Strange lipo problem- 1/2 of battery weakening

For the past month, I have noticed that all of my 6s batteries are getting weak on 1 side. Cells 1thru 3 charge up ok and cells 4 thru 6 are dying. I run 24s, and it seems the 1st and last battery in series discharge more than the middle 2. At first I thought it was because I was bulk charging, so I went back to balance charging individually and have the same problem. I then thought it might be my motor or conroller, so I am using another bike. Same problem. Any ideas?
Thanks, Mark

Can't be bike or bulk charger related. Has to do with the single cells, somehow.
Are the packs no. 2 and 3 inside the pack and 1 and 4 outside? It's cold, they may sag more, but capacity should not change.
Then the cells in single packs getting weaker, is it somehow connected to the other problem?

Sounds like you are drawing too much current. What is the c rating and how many do you have in parallel.

A charger that will graph during charge or discharge will show you the problem. You've got funky cells in some packs.

Temperature is my guess. Cells in the middle get hotter

That's possible if he is only using 5AH and drawing a ton of amps from it.
These batteries don't perform at or near their maximum C rate too well.

Thanks for the feedback, guys. I have been running 1p instead of 2p for awhile now and pull 5000 watts max. Thats only 10c discharge rate on 25c rated pack. It still seems strange that its always the 4th 5th & 6th cell that is going weak. Maybe it is the cold weather. I will go back to running 24s2p, then it will only be a 5c pull on the pack. I will follow up w/ results

Unless they are nanotechs, the ratings are probably optimistic. Who makes them. A good clue of abuse is to measure the temps during use. If they are quite warm to the touch, you are asking too much and have shorten the cycle life. Lipo do not last as long above a certain temp. You are pushing closer to RC use, batts won't last as long. Better if you draw less. Cold weather is even harder on the packs so you compounded the problem.

geetarboy said:

Thanks for the feedback, guys. I have been running 1p instead of 2p for awhile now and pull 5000 watts max. Thats only 10c discharge rate on 25c rated pack. It still seems strange that its always the 4th 5th & 6th cell that is going weak. Maybe it is the cold weather. I will go back to running 24s2p, then it will only be a 5c pull on the pack. I will follow up w/ results

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geetarboy said:

Thanks for the feedback, guys. I have been running 1p instead of 2p for awhile now and pull 5000 watts max. Thats only 10c discharge rate on 25c rated pack. It still seems strange that its always the 4th 5th & 6th cell that is going weak. Maybe it is the cold weather. I will go back to running 24s2p, then it will only be a 5c pull on the pack. I will follow up w/ results

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If those are Turnigy 20C, then they can handle that, but it's kinda beating on them if that 10C draw is constant.

If those are Zippy 20C, then those are more like 10C batteries in reality.. when i bought mine new, they had double the internal resistance as the new Turnigy 20C.

Either ways, cold weather makes lithium sag pretty bad. And for that power level you should absolutely be running 10AH, minimum, of even Turnigy 20C. You'll get vastly more watt-hours out of the battery that way.

I find it very odd that the problem cells are all 4,5 or 6. But odd stuff happens all the time, and the result is superstisions and junk science.

I don't find it odd at all, that 1 P packs have funky cells. All my first lipo packs that I ran hard 1P for a few months developed funky cells early ( about a year). The ones that only ran 2p are lasting a lot longer.

I think you have some issues that are getting magnified by the cold, and are finding out that 1P on 25c cells can find the weak ones faster.

This kind of problem can also be caused by permanent connection of cell monitors and balancers that tend to draw current from selected cells only. Doesn't seem to be your case either.

once i left a 5s pack in the sun, it was a super hot day here in summer.
The pack had black tape around it to, anyway one cell got damaged from being left in the sun, it was the cell that was facing up, always had a weak voltage after that.
So i stabbed it with a shovel, and boy oh boy, i closed all windows on the house because it was like someone threw a smoke grenade in my back yard.


lipo batteries are good smoke grenades lol

+1 on pulling 5000w from a 5ah pack isn't doing you any favours. They'll perform much happier if you only pull a small percentage of their supposed C rate - which usually means running a 10ah pack for something drawing over 1000w and especially up around 5000w.

