From bigclivedotcom.

This is entirely my fault. I bought a mini chainsaw to make a video about, but as I’d featured another variant at that time I held off and ended up storing the unit and its batteries for about three years. When I rediscovered it I found that both batteries were damaged beyond recovery by over-discharge. Usually if the cells have dipped just below about 2.5V they can be gently eased up to 3V and then charged fully. But these ones have suffered long term discharge damage to at least one cell in each pack, causing rogue self discharge.

When a cell does not maintain its voltage after charging (allowing for a slight settling of voltage) then it’s time to let it self discharge completely and recycle it. The losing of charge usually indicates a damaged separator between the electrode layers.

Odd circuitry though. They seem to have gone a different charging direction than usual, with the charge MOSFET drive being level shifted to switch a MOSFET on the other supply rail.
I’d guess that the charge MOSFET output is active at all times, except when an end of charge state is detected, by one of the cells in the battery reaching 4.2V.

The moral of this story is to ensure that all power tool batteries are given a top-up charge every so often. Especially the ones that don’t get used for long periods of time.

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