Consider a micropower comparator driving a MOSFET. Otherwise you would need a latching relay because standard relays would draw substantial power continuously and would drain the battery.
The problem with a comparator is you need a reference voltage. You could use a switching voltage doubler and then a regulator to get a 20 volt reference. This method has the potential to be very accurate.
Check LP2950, it's a micropower 5V regulator. Once you have a 5V supply and reference, a PIC micro with built in comparators may be much more functional than a plain comparator. All this is posible with less than 0.5mA battery drain.
