If your micro doesn't have to do anything other than a2d, then I suggest you drop the idea of using a micro at all.

Try choosing a single chip digital voltmeter.
Do not make a full digital meter, but just use its required function of measuring the voltage of your battery.

Take its digital output and send it over RF 1000 miles across atlantic and I will know that your battery needs a recharge.

Good Luck

Babar/