Batteries drain unless block heater is plugged in

Just bought a 96 5spd 12 valve and if it's below around 40 degrees if I don't plug the block heater in it seems the batteries get drained but if I plug it in it'll start up no problems. I tested both batteries and both tested good had a new starter put on 2 months ago, I'm clueless I've never had anything like this happen before any thoughts or help would be appreciated