How did I end up with an overcharged cell?

[aclysma]

Here is my setup:
* Powerlab 8
* FMA balance board (the one with lots of fuses. See picture: http://revolectrix.com/support_docs/item_1401.pdf)

I charged 2 6s 3000 (Hyperion) and 4 6s 3800s (Gens Ace). All have voltage within .15 when placed on the charger.

At the end of the charge I noticed that all charged the charged lipos except for one were in an acceptable state (voltage 4.17 < V < 4.2) except for one lipo. It's cell voltages were something like:

4.25
4.22
4.2
4.16
4.13

In addition, the red strip for the balance leads turned black indicating that too much current was trying to pass through the balance taps.

Any ideas as to how I got into this state? Using this board is more annoying than 6-way split cables but after seeing this I am SO glad I used it!!

Again, I really want to know how this might have happened so that I can prevent it from occurring again.

FYI:
- I immediately put the battery on a discharge until all cells were below 4.2
- I'm balance charging this battery by itself, carefully monitoring it to make sure voltages are not exceeded.

[Slyster]

Either you have an old tired pack in the mix.. or a new pack not yet fully broken in. When parallel charging it's crucial you only mix healthy (broken in) packs.

[aclysma]

The 4 gens ace are about the same age. The 2 hyperions are a bit older. I think all of them would be considered healthy and broke in. I did not pair my gens ace packs (didn't know I was supposed to do that when I got them)

[Gregor99]

1. Charge each pack independently, checking all cells for IR and proper balance. On the Powerlab, I like to enable the balance bypass screen so I can tell if one cell is lagging behind. What you see is all cells except on have a bypass of 99%. If one cells is way behind the charger will trigger the safety charge mode and fall back to a charge rate of 500ma. For me that might be enough to abort the charge, safely discharge and discard the pack.
2. After the charge, let each pack rest for at least 1 hour. The a use cell checker to see if one cell has self-discharged significantly more than the other. Depending how severe the self-discharge is, it may take much less than an hour for differences to show up. If no issues are found move to step #3
3. Discharge each pack and monitor for any cells that discharge faster than there other cells. You can do this on the Powerlab with a Pb battery in regen charge mode, graphing the individual cell voltages. Or you can simply fly the pack, then us a cell checker for any cell that is significantly lower than the rest. That would indicate the cell is losing capacity.

[aclysma]

I've flown and charged quite a bit in the last couple days and never saw the problem reoccur. Cells are balancing well. Maybe the balance tap wasn't plugged in all the way.

[ahahn]

I think this is the main flaw with parallel charging. You are really depending on the balance wires to be connected. If you lose a balance connection on a pack, then if that cell is on the high side of it charged state, with respect to the other cells in that pack, then you are in danger of overcharging that one cell.

I think it is always a good idea to occasionally check the balance connector to make sure it is still connected ok. Isn't foolproof of course.

[Slyster]

I've had the same issue twice... where a pack with one cell dying made a healthy pack very unbalanced. This is a drawback to parallel charging for sure.

They need to make a high powered charger with like 4 independent outputs at 10-15A each or something like that.

[Gregor99]

This the main reason I advocate using a quality cell checker to check each pack both before and after the charge. Before checks for low cells and broken leads. After checks for over or under charged cells.

[Jotto]

I charge each of my batteries alone once in a while to make sure that they are not acting up.