[puddleglum]

Something else you may want to investigate as an alternative to a block heater is an oil pan pad heater. They are available for 12 volts and draw less power. You would need a fairly flat area on the oil pan. They heat a little slower but the lower power might allow you to use the battery you were thinking of. They do work pretty well since heat rises. They have their advantages and disadvantages compared to a coolant heater.