all cells out of the pack,i need a way of testing then,well there still in groups of 3 cells in parralell,theres nothing to short them so safe as i an concerned!

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2020, 04:32:22 pm
content: OK, my suggestion is,First remove all series connections.Then,1) to save time, do not remove parallel connections; analyze and treat as 12Ah cells; or,2) if you have time, separate all cellsThen, use a cell analyzer, the cheap RC hobbyist analyzers will do; Revolectrix Powerlab is one example from the more expensive side yet quite affordable. It's buggy crap but they all are.Run full charge and discharge cycles, first at low currents like 0.5C charge and 1C discharge absolute max. Monitor cell case temperature while doing this.Any cell with significantly less capacity than the others, or higher temperature rise fails the test.Then run discharge cycles at two different currents, say 0.5C and 1C. Don't need to be complete cycles, from 100% to say 50% is enough. Calculate DC ESR as I suggested in another thread of yours. Again, any significant outlier fails the test.Finally, charge to full charging voltage with low stopping current like C/40. Let sit for a day, record the open-circuit voltage. Let sit for a month or so, the longer the better. Measure voltage again. Any outlier showing increased self-discharge fails the test.At this point, I would be quite confident building a pack out of them. But if you don't know about the history, second-life is always second-life.What cells? Manufacturer and model?Hope this helps and sorry for the rough words. I'm just concerned about the safety of yours and the others living near you.

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2020, 05:11:48 pm
content: i dont know anything about the cells,they have no markings,i know it charges to 54v but my bike has no power,maybe 5mph top speed and .5 mile range before the lvc cuts in,its this battery as my other pack is fine,also i have a 12v tap from both packs for my led lights,the lights with my good pack dont dim under hard throttle do,onthe problem pack they dim badly,i just want to know if all the cells are fooked or just a few.

author: bdunham7
author_rank: Super Contributor
timestamp: October 05, 2020, 06:09:01 pm
content: Quote from: m3vuv on October 05, 2020, 02:08:24 pmSo it would seem that there are legitimate safety concerns, no? For one, I'd recommend you disassemble the pack entirely to single cells, although I don't think you're going to like what you find.QuoteI don't have the power experience that Siwastaja does, but I have torn down a fair number of battery packs, some to harvest cells and some to rebuild the packs. From that experience I can tell you that you probably have a pile of rubbish. I don't know any of the details of your pack or its BMS, so I can't interpret your voltage readings. It is possible the problem is confined to the two banks that have the abnormally low voltage, but the opposite is also possible if the thermal design is poor and all of the 'normal' banks actually have one or two cells that have had the pressure disconnect activated and are now open circuit. You could rig up something to test each bank under load, but unless disassembly is very difficult I'd just separate them all and chuck them all in a device that can charge/discharge/test/measure them. I use an inexpensive AccuPower IQ 338XL, which I think can still be had for $50 or so. At this point I don't think you need to be looking at the fine-grained details like self-discharge quite yet--I think your problems will be readily evident gross malfunctions.

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2020, 07:52:36 pm
content: i think my best bet is bin it and reuse the case,pissed off tho as it cost me £168

author: tom66
author_rank: Super Contributor
timestamp: October 05, 2020, 08:46:44 pm
content: 12V tap for the lights? So you are going to discharge one part of the pack more quickly than the rest. No. Use a DC-DC converter, or a second battery. That isn't an option.With cells that mismatched the pack is quite badly worn out. Was it made from secondhand cells? Has it had many cycles applied? What BMS does it used.Perhaps consider the £168 a lesson learned to avoid burning your home down and appreciate the technology that goes into making safe and reliable batteries is not often replicated by the cheapest manufacturers.I would NEVER, EVER charge a battery like this in my home. If it was to be charged it would be 10 foot away from the house in the garden on the patio. And even then I'd consider the patio written off if the battery pack gets too angry.Did you see this video?That is a battery made by a "reputable" supplier failing under apparently well within rated load conditions (though perhaps that is debateable, it is unlikely the pack was heavily overloaded.) Do you think you could manage with that fire? Is it worth it to save a few £s?