Measuring battery impedance

[lerameur]

Anybody tried or knows a program to measure the impedance of a battery? There a different methods, I believe the DC pulse method would be the preferred one. Looking for a program / algorithym / formula ..
thank you

Ken

[Charles Linquis]

Although I haven't done it, I would think that a small N-channel FET and a resistor in parallel with the battery would work. Measure the voltage of the battery with no (or a known) load. Turn on the FET momentarily to increase the load and measure the voltage. Turn off the FET and do the calculations. It is Ohm's law at that point. If the FET added 1A of current draw, and the "loaded" voltage was 1V less than the unloaded voltage, then the ESR (impedance) of the battery is 1 Ohm. All the formulas you need are here-

http://en.wikipedia.org/wiki/Voltage_divider

[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