If you use an automatic transfer switch it should be held energized by commercial mains power which will drop out (IE open circuit) when a power failure occurs. Another set of contacts ( break before make ) will connect the generator to all or part of your local distribution. There is usually a time delay when the commercial power returns to ensure stability before transferring back. If the switch does not break before make it is possible for the generator to feed back onto the power line so this isolation is a must. A 12kv to 110 volt stepdown transformer will step the 110 back up to 12kv and fry some poor lineman working on a supposedly dead line. This circuitry is not operated from your battery. There should only be a sensor circuit or relay telling your generator when to start. It will then probably connect through another relay operated by its ac output sending juice to the isolation switch. In all of this with the possible exception of the sensor circuit there should be no drain from the start battery. A trickle charger should hold the battery at its proper float voltage. If a battery is the right voltage and the poles properly connected any one should work providing that it fits in the available space.