[Jeff Higgins]

Rats - standard AGM battery. There goes my easy answer. I've had lithium ion and lithium iron batteries that exhibit exactly the behavior you describe, wherein they "die" when sitting. They really don't, with the problem being they build internal resistance when sitting. The "work around" is to load them by turning on the headlights about five minutes before you try to start the car (or in my case, motorcycle) to break down that internal resistance and "wake them up". I've never heard of an AGM battery needing this, though, so it's probably not that.