Sigh. LiPo Problem. 6s -->5s. Looking for explanation/advice

[Kin]

Here's the situation. Well, first. Sorry. I am frustrated to be the person with yet another LiPo thread. Did not expect to have this problem, nor not being able to figure it out. I also have not been able to find my exact problem on ES, though I think I've seen it here once before. It may simply be an internal short in the pack, but I don't understand how that could happen without powerful dissipation of the energy in the cell.

To the details:
12s2p 5ah Lipo. 1p = Turnigy 25C and 1p = Zippy 25C.
Connections:
2P in series with 2P. Each 2P has 6s2p balance leads (i.e, balance leads are connected).
Stored and used in a jumbo LiPo bag from HK.

My cells were at 3.9v, and I jumped on my electric kickscooter buddy for 0.5-.75 mile because I was late for a course. I hadn't used the scooter for two weeks, but I checked the cells voltages with my battery medic very rapidly because I wasn't going to be the fool who overdischarged their cells because they forgot about a vampire discharge. I saw the 3.9v on each cell. What I may or may not have missed is that one of my 6s may have only been showing 5s.

On my ride I felt the scooter was a tiny little bit slow. But I haven't used the scooter for a while, frankly, because the weather is beautiful and I've been enjoying a free rental bike from the school (have to return it periodically, which is why I was using the ekickscooter that day). So I thought maybe my memory was wrong.

Anyway, this is a little bit of detail. I got back to my dorm, and checked my voltages again. Great situation. Cells are only at ~3.7v, it was a very short ride.
I charge for ~15 minutes on a low power charger. I check the cells again, to be sure everything is fine [was suspicious]
But.
Wait!
??
I realize that the battery medic is only showing 5s for one of the 6s2p sets. I'm confused. I check the power lead terminals and...damn. they are showing the pack is only ~19.2v, and but the individual cells that I could see were ~3.8v.........
...

I carefully rip apart my pack shrinkwrap, to see if the fault is in the balance wires, or some other explanation. I test the individual packs at their solder lump. And...One of the cells doesn't show anything. Between the red positive power lead and the next cell there is no voltage....The rest show ~3.8v as expensive

summary
My 6s packs turned into 5s without any explanation. The pack is shorted in such a way that the rest of the pack delivers power still. There was no obvious thermal event. I have no idea what happened.
What is the physical explanation here. Is there one?

Sorry for adding to more LiPo confusion. I'm pretty annoyed that I can't explain what happened. My cells didn't blow up. I have one of the 6s2p packs happy as a camper (the one in series with the set of 6s that turned into 5s). I didn't accidently leave a bms unit or Battery medic for weeks, or at all. Up until that point everything had been staying very well in balance every ride. I have a physical switch disconnecting the controller from the battery during the two weeks of unuse. I really don't see what happened. Pack has probably been cycled from 3.7-3.8 to 4.08v 20 or so times. Maybe a few more. I made it very easy to bulk charge so I would top off a lot without much effort (once I established that balance was well maintained).

 

[neptronix]

That is very bizarre. you are on the right track so far though..

Did you discharge graph ( or otherwise test your pack for capacity ) before using it? you could have had a dud that was missing an amp hour or more.. and you just got lucky in that you didn't overdischarge it, then charge it & get flames as a result.

Another theory... that cell is just defective and has a funky tab connection inside the cell. Or maybe the tab is severed beneath the board.

Whatever it is, that's an odd one. I think i've heard of this problem one other time.

 

[Bazaki]

Have seen it before at another ebiker in my neighborhood, also zippy batteries and also 2 cells to zero volts. So without any reason from 2 x 6s pack to 2 x 5s pack......weird

 

[miuan]

I've had several Zippy 20C 5Ah cells go dead prematurely. I've been buying Turnigy 20C ever since. My Zippies also seem to have double the IR of Turnigy cells.
If you'd drained the other cells to 3.7V, there was no more than 10-15% capacity left. If some cells were duds, it wouldn't take much more to get them to zero volts.
Also your cells are very beat up. You could have damaged the internal structure of one cell, causing it to selfdischarge and take the parallel cell down in the process. Much more likely on first and last cells in the pack that take most of the mechanical shock, exactly as happened to you now. I only buy hardcase packs for this particular reason. And treat the soft packs with utter caution, use a lot of soft padding and solid cases.

