I hope David isn't going to be too upset with me for sticking my nose
in here again, but here goes......:-)

It isn't the average temperature that is the key factor here, but rather
which is better at transferring the heat out of the (presumably warmer
than ground temperature) battery.  Call it a question of thermal
conductivity, or of insulating ability, or "thermal mass" - whatever you
like.  Question - why does a concrete floor feel cooler than the 
surrounding dirt when you place your hand on it?
