Back-up battery charger

Archiver

Does anyone know of circuit (and code) for a battery charger for
lithium Ion cells?

I have a stamp project that normally runs from a 24V power supply but
requires a back-up battery for up to 3 days operation. Using two
3.6V Li-ion cells gives more than enough power but I need to be able
to recharge the batteries when the power comes back on.

Any suggestions will be gratefully received.

Thanks

Helen