Measuring battery impedance

[readitaloud]

First take an open circuit voltage measurement of the battery. Let this equal Vo. Apply a variable load, Rload, until the voltage across the load is 1/2 (Vo). At this point Rload will be equal to the internal resistance of the battery.

You could use a FET as the load, Rload, although you will need to know the transfer function, Id vs Vgs, of the device. You could use a low resistance drain sense resistor, Rd, to determine Id of the FET at each sample of applied Vgs. Pulse Vgs only long enough to reach a steady state value of Vdrain to ground. Adjust Vgs in steps untill Vdrain, same as Vload, is equal to 1/2(Vo).

- Martin

[Ioannis]

The 1/2 Vo method can be dangerous for a high capacity battery. The current that will flow may be the doubleof the nominal battery current.

Pulsed load is better and not necessary to overload the battery. The voltage difference even 1 volt is enough to estimate the internal resistance. But note that this resistance will change in time.

Ioannis