Homemade Battery Tester

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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

Comments

8 Responses to “Homemade Battery Tester”
  1. Eugene Kerner says:

    Hi,
    You need to know the impedence of the load in order to calculate the power output.
    ie if you measure 1.5 volts accross a 10 ohm load then your source is good for 0.15 watts.

  2. Sam Vallance says:

    Hey
    Just wondering if you put up some step by step instructions because I’m trying to make a battery tester and yours seems to be the best.
    Thanks
    Sam

  3. Sam Vallance says:

    PS
    What setting do you have on your multimeter

  4. Rob says:

    Sam, the design is very simple, just a motor with the multimeter in parallel. The multimeter is set to read DC volts @ or below 20

  5. Do not allow rechargeable batteries to drain to zero.
    Some types will not allow you to recharge them after going too negative.

  6. Duke says:

    Some of my rechargeable batteries are getting old and don’t hold a charge very long. Will this battery tester identify one of these freshly charged, bad batteries?

  7. Gourav says:

    What was the resistance of your motor.

  8. Victor Scarlesi says:

    instead of the motor try wiring in a analog quarts watch in parallel with a 4.7 ohm resistor. Start the test with the watch at 12:00 . That way you’ll be able to time just how long it takes the battery to discharge with no need to keep watching and timing it.

    This is a tester like I mentioned.

    http://www.nickhill.co.uk/nimh-capacity-tester.html

    It works quite well.

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