Homemade Battery Tester

Last night my wife came to me with the age old question, “Do you have a good battery for this?“

I didn’t have an answer. I knew I had a box of batteries, but were any of them “good”? I don’t have a battery tester in the house, but from my past experience I knew that none of them seemed to work well for all my different types of batteries (alkaline, NiCad, NIMH, etc.). I did some research and learned that the best thing to do is use a digital multimeter and measure the voltage while the battery is under a “load”.

I decided to rig something up. I chose to use a small hobby motor as my load. This will run fine from 1.5 volts, but will also run (mind you slower) at lower voltages. While the battery is running the motor you measure the voltage on the battery. I used a power connector from an old PC power supply to insert the test leads from my multimeter and the wires to the motor..

Here it is testing one of my NIMH batteries.

An unexpected bonus to this battery tester is that I can hear the difference in the voltages of the battery. When I started testing a box of batteries I noticed that the whizzing of the motor changed from battery to battery. I could very easily hear the difference between a good, medium, and bad battery and didn’t have to look at my meter!

Footnote: I remember as a kid my first experience with rechargeable batteries was in my Tyco Turbo Hopper RC car. I’d run the batteries “dead” but after they sat for a few minutes I could get a little bit of juice out of them. While this was fun with my car, it makes testing batteries more challenging. For example, when I put a NIMH battery in the tester I can get 1.1 volts at first, but as the motor spins I see the voltage drop: 1.0, .9, .8….. and down to the point the motor stops spinning and I’m at about .6 volts.

Comments, Questions, Suggestions, (or warnings that something I made is about to blow up) shoot me an email!
rtl@nifty-stuff.com