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I was startled to hear that there are devices out there that can measure the internal resistance of batteries, in circuit, while the batteries are being (trickle-)charged.

For example, Hioki markets industrial IR meters for UPS batteries. The lowest range is 3 mΩ with a precision of 1 µΩ (!), and they claim you can measure the battery while it's online. I'm puzzled here - isn't the charging current going to interfere with your readings?

I can imagine that if you compare battery voltages with some small and large loads applied (e.g., 1A and 10A), you can estimate IR, but you won't get µΩ precision. But at least, at 10A, the trickle-charging current is probably insignificant and doesn't interfere a lot.

However, all manufacturers seem to use an AC testing method at 1 kHz (Hioki uses 150mA @1kHz at the lowest range). I'm not exactly sure what that means. Wikipedia suggests that these are simply ESR meters, but reading on how an ESR meter works, it can't be possible, since it starts with the assumption that the DUT is completely discharged.

So, my question is: what this AC testing method is, exactly, and how does it isolate the charging current out of the equation?

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The internal resistance of a battery can be determined with AC techniques. One method is to inject a current into the battery using a transformer and a decoupling capacitor to isolate the battery voltage from the transformer. Then the AC voltage across the battery is measured. The resistance is calculated by dividing this voltage by the driving current. A description of this method is given at http://www.cdtechno.com/pdf/ref/41_7271_0512.pdf. This is done with the battery being charged.

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I can imagine that a battery sat there with no load could have it's internal ESR determined by placing it on load (say 1 amps). It only takes a milli second or so to determine this and if this interrupts the charging process fractionally then that isn't a big deal in the bigger picture.

So, reduce this problem to determining ESR when not being charged and then alternate between charging and testing with the majority duty cycle being taken up by charging.

To get micro ohm resolution in one shot is unlikely but you should get accurate to within a fraction of a milli ohm. Average this enough times over a period of ten seconds and you are probably going to get something like micro ohm resolution.

I'm unsure what the AC testing method is but it might be something like I've described.

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