DIY 3.6V LS 14500 Battery Tester

This simple design arises from the need to test industrial-use 3.6 V LS 14500 batteries several times a month at work, which are installed in wall-mounted detector equipment; a common failure I encounter is a dead battery, so having a tester makes my work much easier.

It visually and "quickly" indicates whether the battery is OK or dead, since, for example, a multimeter actually measures without a load, showing an "unrealistic" voltage—indicating the battery is at 3 V, for example—but when the circuit draws power from the battery, it drops, and it is actually at 1 V.

With the battery disconnected from the circuit thanks to its connector (it is not visible in the photos because I disassembled it to solder it to another battery), a load is applied and it visually indicates with the LED whether it is OK.

We see in the electrical schematic that the number of components is minimal: R1 100 ohm 1/2 W resistor, R2 47 ohm 1/2 W resistor, blue LED (I use this color because it is much more tolerant of voltage variations and because the voltage being tested is close to the LED's 3 V excitation threshold), 1 push-button, and 2 alligator clips, red and black.

Circuit view; the same, when pressing the center button, goes on to connect the LED with its protection resistance, applying a draw of 60 mA (0.06 A) to the circuit, enough to see the actual state of the battery. I have left output wires to be able to connect a multimeter so that the measurement is more complete with a DC voltmeter.

Photo 2: initial idea with the wires + alligator clips at the same length; later I saw that it was much more useful to have the wires at different lengths to be able to "hang" the circuit from the battery.

Photo 1: Exploded view of the 1st cable configuration with alligator clips, to test the batteries I was interested in; in my case, it was much more useful to leave one cable longer than the other.

Photo 2: View of how the final circuit turned out with the Black cable longer, to be able to perform the test in a vertical position "hanging from the dead battery".

In my case, I made this design to check a battery installed vertically in a wall-mounted box, so I adapted the shape and length of the alligator clips to the position in which the battery is installed in the device.

The cable and red clip are shorter since the device hangs from them, and the black clip and cable are longer so that once the red one is connected, the black one can be connected easily.

Exploded view of the PCB with the jumpers, the two 100 ohm LED protection resistors and load resistor, 47 ohms applied consumption to the battery under test, and the LED indicator light. By pressing the button, we apply a consumption of 60 mA = 0.06A to the battery.

Photo 2: To provide oxidation resistance to the PCB solder joints and protect against possible short circuits, since I have left the PCB exposed without a case, I have applied clear nail polish.

Tester explanation video:">Tester explanation video: CORRECTION: MINUTE 2:06 THE LED POLARIZATION VOLTAGE IS HIGHER THAN THE VOLTAGE SUPPLIED BY THE BATTERY, (Not lower as I indicate) WHICH IS WHY IT DOES NOT LIGHT UP.

When measuring any type of battery with a multimeter set to DC voltage, the device will always give a voltage reading higher than the actual value.

Connecting the battery test circuit board applies a small load to the battery, which drops the voltage. Pressing the button on the board connects the resistance and the LED, effectively applying a real load to the battery; if the battery has low capacity or is a "worn-out battery," the voltage will drop to show the real value.

With a good battery, the blue LED will light up without issues, and the voltage will be approximately 3.6 V.

With a partially depleted battery, pressing the test button to connect the LED will show a voltage below 2.9 V, and the LED will light up dimly.

With a completely "dead" battery, pressing the test button to connect the LED will result in the LED NOT turning on. Photo 3.

We can use the circuit independently, with only the LED indication, or in conjunction with the multimeter, which will provide the real voltage value of the battery.