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I need an estimate of how many watt hours a battery has gained after charging to a certain number of volts.

With a 12 V, 50 Ah lead-acid car battery, charged from 12.5 V to 13.0 V, is it as simple as this?:

(13.0 V - 12.5 V) * 50 Ah = 25 Wh

This only needs to be an estimate. The battery powers a load with a constant draw that turns on and off 5-20 times per day, so I know how long it can run on a given number of watt hours, and just need an idea of:

'If I charged this many volts, I can expect this many extra watt hours available for spending'.

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    \$\begingroup\$ Not like that because that equation implies that the battery is dead when it reaches 0V but the batter is dead long, long before that. If you're only going off voltage, you'll just need to find and use charts for this. SOmething like this: powertechsystems.eu/home/tech-corner/… \$\endgroup\$
    – DKNguyen
    Commented Mar 16, 2022 at 21:22
  • \$\begingroup\$ If you can manage to add a "smart shunt' to your system you will get much better results. Otherwise using a chart like DKNguyen said will give you a very rough estimate. Some things to expect if you do this are that the estimate will sometimes be off. Also, the battery voltage is going to go up and down with the load. Higher when load is off, and lower when load is on. This can give a false reading. \$\endgroup\$
    – user57037
    Commented Mar 17, 2022 at 4:02

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This is not an exact answer the best solution would be a coulomb counter. However you can get a reasonable but not super accurate capacity remaining with a little math and a voltage measurements. The voltages given below are open terminal, ie no load or charging occuring at the time of measurement. Giving it a little time prior to the measurement will help. Your battery should not get over about 13V unless it has what they call a surface charge which will dissipate rapidly when a load is applied. The actual voltage is battery dependent, there is a range for each of the points which will change from battery to battery and age. Using voltage to determine the state of Charge of Sealed or Flooded Lead Acid battery. 100%~12.70, 75%~12.40, 50% ~12.20, 25%~12.00. These are an approximation not an exact number.

As you use the battery, assuming your load is relative constant you can determine how long it takes to go from one point to another, this over time will give you an rough approximation of the battery state/capacity. Some testing will give you a much better picture then I can of your battery and its performance.

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