Geoff Norfolk brought next idea :
What on earth is "reverse charging"? I always understood that you were
supposed to completely flatten the battery before recharging and that you
shouldn't recharge a partially charged battery due to the "memory effect"

Some suggest that the memory effect is a myth, however....

Reverse charging is what happens when you have several cells in series
and you discharge them to the point that the weakest ones voltage first
falls to zero, then starts to become charged with the wrong polarity as
the stronger cells continue to discharge through that cell.

The charging process pushes the current through one way, during the
discharge the current flow is the opposite way. You can never get
perfectly balanced and equal capacity cells, therefore the one with
least capacity will become reversed charged if the discharge process
were to continue after the weak one had fully discharged itself.

So for best life, you should always put the battery pack on charge at
the first hint of the drill slowing down.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk