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Jumper across a completely dead cell in a LiPo pack

evranch

Well-known member
#1
I have a 3s pack here that got punctured in a crash over a year ago. I sealed up the hole with hot glue, and it actually made it an entire year until the punctured cell completely died - it now measures 0 volts.

I could tear it all apart and make a 2s pack out of it, but I feel like it would be easier and maybe even safer just to put a jumper across that dead cell. If it's truly only producing 0 volts, there should be no reason you can't, right? And before you mention hauling the dead weight of the cell, I'd just be using this 2s pack to power my goggles or my R9 transmitter module.

Once a cell is discharged to 0, it contains no energy at all and could never spontaneously recover... right?
 

quorneng

Well-known member
#2
Are you sure the 0 volts is not because one of the cell tags has fractured? If this is the case then the cell could still have significant energy in it.
The only sure way to absolutely discharge a LiPo is use a bulb as a jumper when it has energy in it. Then when it shows 0 volts with the bulb still in pace you are as sure as you can be the cell is truly 'empty'.
If you do use a jumper make sure it is soldered to dead cell terminals so when the battery is in use there is no possibility of a poor connection allowing a small current to put some charge into the dead cell. It could then do something nasty.
Remember there is nothing wrong with the power in the good cells. Working around live cells is not recommended.
 

evranch

Well-known member
#4
The entire pack reads 8.2v on the main leads and the balance lead gives me 4.1v, 4.1v, 0v. So the circuit can't be open and it's pretty likely the cell is completely discharged.

I've done some surgery on packs before, and it's a lot easier just to slit open the connection end and do some soldering than it is to split all the cells apart and then repackage them. I'd also say it's likely safer because all the connections are constrained by the pack and you don't have loose cells kicking around to short out.

Installing a jumper is simple, just soldering a chunk of wire between two of the terminals and closing that end back up. It's a good idea to use a bulb as a final confirmation before shorting the terminals and I'll do that.

As quorneng states you have to be very careful with live lipo cells to avoid a fire, and if I hadn't been playing with high power devices since I could hold a soldering iron, I'd probably just dispose of the pack.
 

evranch

Well-known member
#8
In the end it was irrelevant because it turned out once I opened the pack, the dead one was not the glued one but another cell with a tiny hole in it that I missed! Looks like hot glue is actually pretty effective at prolonging the life of a punctured cell - at least when the cell was punctured in the cold and the glue is applied within minutes.

Why put the effort into rebuilding a pack with a questionable cell in it, so I just tossed both cells and have a 1300mAh 1s left. Not nearly as useful as a small 2s pack but I guess I can run an FPV AIO cam off it.

Thanks for your input everyone.