Battery Capacity Tester





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.



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    15 Discussions

    Is there a glossary or data sheet for the various wires and connectors you can use?


    1 year ago

    and whats the chances i will get a electric shock while creating this stuff ?

    If you discharge a battery fully it will never return to full capacity.. It continues to fall dramatically over its life span - usually about 3 years , until it completely fails catastrophicaly . So far I have not found any type to be different but LiPo4 are better

    4 replies

    yes, except discharging a 12v battery to 11v is NOT fully discharging it.

    damage only occurs when you go below a threshold (that, for example, in a 1.2v NiMH battery, would be about 0.9 - 1v). so since you can not safely go below there, you may say that when it reaches 0.9v, you've discharged it "fully" - in reality it still has some juice, but it's not "usable" juice (unless you don't care about the poor battery).

    I honestly don't know what the threshold would be for a 12v battery, but I bet 11v is quite above it.

    re: lifespan, that has quite a lot more to do with cycles than with time (at least in NiMH and lithium, I'm not that sure about the ones designated to operate continually at full voltage, i.e. backup systems made of NiCd and others). also with recharge conditions - batteries don't like being fed current when already full (though backup ones are designed to work like that - and probably that's why they have a lifespan counted in years and not cycles).

    Lead plates are continually dropping lead off the plates and after 3 years if not shorted at the base already the plats will be so thin as to give up with the slightest vibration

    See here -


    Depth of Discharge

    Lead acid discharges to 1.75V/cell; nickel-based system to 1.0V/cell; and most Li-ion to 3.0V/cell. At this level, roughly 95 percent of the energy is spent, and the voltage would drop rapidly if the discharge were to continue. To protect the battery from over-discharging, most devices prevent operation beyond the specified end-of-discharge voltage.[/quote]

    1.75 V/cell = 10.5V for a 6 cell battery. 95 % is gone so if playing like you are stay above 11.0 Volts

    From info l got from Yuasa the lowest a 12 volt should be taken is 10.6 volts. Lithium of course give more but how much is not yet agreed.


    1 year ago

    Really, Well done. Maybe you must explain a bit better how the switch module works. Anyway, after reading the Ebay info, it was easy to understand the video.

    About voltage levels:
    Some manufacturer says:
    After topping a battery, keep at the maintenance stage of 13.2 volts with a periodic increase to 13.8 to top off the battery.

    Regarding lower level, it is related to the discharge rate. The faster you go, the lower you can go, but be aware that you need also to go to recharge station fast before the batt get damaged.
    As an example, if 72 or 100 hours discharge time, you can go as low as 11.40 Volts. But if your discharge time is around 3 hours, the values is near 10,5V (always talking about 12v - UPS gel - lead batteries).
    Hope this helps.

    The one on the amp meter, the first one is called a T plug and is mainly used in RC

    You mention that the module has 8 operating modes but don't explain what each mode is other than #3 which is voltage control relay mode. What are the others? Is there a datasheet for it?

    Fantastic, simple, & useable... Thanks.....!!!!!

    Nice! Thanks for sharing!