li-ion battery overcharge failure mechanism

[zilog]

I have discovered a flaw in my prototype which kills batteries. I want to fully understand what is happening before I move on.

I am charging a 4.2V li-ion polymer battery cell with constant current/constant voltage until the charge current is 10% of C, then a taper off condition shuts down the charging completely.

The problem is that sometimes this taper off-condition is not acted upon by the charging circuitry, and the cell sits at 4.2V constant voltage (with very little current) for many days, sometimes even days before the taper off shuts down the circuit. This is because of a current drain on the battery that fools the charging circuit to never observe the taper off condition.

I have observed batteries very rapidly losing ther capacity, and after charging a number of additional times, start to leak the electrolyte. Is this behaviour caused by the "prolonged charging" I sometimes subject the battery to, or do I need to look for other problems to solve aswell?

[Mooly]

Li-On cells as far as I understand should never be trickle charged... the chemistry doesn't support that and the cell is destroyed quickly due to breakdown off the active materials.
It may also be worthwhile to only bring the cell up to say only 95% capacity to allow for some safety margin... this is often done with laptop charging circuits.

[djQUAN]

I also use the CC-CV method for my batteries but right down to 0.02C. no problems encountered yet after a few years of use.

other specialized chargers also incorporate a 2hr time limit once the CV mode is reached. so it stops either current drops down to 0.07C-0.02C or 2hrs when it reaches CV mode whichever comes first.

that could be a solution to your problem of overcharging batteries.

on the other hand, I have a discrete charger that does not have a charge termination feature. it just does CC-CV and has an LED that dims as charge current decreases. that one has damaged two Li-Ion CR123 cells already by overcharging. I need to do something about that.

[star882]

Redesign it to stop as soon as the voltage reaches 4.2v per cell.