It seems as if you never have enough on hand. And those that you do have get drained in the blink of an eye, it seems. Which can get very expensive, very fast.
So you'll be glad to know that there's now an AA battery with a built-in intelligent charger that you can recharge from any available USB port (and approved chargers, if you prefer). AA is the most common size used by most devices, as if we had to tell you that.