I posted something similar to this in the hazards section, and it concerns making sure that there is no power to the cell when the engine is not actually running. The simplest solution I've found is a product called PWM30A from HHO-PlusUK which you can see here on uk ebay http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=271219811126
This uses an extra pair of sensor wires on the battery terminals to detect when the alternator is charging the battery. It only provides power to the cell when the voltage rises above 13.8V
I think this is such a simple and clever idea. There is also an enable input that can be used with a float switch in the reservoir, a thermal switch or both.
This idea could be used to control a relay if you don't want to buy their controller. Obviously there needs to be some kind of hysteresis in the sensing circuit to prevent oscillation because the voltage is bound to fall when the cell starts drawing current.
I've attached their manual so you can see how this connects in practice.