[_Jim]

re: “ If it has the protection circuit board built into the pack, it can most likely be ruled out. “

No.

During testing of Li-Ion chargers and using used cells (from a laptop battery pack), I have had cells which never saw a terminal voltage over 4.1 Volts begin to “self heat” (as it began to self discharge due to an internal short that developed AFTER having seem a full charge).

Fortunately, I didn’t have any ‘flame on’ happen or ‘cell swelling’ (due to internal gas buildup) take place.

[Ol' Dan Tucker]

During testing of Li-Ion chargers and using used cells (from a laptop battery pack), I have had cells which never saw a terminal voltage over 4.1 Volts begin to “self heat” (as it began to self discharge due to an internal short that developed AFTER having seem a full charge).

During your testing were you monitoring individual cell voltages within the pack? IOW, were you testing individual cells or full packs?

Here's the rub.

LIPOs are also sensitive to over-discharge. If one of the cells in the pack is out of balance with the other cells, the voltage can drop below 3V. This can (not will) trigger internal shorts.