How complicated do you want it?
Assuming that the battery is NiCad, it can
be as simple as a dropping resistor and a
D.C. power source. Or as complicated as
a triple-mode, delta-V, pulse charger with
conditioning profiles.
There are some examples at:
**broken link removed**
If you are unsure, use a small 12v lamp
as the dropping resistor, and a 12vdc
supply. It should be safe, if slow.
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