You have a pack of rechargeable batteries which you use to power up your devices.You think of charging them again, only when you find out that the device isn’t working. Since there is no way to determine an uncharged battery from a charged one here’s an idea from Mac Funamizu that lets you know about the battery condition before it is completely drained out.
The new batteries called the Hungry Batteries starts getting thinner as their energy gets used up. So you can spot the difference that easily. Moreover such thinning of batteries also makes it easier to be removed from the battery holder while replacing.
[source : tuvie.com]