Looking to understand what happened in over-charged 6s pack

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Kin   10 kW

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Looking to understand what happened in over-charged 6s pack

Post by Kin » Mar 25 2020 10:54am

So I left a 6s pack around 4v for a couple years in the basement. I decided a while ago and started acting recently that I need to get rid of my derelict lipo packs bc wtf am I doing with fire hazards in the basement.

Most packs are ok. One pack has corroded balance wires and I cant seem to figure out the mechanism by which it seems to be reporting low voltage on one cell but ridiculously high voltage on the other.

When I check with the voltmeter...I dont even get voltage across 2 of the taps side by side.

When I check with a battery medic, one cell shows up as 0.07v initially, and the other cell as 5.85v!! Holy shit.

I am discharging very slowly right now but I think this must be some sort of weird but explainable phenomenon either with the battery medic measurement or something else. I'm not entirely sure what though.

If I get the pack down a bit I'll be comfortable cutting it open to see maybe where the balance taps are right now. But at the moment I'm just watching the 5.85v cell because that's a little frightening. My cell log wont report anything, I just get cell error. I suspect if I get to the bottom of this, the difference in how the battery medic and the cell log operates might tell me something.
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Kin   10 kW

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Re: Looking to understand what happened in over-charged 6s pack

Post by Kin » Mar 25 2020 10:57am

Needed to get on computer to downsize the photo. Attached here
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scarycells.jpg
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New to endless-sphere? Notice a lot of signatures recommending Ebikes.ca? That's because it's the best place for a quality and manageable, (but still 'real'), first build. Justin is a solid supporter of electric bike development, and a good source of equipment.

Kin   10 kW

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Re: Looking to understand what happened in over-charged 6s pack

Post by Kin » Mar 25 2020 11:00am

I think I might understand the mechanism. This is minutes after I posted, but I was staring at this pack for like 30 minutes before I gave up and decided to post asking for halp explaining.


My theory:

If one of the taps is broken, then maybe it is working like this:

battery medic:
Cell 1 Gnd
Cell 1 Positive <--> Cell 2 Gnd
Cell 2 Positive

If the tap on Cell 1 positive is broken, then Cell2 may be using Cell 1 ground, and Cell 1 will show up as "0v."

In the battery medic I showed, it wasn't the Cell1 and Cell2 that were odd, but I used the numbers 1 and 2 for simplicity.


Does this make sense? This would also explain why with the voltmeter measuring 2 taps next to each other at a time, I would read 0V twice. Each time that I am reading on the broken tap on one leg.
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spinningmagnets   100 GW

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Re: Looking to understand what happened in over-charged 6s pack

Post by spinningmagnets » Mar 25 2020 11:08am

Try measuring the actual individual cell tabs, if you can reach them.

Sounds like the harness is not wired right, or something shorted.

If they were truly at 5.8V, I am sure they would have caught on fire, or at least would have no capacity now, since storing at anything more than 3.3V is inadvisable.

If the two cells are actually at 2.9V, that sounds realistic.

Kin   10 kW

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Re: Looking to understand what happened in over-charged 6s pack

Post by Kin » Mar 25 2020 11:14am

Unfortunately these are hobbyking 5ah packs that a while ago I encased in a fire retardant polypropylene sheet and doused in hot glue at the ends. So it would be relatively violent to check the tabs.

I do know that the harness *once* was correct, so I think the theory of a broken tap and the medic defaulted to the accessible ground makes enough sense to me now that I'd probably feel comfortable taking it down to the tabs. But I'll keep discharging for now in case I break balance taps and make it harder to discharge. I'm going down to 0 because these are very old packs and I dont want to hand them off to the recycling depot at charged voltages since lipo recycling centers around me seem pretty haphazard and mostly setup for 18650s not rc packs
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motomech   1 GW

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Re: Looking to understand what happened in over-charged 6s pack

Post by motomech » Mar 25 2020 12:44pm

I go thru this all the time. Balance leads short out or /corrode/break and the only way to know what's going on is to physically inspect the tabs and the leads coming out of them, then measure each cell ( as SM said one or two cells could be low/bad), tab to tab and then tab to the lead at the connector. It's not unusual to have to resolder a lead back on it's tab or the even replace the whole lead ASM because the connector is bad (corroded inside).
If you can't access the tabs, I think the brick is a throw-away. The best way to despose of them is is to clip the leads off (both power and balance) one by one (so no sparks) for future use and then let the brick sit in salt-water for several hours to discharge and it's safe to put in the garbage or recycle.
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Kin   10 kW

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Re: Looking to understand what happened in over-charged 6s pack

Post by Kin » Mar 26 2020 1:09pm

I remember reading the salt water, but I think it might still be a good idea to discharge first. The first pack I disposed like that recently, had quite a bit of corrosion on the cell tabs that I didn't discharge before tossing in the water.

So atm I'm trying to get them to <2V each, then drop everything into the bucket. These are all packs that I'm trying to get rid of - some of them are even ones I've already done the de-gassing on so they're well past their end of life :P.
New to endless-sphere? Notice a lot of signatures recommending Ebikes.ca? That's because it's the best place for a quality and manageable, (but still 'real'), first build. Justin is a solid supporter of electric bike development, and a good source of equipment.

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