 

[neptronix]

Miuan; zippy packs are definitely overrated in C, by about a factor of 2x. Mine are about double the internal resistance of the Turnigy as well.

For me, they tend to stay in balance better however.. and they're quite a bit smaller too. That's the only reason you'd ever really want to use them. I like how my 6S 15C zippy are within 3% of the size and weight of the 5S 20C turnigy packs!

That pack looks like it has hail damage. I bet this happened at the factory. Unless you rest all your packs on a bed of pebbles. :lol:

 

[dogman dan]

Suprising that you aren't more aware which one. I would have expected some puffs. Seems like I saw somebody else have similar problem, a cell just spontaneously discharge and die. You can take the dud cell out of the 6s packs and have 5s, or try to repair them by buying something to use as a source for a new cell. I'd just get a new 6s pack, then combine the good cells from the others to make a good 6s pack out of cells the same age.

 

[Kin]

Maybe I should have waited before posting this. It looks like I won't have time look more into it and maybe take it apart past the little board, until saturday

Important point: I think the 6s to 5s transition happened before I left the apartment, i.e, while the cells were still at ~3.94v. So *not* overdischarged, at least not during my ride. My guess is that looking quickly I didn't notice the difference between the battery medic showing 6 cells and 5 cells. My reason for this is because I thought the speed was just going a bit slow, and in a 12s string, having only 11s would amount to ~9% reduction in speed

Did you discharge graph ( or otherwise test your pack for capacity ) before using it? you could have had a dud that was missing an amp hour or more.. and you just got lucky in that you didn't overdischarge it, then charge it & get flames as a result.

Another theory... that cell is just defective and has a funky tab connection inside the cell. Or maybe the tab is severed beneath the board.

I think I'll be able to look into the severed tab theory tomorrow. I didn't discharge graph them (cheapo Eco6-10), but I did run tests early on with this pack on capacity. At the end of the voltage curve there was a difference in the cells, but overall they did have decent capacity. My suspicion here however is that my 6s was already 5s before discharge. Also, I'd been pretty happy/comfortable with the behavior I'd see. Rarely if ever did I need to balance out the back when used within specs. While I said '20" some cycles on my first page, in reality it must have been 30 at least. The one time I did overdischarge I got lucky and LVC cut off the batteries at something like half 3v half 3.3v.

Suprising that you aren't more aware which one. I would have expected some puffs. Seems like I saw somebody else have similar problem, a cell just spontaneously discharge and die. You can take the dud cell out of the 6s packs and have 5s, or try to repair them by buying something to use as a source for a new cell. I'd just get a new 6s pack, then combine the good cells from the others to make a good 6s pack out of cells the same age.

I'm not sure what you mean by "aren't more aware which one" I can see which one it is, by checking the voltages. But I suppose you could be saying, the fact is surprising a 0v cell doesn't show more evidence of puffing/disturbance. :S I might pick up a 3s 5ah pack (3s instead of 2s in case I screw up). The reason why I hesitate using similar age cells is because I don't want the 5s2p to become 6s1p. That's unfortunate. But it's all ugly, since the issue is split between a turnigy pack and a zippy pack.

Miuan; zippy packs are definitely overrated in C, by about a factor of 2x. Mine are about double the internal resistance of the Turnigy as well.
For me, they tend to stay in balance better however.. and they're quite a bit smaller too. That's the only reason you'd ever really want to use them. I like how my 6S 15C zippy are within 3% of the size and weight of the 5S 20C turnigy packs!
That pack looks like it has hail damage. I bet this happened at the factory. Unless you rest all your packs on a bed of pebbles. :lol:

I like to practice upright needle-bed meditation with my Lipo packs. I'm not sure where the hail-like damage came from. I initially wasn't too nice to the packs, in terms of padding, but since that brief period of a week or so I have been keeping them in a LiPoSack that protects them from the bottom of their steel container.

