I have a solar-powered LED number display whose rechargeable batteries have apparently died. I'd like to hook it up to AC power using a 3V adapter (i.e., cutting the leads from the batteries and soldering to the adapter output), but I'm unsure whether anything special needs to be done to prevent the charging circuitry from frying the adapter. Presumably a simple diode would solve any such problem, but do I need to worry about it in the first place?
Just use the adapter and cut the leads from the solar panel.
If the batteries "took over" supplying energy at night time then you needn't worry about the solar charging circuit affecting the batteries when dark and this applies if you power from an external 3V.
Cutting the leads from the solar panels stops any charging circuit causing any possible problem and emulates the night-time scenario. If you don't want to cut the leads from the solar panels, mask them off optically.