BreakPoint
Testing AA cells
I'll have to try and find two batteries of the same brand to try this out.
Usually a little in-line resistor to put the battery under a load is the best way to tell.
...or if it's a 9V just lick it. 
Somebody with a myRIO and a strain gauge should build a little test rig to quantify the number of bounces with the charge level.
Discharge the battery over a 1Ohm power resistor, bounce it every minute and record each strain trace on an intensity chart.
(Fully discharged batteries do expand slighly. If you ever left them in a Maglight, you'll notice that they will no longer slide out.)
Things are much easier with the non-cylidrical sizes (4.5V, 9V, 6V, etc) where the contacts are closer together. You just hold the contacts against your tongue and you'll know immediately if there is any juice left. 😄
Never knew of 4.5V, and I think the 6V terminals were more than an inch away so licking might be more difficult. But then again I haven't seen one in a while.
Just watch out for the 22V photoflash, or some that were higher for running the old portable tube sets!
I've been doing the licking of the 9V battery for a very long time. I might have done it a few too many times during my tenure at Radio Shack.
Another trick I learned there was to take 2 9V batterys (preferrably when they are already on the low side) and directly connect them. Makes a really nice hand warmer. Perfect for these -20F wind chills we have had lately.
@crossrulz wrote:
Another trick I learned there was to take 2 9V batterys (preferrably when they are already on the low side) and directly connect them.
I did this once with two 9V batteries as a kid. The terminals just plug right into eachother. I had a DC motor near by so I wired it up and I swear the motor spun really fast, and the batteries got really hot. After disconnecting them both 9V had no juice at all. Electrically I convince myself that the motor didn't move at all but that's not how I remember it.
