Originally Posted by Kent10
Thanks bobelectric. That would work. It is just that I have a couple of kids and a wife who like to mix used batteries and new batteries and so it would be nice to quickly test them. Also I have clocks, for example, that may use up one battery but the other is still good so only one will need to be replaced. And it is kind of fun for us to test the batteries. Thanks.
I've read through this entire thread and agree with much of it as far as testing, the series resistance of the battery, and the use of various testers. The idea of reverse charging a dry cell battery with another in series is a little far fetched.
Kent, OP, in this clock mentioned above, how did you determine that only one battery was dead or weak? If the clock has two separate circuits then yes, only one may be bad.
Batteries in series, like a two cell flashlight, with discharge evenly assuming there isn't some other issue with one of the batteries. Weak batteries will have a low voltage when measured with a multimeter. When a 1.5 volt standard battery, i.e. AA, C, D and the like, measure 1.2 volts, it is pretty much dead. Some equipment won't work when the open circuit voltage of a battery is less than 1.3 v. Button batteries come in several voltage levels so their application will dictate what is the cutoff level.
When a flash light battery gets weak the light just gets dimmer and dimmer, but with electronic circuits, the device just quits working.
Use your multimeter to test batteries. The more you use it to test various batteries the better feel you will get for what is good and what needs replacing.