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My 4S fell from the sky and is now a 3S?

#1
My first flight on my new quad and I accidentally disarmed about 5 feet up over gravel. Everything seemed OK but the battery had a few minor scratches in the plastic and the outer most clear layer was cracked. I was coming in to land anyway so I just put the battery away. When I put it in my charger at home the charger reported an error. I checked the cells and the 3rd cell reports the same voltage as the second and the total pack voltage reads at ~11V. So using a multimeter on DC voltage, I put the ground on the first pin and they read: ~3, ~7, ~7, ~11 which does not seem right. I also did a continuity test and both of the pins 2 & 3 on the balance connector report being connected. What is going on? The only thing I could think happened is the cells got bridged internally and, luckily without exploding, balanced each other out.

Is that possible? I looked real close at the balance connector and the pins don't look bridged there. I don't know how to take it apart safely to be 100% sure. Right now the battery is in my bat safe, but is my best option to discharge and toss it?
 

FDS

Well-known member
#2
It’s hard to check individual cell voltages with a multi meter, a cheap lipo battery checker will do a reliable job. You want the individual cells to be +/- 0.1v of eachother. You could well have destroyed a cell.
Until you fully know what’s up with it you are right to be cautious. Usually if one cell goes then the pack is dead and needs to be replaced.
 
#3
I had a similar issue when I crashed a plane and the center cell of a 3S pack was stabbed by the motor shaft. Nothing was obviously wrong until I went to charge it the next time and the center cell was completely dead. I was able to salvage the other two into a serviceable 2S pack for my 3D printed F1 car. Be sure to take all sensible precautions if you decide to cut the cells apart and make a new pack.
 
#4
My cheap lipo checker did not like the pack. It reported ~85v on that pin. I didnt think the decimal could move for cell voltage. Anyways, it seemed less trustworthy.

Turning into a real 3s woudnt be so bad but I would not trust myself opening the pack and if the cell is really damaged, or bridged, leaving it there doesnt seem safe either. I guess I should drain it and bring it to the haz mat drop off as even the nice 4s pack is not worth a fire.
 

FDS

Well-known member
#5
Absolutely. I use a cell checker/balancer for all testing, some chargers also have a cell reading capability. Mine cost about $15 and has been well worth it.
 
#7
I was going to try Batteries + near my house. The county also has a haz mat collection event every quarter that also takes things like paint thinner, CLFs, etc. It just takes planning a trip on that particular day and remembering to go.

RIP my wonderful 4S. I'll replace you but you will be missed.
 

Bricks

Well-known member
#8
I had a similar issue when I crashed a plane and the center cell of a 3S pack was stabbed by the motor shaft. Nothing was obviously wrong until I went to charge it the next time and the center cell was completely dead. I was able to salvage the other two into a serviceable 2S pack for my 3D printed F1 car. Be sure to take all sensible precautions if you decide to cut the cells apart and make a new pack.
I did the same with 2 3s 2200 packs pulled them apart and made one good 3s pack now I have 1 cell waiting until I destroy another 3s pack.