We are stocking a portable power pack for USB charging. We tested them when they first arrived and found no issues but after 3 months we tested some of the remaining stock and found about 50% of them were dead. I took a few apart and found the cells were 2.5v (+/-.1v) There are two lithium-ion cells connected in parallel.
I charged a few of them back up a little and tested them every few days. They self discharge back down to 2.5v and then stop.
I believe this is because of poorly balanced cells and at 2.5v the stronger battery no longer has enough energy to overcome the internal resistance to charge the weaker battery so rapid self-discharge stops.
Our manufacturer is claiming it was a fault in the circuitry and they changed a capacitor to solve the problem. I do not believe this because it seems like any circuitry problem would keep discharging the cells.
Am I correct in how two unmatched lithium-ion cells will behave in parallel? Will they self discharge to a point and then stop?
Also what are the potential dangers of this? Is there increased risk of fire from charging batteries that are not well matched?
Is this an obvious case of mismatched cells or could it possibly be a circuitry issue?