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# Need to measure battery internal resistance

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#### Ultor

##### Newbie level 3
Does anyone know how I can measure a battery's internal resistance while it's still connected to a circuit? I have a need to measure a lead-acid cell's internal resistance.

Anyone with some ideas?
Thanks
Robert

As far as I know, this is done on a per-cell basis (if individual cells have external connections) or for the entire battery.
In any case, a certain load is connected across the cell or battery and the voltage is measured. By measuring both the no-load voltage and the loaded voltage you can easily calculate the internal resistance of the battery

Rbatt=ΔV/I

where ΔV is the difference between the open-circuit voltage and the voltage under load and I is the actual current when the load is connected.

If the battery is in the circuit, you can measure the voltage difference for two different currents, for instance, but you need a way to change the load current and a way to measure accurately the change. Measuring the voltage change is not a problem, a voltmeter will do.

### Ultor

Points: 2
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VVV said:
As far as I know, this is done on a per-cell basis (if individual cells have external connections) or for the entire battery.
In any case, a certain load is connected across the cell or battery and the voltage is measured. By measuring both the no-load voltage and the loaded voltage you can easily calculate the internal resistance of the battery

Rbatt=ΔV/I

where ΔV is the difference between the open-circuit voltage and the voltage under load and I is the actual current when the load is connected.

If the battery is in the circuit, you can measure the voltage difference for two different currents, for instance, but you need a way to change the load current and a way to measure accurately the change. Measuring the voltage change is not a problem, a voltmeter will do.

VVV, did you check these ? How much must the current be?

xxtigerxx said:
VVV, did you check these ? How much must the current be?

The more the better.
Let's be reasonable, we are not talking about 1000A

yes not 1000A.

but for a 12V/12Ah a current around 1A is good?

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