Help diagnosing battery that won't fully charge??

I have a 16s5p pack with about 450 cycles. I have charged this pack pretty consistently to 67.2 sometimes a little lower.
Overnight a couple of days ago my pack would only charge to 65.8v. I rode the bike and tried charging again and again 65.8.

I took the pack apart at and measured cells and all were 3.57-3.58v
. Put back on charger to try to charge fully but again charge stops at 65.8v
I then disconnected the sense wires while the pack was charging and plug back in and pack started charging. I was able to get the pack up to 67v then shut the charger off. I then removed the sense wire at full voltage and let the pack sit overnight. When I inspected this morning the pack voltage had dropped down to 65.8 again.

I don't know what is going on, can anyone guess what the problem might be?

Well, 65.8 / 16 is 4.1125v per cell, so if you are only reading 3.57-3.58v per cell with a meter then you might want to check the battery in the meter first, as something is wrong with either the meter you are checking the pack voltage with, or the meter you are checking hte cell voltage with, or the way the measurements are being done.

Then make a complete list of each cell voltage here in the thread, including which cell number it is, starting from the most negative cell.

Make one list before you do anything to the pack.

Make another list during charging, while charge current is still high, if you can.

Make another list at "full charge".


You might also want to post what chemistry your pack is (cell voltages are different for different types).

First measurement take was with pack voltage at around 55.8 volts
All 3.4 but cell 3 was 3.5
At end of charge when charger shuts off
All 4.05 but cell 3 is 4.19v

When charging all cells were equal but cell 3 was .05volts higher.

Pack is 500 shallow cycles li-ion Samsung 25r
165p

My best guess is that 4.1v per cell is full for that pack by now. Unless you see all the voltage drop on one cell group.

After 500 cycles, sounds about right to me.

Simple818 said:

At end of charge...
All 4.05 but cell 3 is 4.19v

Click to expand...

Cell group #3 (4.19V) probably has high IR. And/or, it’s simply out of balance for some reason and shutting the BMS down early.

If it were my pack, I’d probably rig a simple load (lightbulb or discharge setting using RC balance charger) to drain #3 down to around the other cell group voltages. 4.05V or whatever you get for the majority.

Then charge everything back up and see where it sits? The goal being to get group #3 close to the others at end of charge.

To check battery you need to fully charge then check each parallel set of cells. Write down on paper and post here.
1. 3.85v
2. 3.95v
3. 4.10 v


16. Xxx volt .
Post here.

I fully charged my battery by unplugging and plugging back in the Sense wire connector on the BMS. My pack would not charge past 65.8 so I guess by doing this I forced charged it somehow. I took measurements again and I don't know if you call it cell 3 or 13 (don't know which end you start at) measured 4.32. Wayy past what it should be.
I'm not familiar with going about discharging a single cell. ( I don't own an RC charger). Can somebody explain to me in detail on how to do this. Maybe without a RC charger

this was long time ago , but this guy not even had more repplies, took a dangerous path and a cell was charged past 4.2v