Some batteries won't hold their charge for a year even if they're just left on the shelf!

Exact!
And some batteries don't like it to be stressed with higher currents, even if the duration is rather short. Another point is ambient temperature! If batteries work at specified temperature limits, capacity can be drastically degraded!!

The choose of the right battery can make your project totally fail, even if your electronics was properly designed. This battery issue is demanding! And a look into datasheet of battery can prevent you from some nasty surprises...

Kai

PS: Unfortunately, at this very moment I cannot give Andy a positive Karma point. But I will do this later.