Why AA batteries run exactly 1.5V?

 We all run into a situation when batteries in our remotes, toys, keyboards/mice run out. If we don't know how to check a battery we might throw out a perfectly fine battery (especially when we have a pile of them somewhere in the drawer).  

This electronics tip has to deal with checking common alkaline AA/AAA batteries or AA/AAA rechargeable batteries for proper voltage with a voltmeter.

Disclaimer : some people might say that a battery should always be tested under load but I have found that in most common household applications this is insignificant and will not change the results of the testing too much.