1 Bad cell after 14 months

[athletic91]

one of my turningy 5s 5ah 20c packs develop a bad cell after 14 months of use. Always charge to 4.1v/cell and never below 3.75. after about 100 of so cycles one of the cell discharge much faster then the rest. I never drew more than 3c for it . Am i considered lucky or unlucky?

 

[dogman dan]

It's odd, my experience has been more like, good for 2-3 years, or dud in days. One thing that could cause it though, would be a ding in the pack. Dropped on the corner, is what I mean, or rubbed wrong in a battery box.

 

[Gregory]

Running 1P is pretty hard on the lipo packs. I found I got better results with 2 or more in parallel. Less likely to over discharge, halves the load on the cells.

The penalty being a physically bigger battery.

 

[bionicdan]

iv had a low self discharging cell in 3 of my nine brand new multistar batteries and one puffer. Im renowned for awful luck so your situation sounds amazing compared to mine

 

[dogman dan]

Nothing you did was wrong. 3c discharge from 5ah is only 15 amps, that's still pretty easy on the pack. But because it did not die right away, I suspect physical damage a lot more than defect.

 

[athletic91]

Im currently using a 13s 5ah setup in my topeak saddle back for a 20km commute each day. I inspected the pack but found no physical damage. Could temperature be the issue? It gets as hot as 35'c in the afternoons here

 

[bionicdan]

Are you still bulk charging only?

 

[athletic91]

yes only bulk charging , but i will manually check all cells are about 4.1v after every bulk charge

 

[dogman dan]

Heat does affect lifespan, but mostly that's either A, heat caused by a small pack at high discharge rate, or B storing the pack fully charged to 4.2v in an even hotter environment, like a garage that gets to 50c. I live in a very hot climate myself, and get 2 good years, then one year with low capacity out of lipo.

What would be odd, is heat causing just one pack to fail before the others.

Well, whatever random cause it was, just replace it and carry on. And sort of, expect the rest of them going in the relatively near future is possible. Very often it's just one cell on the end that goes bad, so replacing just one cell is possible. Buy a cheap 2s soft pack to get the one cell. Or just replace the whole pack.

 

[TheBeastie]

I had a pack a few years ago that got a dodgey inner cell in the lipo pack, thinking back I know exactly what did it now.. I had the pack sitting on the boot of the car and it slipped off and hit the towbar every outer end edge, it was the very middle cell of the lipo pack it dinged and it was barely visible at all and if I hadn't of seen it I would of never known.

But sure enough the cell went crappy. I don't even think it went bad right away it just crept in over many cycles and until one day I noticed it was clearly stuffed.
I think its hard to accept of believe that a non visual ding can seriously hurt a lipo cell but in my opinion it most certainly can, this is a major upside of 18650 cells as they cylinder canister cell is made out of steel and plated in nickle, you can stand on them and not deform them..

 

[Ykick]

It happens. Out of +40 20C 5Ah hardcase packs in the last 2 years, 2 developed issues. One of ‘em, the tab simply pulled off the cell and the resulting large current trying to flow through the sense wires smoked the BMS. Joy…

Another one, appeared to corrode from inside-out and quickly went to shit capacity/IR. BMS tripped and a quick smell confirmed it needed to come apart.

I do a lot of DC IR testing/matching so plugging in another brick/pack is not much trouble. 4S bricks cheap enough, good to have spares.

My hope is for 18650 cell welding services/sources to reach a level of convenience/affordability soon? If/when that happens, I’m gonna take knowledge gleaned from RC Lipo and BMS to apply to better quality cells.

 

[dogman dan]

Oh, yeah, you can easily ding packs, including hard cases. And the damage doesn't always show. If you smell it, you know a cell leaked.

Typically, a contaminated cell will die sooner, usually puffing some first then croaking. So the probability is that somehow he did ding a cell.

Re 18650's, I'd love to see cells for sale in various parallel configurations spot welded, including the insulator under the positive side nickel strips. So you'd just buy 5p or 10p, whatever, in modules ready for use just like single cells of 5 ah, 10 ah, etc. Then the DIY crowd need only solder to a few strips to make the series connections.

RC packs, of 4s or 5s 2p etc would work too. We'd handle them exactly as we do lipo now, without as much need for the fire box storage area.

Or similarly, 13 or 14s 1 p strings. Either way, you'd have that modular convenience to run a 5 ah pack one day, a 40 ah another.