On a related note of lipo oddity, I built a pack for a guy that had one cell die for no reason - one day he went to ride and checked the balance to find them all 4.15v with except 1 at 1.5v (clearly dead). Because the 10ah pack was made of 5ah bricks hard wired in parallel it meant 2 dead cells but I ended up replacing it with 2 new 6S bricks out of convenience rather than replacing the dead cells. 2 months later the exact same thing happened - same sub pack, same cell that died. Coincidence ?? It was running no BMS that could have gone faulty and leeched away at one cell, it was bulk charged in 6S parallel groups and the balance was checked only momentarily with a battery medic so no parasitic draw there. It was charged up and balanced fine at the start of the ride when 7ah in that 1 cell suddenly died - reading 1.8v while all the rest were 3.8v

Bizzare...

I heard of BIG FOOT in OZ land so no worry mate.

Three Turnigy 6s packs that failed for Pdf all had the same cell fail--2nd from the end. No particular reason AFAICT, just did. 0V dead. One of the same kind from Mdd0127 had the same cell dead, too.

I'm sure there is actually a good reason for it, but I don't know what it is.


I would guess that statistically with only six cells in a pack, and the number of defective cells that they seem to ship with (even if percentage-wise that's a small percentage, they ship a LOT of packs, so there's a lot of defective cells), that there will be a lot of packs with the same cell(s) bad, and that it could happen occasionally that all (or most) of the ones someone has end up with the same ones bad. Not often...but I'm sure it happens. If anyone with statistics experience wants to work some numbers, it'd be interesting to see what the chances actually are.

amberwolf said:

Three Turnigy 6s packs that failed for Pdf all had the same cell fail--2nd from the end.

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See that's interesting - getting 3 duds is one thing and statistically it just reflects a certain degree of poor QC or a bad batch etc and a bit of bad luck in getting as many bad ones as you do.
But having the same cell die in different packs without reason is significantly less likely to happen. If it was always the outer cells to die you could blame external damage, or if it was always the inner most ones you could blame heat but the fact that different people are having the same individual cell die in multiple packs is quite strange. If it was a BMS you'd blame that, but most of us don't use them.

...If anyone with statistics experience wants to work some numbers, it'd be interesting to see what the chances actually are.

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I did do one statistics module at uni many years ago but it was the most boring subject I took so kinda glazed over for most of it
Using laymans stats though the chance of any given cell being dead in a pack is 1 in 6 (16.6%) (assuming 6S packs of course) The chance of that same cell being dead in 2 packs is 1 in 12 (8.31%), or 1 in 18 (5.5%) for 3 packs.
That is assuming all the packs you're talking about are known to have one faulty cell.
To look at the overall liklihood you'd need to know what the failure rate is across the board and then the probability of getting the same 2 cells go bad is much much less likely.
Eg if you said the overall dud rate from HK was 10% that reduces the chances of getting 2 packs with the same 2 cells being dead less than 1%. 0.83% to be exact, or 0.055% for the cell being bad in 3 packs. So if hypothetically speaking the failure rate out of HK was 10%, you'd have to buy 180 packs before you got 3 packs with the same dead cell...
Clearly this isn't the case, so there must be some external factor affecting these. But as amberwolf said, we just don't know what it is!

Well, after charging individual cells across the balancer leads I was able to get almost all of them fully charged. And after running 2 cycles on 2 different sets of 24s2p, most of them stay balanced, I've probably ruined 5 or 6 packs. I think it was a combination of cold temperature and too high a discharge rate. Last year I was charging inside at room temperature and always ran 2p. This winter I've been charging in the cold garage and mostly running 1p on shorter trips.

geetarboy said:

Well, after charging individual cells across the balancer leads I was able to get almost all of them fully charged. And after running 2 cycles on 2 different sets of 24s2p, most of them stay balanced, I've probably ruined 5 or 6 packs. I think it was a combination of cold temperature and too high a discharge rate. Last year I was charging inside at room temperature and always ran 2p. This winter I've been charging in the cold garage and mostly running 1p on shorter trips.

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For the price of 5 to 6 packs, you could have bought some test equipment to prevent this from happening, like the graphing chargers i mentioned, which will measure IR, whether you have a bad cell in a pack ( missing amp hours, screwy discharge curve, wrong C rate, etc).

No more guessing games - when we are dealing with a battery that vents massive amounts of flame and smoke when mistreated, what's going on with those batteries needs to be understood, and you were probably running a dud or two in parallel with some others, causing more packs to die early.

Sorry to badger you, i am just concerned for your safety.