Is there any truth to storing a battery on the ground being a bad idea ?
I think it really depends on what type of batteries you’re going to store. Generally storing things on the floor make them hazards – you might trip over them or forget they are there entirely. In addition, you generally want to keep them safe – cool and dry – and to do that, it would be much better if they are in storage in a container and on a shelf, and in an environment where you can control so that they are kept in a stable situation.
Yes, When stored on the ground the battery will assume the same temperature as the ground. When warmer moist air comes in contact with the battery case, moisture can condense out (like an iced drink in the summer) and increase the amount of self discharge across the top of the case (between the posts). To see this in action take a voltmeter and attach one lead to a post and drag the other lead across the top. You will note an increasing voltage as the probe heads to the other post. To reduce this aspect of self discharge always store batteries on a shelf or on some form of insulation.