What happens if you use a smaller than the original 9 volt charger with
a 7.5 volt Makita drill, which currently is totally dead.
I just don't have time to find a 7.5 volt wall wart, and it's not worth
the money to buy new.
My guess is that you'll overcharge the battery if it happens
before you smoke the charger.
Primary failure mode of a drill battery that's been discharged for a while
is shorted cells...sometimes ALL of them.
Even the right charger may smoke itself while overcharging the
remaining cells that aren't shorted.
Since you said the charger is smaller, it probably is rated at less
current and you may burn it out.
If you try it (assuming the polarity is correct)
carefully monitor the charger and the battery for overheating and
disconnect at once if they get hot.
The slight over-voltage is probably not going to be an issue if there is
a regulator chip in the drill's charging circuit.
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