Intro: Add Battery Test to a Multimeter
The photo shows an inexpensive (often free) multimeter from Harbor Freight. I added a white dot to the indicator. Notice that the white dot points to a battery test function for 1.5 volt and 9 volt batteries.
Batteries should normally be tested under a load because a no load voltage test could read normal, but drop considerably when the battery is under load. This multimeter's manual indicates that the battery test circuit adds 360 ohms of resistance as a load. According to the label on the meter, a 1.5 volt battery in good condition should show a current draw of 4 milliamps or more and a good 9 volt battery should show a current draw of 25 milliamps or more. There is nothing mysterious about this. It is a simple application of Ohm's Law, which says that voltage (E) in a direct current circuit is equal to resistance (R) multiplied by current (I), or E = IR. Factored, that is current (I) equals voltage (E) divided by resistance (R).
Step 1: No Battery Test Function on Your Meter
This is not my better meter, but one I needed in a second location. It does not have a battery test function, but I decided it would be handy to add one. I can use the 200 m function. But, I will need to add about 360 ohms in resistance when in use. (My better meter also does not have a battery test function, and I made one of these resistor probes for it, too.)
Step 2: Solder Resistors
It would be ideal if I could have gone to Radio Shack to buy a pack of 360 ohm resistors, but they are not available. I rummaged through my junk box and found a 270 ohm resistor and a 100 ohm resistor. Soldering these two end-to-end (series) gave me one resistance of about 370 ohms. (Resistors are usually not exactly their stated value, but vary just a little. Check the actual resistance with the Ohms function on your multimeter. My chain of resistors is actually 367 ohms. Also, resistors use a color coding system to indicate their value. The colors shown on the resistors in the illustration are accurate for the values indicated.)
See the second photo. I soldered a small alligator clip to one end of the resistors. I slipped a soda straw over them and added some hot glue at each end to provide some physical strength. I scraped the resistor lead opposite the alligator clip to remove any hot glue that might act as insulation.
I could have brought the 367 ohms down to exactly 360 ohms by soldering another resistor of 19000 ohms in parallel to the two resistors. Resistances in parallel function differently than resistances in series. (Adding 19,000 ohms, if such a resistor were to be available, is more of a hypothetical example than a serious suggestion.) Here is an on-line calculator for parallel resistances.
To use, just attach the alligator clip to one of the test leads. Set the multimeter to the 200 milliamp scale. Use the bare resistor lead as the probe. Touch the probes to the battery under test.
Step 3: Make a Label
It is easy to forget what the thresh hold figures are for testing batteries. I made a label for the back of the meter. My figures are not quite accurate, but are close. It may well be that a battery considered depleted in one application would still function in a less demanding application. See the second photo. The battery that read 3.9 milliamps in the photo for the Introduction with 360 ohms added reads 3.8 milliamps with my 367 ohm resistance added to the circuit.
You can easily make your own resistance load from resistors in your junk box for checking common batteries under load with a meter you already own, even though it may not have a battery test function.
Step 4: One Caution
This meter contains a fuse. It is easy to blow the fuse if the selector was set to the wrong range for a test you performed recently. If the fuse is blown, the milliamp scales will read zero (0). At first you may wonder what is wrong. You simply need to install a new fuse. The fuses used by this particular meter are not easy to find locally. I ordered a package of five from Amazon.