To know your battery intimately:

http://www.cadex.com/prod_analyzers/7000_overview.asp

Commercial level battery analyzer, far more than what any "easy" circuit can do.

You may also find a system like the Triton Charger/Analyzer which does load testing with high current, capacity measurement, charging, and cycling for Nickel based batteries, supports every type from Lithium to lead acid though.

If you disassemble the pack, you can verify all cells are taking an equal charge, and show very close to the same voltage when the pack is under a 2 Amp load (8 Ohm power resistor, etc).

 

This may be of use/interest to you: http://www.eevblog.com/2010/08/01/e...my-load-for-power-supply-and-battery-testing/

 

A battery test results in a bunch of numbers. Who will decode the numbers?
I recently had my car's battery tested and they said I should buy a new one (of course from them). It measured "only" 306CCA instead of its rated 550CCA (when brand spanking new).

Guess what? My car battery is over 5 years old and still works fine. It might fail in a couple of years or more so why should I replace it now??

Why do you want to test batteries for cordless tools? They work or they don't and a test won't tell you anything.

It would be nice to know your car will start in the winter. Power tool batteries it would be nice to know how long your tool will work.