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I hope this is not off topic.

Consider I buy brand new 12V battery. Normally, will be 13.8V when it fully charged. According to the info written in the cover, it capacity is 42Ah. After used for sometimes, based on the calculation of all the electrical devices connected to it and the duration of they are connected (using power), there still left at least more than half the current. But the fact, the battery is finished. It need to be recharged.

We are used to measure the voltage with multi-meter (AVO meter). Then my question is, can we measure the actual capacity of a battery, i.e. using an AVO meter?

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Battery capacity can't really be represented as a single number, like 42Ah, unless all the other details are specified as well. For instance, a car could be said to get 25MPG fuel economy. But if you are driving at 150MPH, that number is incorrect.

That said, if you want to measure battery capacity, you need to decide a few things:

-What represents "battery full","battery empty", and what load current or load resistance you want to discharge the battery with. This is probably something that is representative of how you will use the battery in your design. For instance, if you were designing a flashlight, you would discharge the battery using the bulb in your design (or current level that would be similar). Maybe your "battery empty" state would be a certain brightness level of the bulb, or maybe it would need to be a certain battery voltage. It depends on your design.

Once you have decided these things, you can make the "battery full" by charging it, then attach the load and make periodic measurements of the current and voltage until the "battery empty" state is reached. Do a little arithmetic and you can come up with your capacity number. You may need two AVO meters, one in series to measure current constantly, and another to measure voltage.

Depending on how sensitive your design is, you may also need to specify other things, like temperature during charge and/or discharge.

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  • \$\begingroup\$ Actually my question is the method of measurement without knowing the load. Say like after starting a car, I want to know what is the consumed power for one start, by measuring the battery capacity before and after starting. \$\endgroup\$ – AirCraft Lover Feb 21 at 6:51
  • \$\begingroup\$ @AirCraftLover measuring the battery capacity before and after starting As explained, you cannot "instantly measure" a battery's capacity / how much energy it has stored. The energy is "locked inside" in a chemical form. Only by monitoring how much goes in (charging) and/or how much goes out (discharging) can the amount of energy be measured. So when starting a car, you need to measure, current and voltage over time while the car starts. Then the amount of energy taken can be calculated from that information. \$\endgroup\$ – Bimpelrekkie Feb 21 at 8:44
  • \$\begingroup\$ You should put this comment in your own comment so I can accept as the answer. That is actually my question. \$\endgroup\$ – AirCraft Lover Feb 21 at 8:52
  • \$\begingroup\$ But if it can not measured so easy, so how do all mobile phones and computers calculate the remaining capacity of their battery? \$\endgroup\$ – AirCraft Lover Feb 21 at 8:54
  • \$\begingroup\$ @AirCraftLover phones and other battery powered devices don't measure the capacity. They measure the voltage. When the battery gets depleted, the voltage drops, as you know. When it gets below a certain threshold, it goes to low battery, then as it drops further it gets to a flat battery state. So, they all measure the remaining voltage of the battery, it has nothing to do with measuring capacity \$\endgroup\$ – MCG Feb 21 at 8:58

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