I've had several Zippy 20C 5Ah cells go dead prematurely. I've been buying Turnigy 20C ever since. My Zippies also seem to have double the IR of Turnigy cells.
If you'd drained the other cells to 3.7V, there was no more than 10-15% capacity left. If some cells were duds, it wouldn't take much more to get them to zero volts.
Also your cells are very beat up. You could have damaged the internal structure of one cell, causing it to selfdischarge and take the parallel cell down in the process. Much more likely on first and last cells in the pack that take most of the mechanical shock, exactly as happened to you now. I only buy hardcase packs for this particular reason. And treat the soft packs with utter caution, use a lot of soft padding and solid cases.

This is definitively a possibility. Maybe there was some small problem that causes the gradual self discharge. What would that physically represent? Because if there's any internal short..I mean, how could that be slow? If the resistance of the short is really high, I suppose it could could be a slow discharge. However, there are very few materials in the cell. They're either strong insulators or strong conductors. I don't think there can be a high resistance short that isn't part of a very small amount of material (which should therefore rapidly heat up), or if there is, I don't know what it would be. This is just frustrated curiosity speaking.

 

[parabellum]

This is definitively a possibility. Maybe there was some small problem that causes the gradual self discharge. What would that physically represent? Because if there's any internal short..I mean, how could that be slow? If the resistance of the short is really high, I suppose it could could be a slow discharge. However, there are very few materials in the cell. They're either strong insulators or strong conductors. I don't think there can be a high resistance short that isn't part of a very small amount of material (which should therefore rapidly heat up), or if there is, I don't know what it would be. This is just frustrated curiosity speaking.

LFP mentions often possible "contamination in assembling process".

I have now new hard case pack, where 1 cell is missing ~0.1V after 1 months on the shelf, no puffing or other problems, capacity is equalized.
Before I received one loosing ~0.1V in 1 week, I removed this cell.

 

[dogman dan]

I suspect a dud cell self discharged to 0v. Had it been 2v, you'd have noticed it sooner. I think you are right, you just saw voltages and didn't notice it wasn't 6 readings.

Yeah, I meant puffing, discharged to 0, I would have expected to swell like crazy.

I doubt you have a cut or vaporized tab, the pack would be at 0v then.

Those do look pretty dinged up. do something about it. I like making little boxes out of coroplast political signs.

 

[Kin]

Hey everyone. It looks like the best guess is self-discharge.

I'm really surprised I didn't notice the problem over the course of while I was using the scooter. Two weeks is long time, but I should have noticed discharge when I was sometimes not using the scooter for a week. Everything stayed in nice balance, but was probably hidden by my parrelling leads.

Le Sigh.


I don't have time to deal with this for several weeks, because I was riding my long board while extremely tired and let it slip under me and into a car. Car kept going, either didn't care or didn't want to deal with it (not that it was their fault- though I am kind of angry they hit it and didn't give a shit to find out what they hit). Now i'm focusing on repairing that (it cracked along the grain, so I think the essential strength direction is still ok.)

I'll probably simply cut out the cell from below the board, and short underneath the board with 8awg wire [depending on if that looks feasible]. Only problem will be having the position charge lead stay on the tab while I heat up the solder.

I'll take the two 'new' 5s 5ah packs and turn them into awkward 10s 5ah pack (with connectors). If I want to put this along with my 12s pack, I'll buy a 2s 5ah battery. It appears to be the cheapest solution, although will require even more active monitoring.

 

[shock]

Holy crap! What did you do to those cells?? Looks about one more pin hole away from explosion.. I think the answer is pretty obvious here. Physical damage.

 

[Kin]

They do look really bad!

To be fair, though, lipo has a rather strong separation. True if I punctured it would be catastrophic. But there was no puncturing. I would not assume that that damage is *not obviously the answer.

Edit: I updated my post to include "not obviously" because that was my intended meaning. (

 

[dogman dan]

Dinged as they are, I don't think that was the cause. You just have had the random cell that died lipo experience. Not particularly rare, some of us have had it at a rate of about 10% of all packs purchased.

So after it self discharged awhile, it was either dead already, or you rode the last 50wh left out of it. Most likely dead when you left the house.
Chalk this one up to your 10%, and buy some more.