LiPo bank failure- What Happened?

**teddillard** · 10 April 2012

So, I have 5 banks of 4 paralleled 4S lipo packs, giving me 20Ah @ 74V nom. One bank was not balancing well, by that I mean, out of the 4S I'd get maybe .05V difference, sometimes .1V, even though I ran balance charging on that one bank several times.

(stupid alert)

OK, apparently I left the main power cutoff "ON" for maybe as much as a few days. The contactor was not on, nothing except the (analog) VM was in the circuit. I saw my VM was reading really low voltage, and after getting control of my panic attack I slapped my pack charger on it, and it came up to a reasonable voltage within minutes - around 60V. I then started charging each bank, and they all responded normally except for that one that had been giving me trouble. That pack would take a charge, then immediately the voltage would drop (stabilizing around 7V). I freaked again, and pulled that bank out, pulled out each battery pack and tried charging them individually... same result. All 4 packs in the bank were toast.

So, what happened?

My guess is that I had a bad pack within that bank, and it took the rest of the packs it was paralleled to down with it. Somehow having the main cutoff "ON" made this happen? Can someone explain this to me, in small words?

**Coninsan** · 10 April 2012

You main cutoff most likely didn't have anything to do with your bank failure.

What happened might be that one cell in one of the paralleled batteries collapsed some time during storage. When this happens its voltage drops to zero and that cell becomes a black whole that pulls current from the cells/batteries that it is mated with. First the cells within the afflicted battery go down, then the paralleled batteries go with it into matyrdom.

Had one battery go down like that recently, one cell collapsed and dropped to zero, the rest of the internal cells soon followed and now I have a interresting looking paperweight.

**Allen_okc** · 10 April 2012

knowing how expensive batteries are, im sorry to hear that fellows, my heart goes out to you... seriously...

**NonPolluter** · 11 April 2012

Your experience (similar to several I know) highlights the biggest major problem with Lithiums with BMS -- a single mistake could "brick" your ride.

Even the Tesla cars have been "bricked". Surf for the stories, which may still be on the Internet, if Tesla's lawyers haven't yet suppressed them.

Re-read the bricked Tesla articles, so those of you with BMS'd ecycles don't have this same costly mistake.

The cost of a "bricked" Tesla may explain why used Tesla's and other EV's (our hobby, boo-hoo) are sold for cheap on ebay.

You can allegedly "brick" a Tesla in only a few days if you park it discharged.

The answer is a bike with no BMS. Is this possible? YES. The repair guy at E-Cycles, Philip Monchamp, has a totally custom LIFEPO4 ebike that has a 140 km range, no BMS. He manually balances the cells 1X a month.

However, Monchamp's bike's Thundersky's are easily accesible from two sides with flip-open fairings. This is one weird-looking scoot, but Monchamp is a near-genius and has solved many difficult tech issues at his place of work. (That's why they pay him the big bucks, no joke.)

**billmi** · 11 April 2012

Not sure I follow your logic here.

How is a BMS related to the cell failure on Ted's motorcycle?

**DRZ400** · 11 April 2012

Originally Posted by billmi

Not sure I follow your logic here.

How is a BMS related to the cell failure on Ted's motorcycle?

ohh ohhh let me guess, Ted's bms was built by Telsa and it resembels a brick recently sold on ebay?

**liveforphysics** · 11 April 2012

Leaving backlit voltage monitors, or an RC balancer plugged into a pack destroys it in a short time.

NEVER just charge an RC LiPo pack that is reading extremely low voltage with out checking each cell for any near zero (meaning they decomposed under low voltage to the point of things internally shorting). You also should never just plug in a charger to a pack that was over discharged without observing the condition of each cell group just so you can actually have an idea of how bad the damage is. If your cells were below 2.5v for an extended period, they are damaged, and should be recycled. You can never trust them again. The best advise would be to write that pack off and start over. The sketchy not-smart-advise that someone would do if they were desperate would be individually testing each pack on an RC balance charger, and throwing out anything that looks sketchy and trying to work with the rest. Then never charge that pack anywhere you wouldn't mind watching burn down.

**liveforphysics** · 11 April 2012

This page has some decent info, and shows you what happens at lower voltage.

http://www.mpoweruk.com/lithium_failures.htm

**teddillard** · 11 April 2012

Thanks for that link... I'll give it a good read.

I still can't figure out what happened. There was nothing on the pack causing any drain, it just seems like that one bank crashed. I'm pretty reluctant to build another entire pack from these, if it's the case that one 4s pack failing brought down the whole pack.

**liveforphysics** · 11 April 2012

Originally Posted by teddillard

Thanks for that link... I'll give it a good read.

I still can't figure out what happened. There was nothing on the pack causing any drain, it just seems like that one bank crashed. I'm pretty reluctant to build another entire pack from these, if it's the case that one 4s pack failing brought down the whole pack.

What did the voltage read before you plugged it in and saw the scary low voltage.