I've recently started trying to keep batteries only in those mods that I'm going to use today, because I've had too many batteries that I had to retire after they sat in chip mods for months or even a year. The voltage got too low to accept a charge. Since I only use two-battery mods these days, I've also got a fair number of batteries that I don't use because their mates died. I give them a maintenance charge for safe storage, but eventually they'll die of loneliness. Then there are the paired batteries that are still perfectly good but which I don't use because they're lower in mAh than the ones I use. They also sit in plastic battery boxes with their mates, waiting to die from inactivity. Morbid!