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?