Introduction: Battery Capacity Tester

This time I'll make a battery capacity tester from a bulb, watt meter and voltage control module.

How I did it - you can check by looking DIY video or you can follow up instructions bellow.

For this project you will need:



12V voltage control module

12V 55W light bulb




Step 1: The Need

I bought four, used 12V AGM lead acid batteries for my solar system. Because those batteries are 2 years old and have been used before, I want to find how much capacity they still have.
For this test I could use my ImaxB6 smart charger and discharge each battery. The only issue is that this process will took way too long, because it could discharge 12V battery only at 0.4A of load.

So I decided to make my own discharge capacity tester.

Step 2: The Idea

Capacity tester is combination of three devices with different functions. A power meter, automatic switch and a load.
So here it is, a meter, an auto switch and a load. To measure and monitor capacity I used simple watt meter. I bought it with bare wires and soldered connectors, because I want to have it modular and will use it in various future projects.

To control and disconnect discharge process at specific voltage, I used 12V voltage protection module.

As a load I used regular 12V 55W bulb from car headlight.

Step 3: Wiring

Made quick wiring regarding showed diagram and we’re ready to go.
For testing I used small 12V battery.

Step 4: Operating Mode

This voltage control module have 8 operating modes. I’ll use third - voltage control relay mode.

Step 5: Voltage Limits

Now display show battery voltage 11.7V.

First it’s need to set upper limit voltage value (11.6V). This value must be a little bit lower than current battery voltage, because only then relay will be activated. Set lower limit values of voltage (11.0V). The lower limit voltage can not exceeds the upper limit. After relay was activated, it’s need to increase upper voltage limit to the number, which will not be exceed (13.6V). This will prevent from close loop cycle discharge.

Step 6: Low Voltage Control

To reach lower voltage limit quicker, I connected additional 12V bulb to the battery.

Voltage drops below set value, relay disconnect a load and meter shows how much power was used.

Step 7: Conclusion

By using components for around 15 euros you could get cheap and accurate battery capacity tester. One more great feature, that by using bulbs with different power ratings, we could get specific load amperage. For example to increase amps - it’s need to add mode bulbs in parallel, to decrease amps - use less powerful bulbs.