Simple 12V MN21 remote control battery tester.

I have dedicated many battery tester models to most of the batteries on the market, at least all those I use for work or personally. I had one last challenge left: creating a tester for the 12V MN21 batteries, which were widely used in remote controls between approximately the 80s and 2015.

It is a battery that can be easily checked with a multimeter, but why not make a homemade tester? "Challenges sharpen the wit."

I have created a design with 4 blue LED diodes whose voltage is very close to 3V and a 100-ohm resistance for protection, so as soon as the voltage drops below 12V, the diodes stop shining at 100%, indicating that the battery is depleted (voltage below 11V).

Assembly process during testing with the diodes separated to make various checks easier; in your case, since we already know it works, you can proceed to step 5.

Assembly functionality test; first, I perform a test by soldering the LEDs in the air with some separation to see how many LEDs it is most appropriate to use, the design ending up with 4 blue LEDs, just as we see that they light up at 100% with a 12 V dead battery.

I will show you 2 options that I initially evaluated and discarded.

1st: Using a PCB as a support; it can be used, but it adds more volume to the assembly.

2nd: Choosing between 3 LEDs, or 3 blue LEDs and 1 red (the latter has a lower voltage), or 4 blue; in the end, I discarded the red and opted for the 4 blue.

Final design assembly, diode soldering process, Anode - Cathode - Anode ......

A soldering stand is a great help in these cases.

Measurement of the negative terminal that will touch the battery's (-) pole and the component soldering process.

View of the final assembly and comparison with the diagram.

To add hardness and consistency to the assembly, I apply a layer of fast-drying 2-component glue. To hold it in a vertical position, I use pliers for support.

Photo 1: Assembly test, a battery in good status with 12 V lights up the 4 LEDs at 100% or almost 100%.

Photo 2: A battery with low capacity barely lights the LEDs dimly.

Photo 3: A battery below 10 V does not light the LEDs.

View of 2 low capacity batteries under testing.

Other homemade battery and cell testers I have published:

End of Guide.