On Sun, 29 Dec 2019 16:47:45 +0000, Adrian Brentnall wrote:

Plug it into the cradle, leave is as long as you like.
Remove it from the cradle, pull trigger, it runs for maybe half a second
and then stops..

Does the "charge" LED behave as it should? ie steady on for a period
of time then go off? Or does it flash slowly or quickly?

It might be a duff cell in the battery, though more likely is a
"confused" battery management system. Think I'd open up the battery
and measure the terminal voltage of each cell and zap the lowest with
a PP3. Then place on charge and see if the charge LED behaviour has
changed.

Not sure how multi-cell Li batteries arrange the battery management.
I wouldn't be surprised if it's on a per cell basis rather than whole
battery as you can't "protect" a single cell by only monitoring the
whole battery.

The charge LED on the wall wart may just reflect the current drawn by
the connected unit. Above a given amount it's on, below off. The
battery management controls it by simply drawing more or less current
to indicate the state (charging (on), charged (off), fault (slow
flash), temperature (fast flash)).

--
Cheers
Dave.