M
MooseFET
It's not actually very difficult. Microcontrolelrs typically have A-D converters on
board now. Add a serial EEprom for data backup and you're away.
The battery he is using can provide the backup power. You only need a
way to deal with the rare case of charging back up after the battery
was taken totally flat. If he is running a low power PIC of one of
the small TI processors, he can use a very sloppy and low power
regulator to keep it alive during the "off state".