Reading the instructions for one it recommended charging at least once a month to maintain the batteries over the long term. Does anyone have any idea on whether this helps? It's easy enough to do so I'll give it a try. 
Terry 
Funny because we just had a thread bout this yesterday.  Basically what happens is batteries self discharge due to their internal resistance.  What can happen when a battery gets too low is it can leak or reverse its polarity.  So you need a maintenance charge on anything battery powered to keep them from getting too low.