You can charge while connected but you need to provide for increased or
reversed voltage during charge. Fuses are mandatory for rechargeable
batteries, they can supply enough current to have your circuit literally
catch fire. The most commonly used built in charger is a NiCd or NimeH
trickle charger (constant current) that can be plugged in. In this case
two fuses, a zener to limit voltage if there is no battery, is all it
takes. The fuses protect one the +batt into the device, and the second the
charge plug to +batt. The zener is mounted at the input of the device
(after the fuse). You may have to do more than this if you use a fast
charger (which can cause voltage raise).

Peter

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