Always check your batteries before each flight, even if you know they're okay. When your batteries start going bad, they'll give you plenty of warning, but you have to be paying attention in the first place. Just because the ESV says they were fully charged before the last flight, doesn't mean they've got enough power this flight. When packs fail, they usually fail due to a cell shorting (a cell opening can happen too, and is unsurvivable, but it's pretty rare). When a cell shorts out, the pack will still take a charge.

A freshly charged pack with a bad cell will always read fully charged on an ESV at the beginning of the day. As the day goes on though, that pack is going to weaken very, very quickly and you may be lucky to get two flights out of it. ALWAYS CHECK YOUR BATTERIES BEFORE EACH FLIGHT. There is no better insurance you can buy. If you do this, and nothing else, you'll probably never lose a plane due to a dead battery, at the very least you can see when the pack needs cycling.