How do I calculate the effective capacity of a switched battery power supply?

Question

I have to calculate the effective capacity of a Li-Ion battery with a switching regulator. What the battery-fed supply does, is switching its output between 7.2V and 0V, with the ratio between off and on determined by its setting; at 75%, it's supposed to be switched on 75% of the time.

I can't use the 100% setting to get 100% output: it still switches off and on. I think it's to prevent the battery from shorting out at high loads. That's the reason I'm asking this question.

So how do I calculate the effective output? Is it the average of all voltages (I use a data logger) during the entire unloading of the battery (e.g. 7.2V, 0V, 0V, 7.2V, 7.2V, 7.2V etcetera), or is it the RMS value? RMS gives a higher value (of course) but is meant for AC power (where a simple average would give me 0V) so picking the right one is essential to calculate the battery's capacity.

Answer 1

The capacity of a battery is given in Ah  (ampere-hours) and is quoted by the manufacturer.  There is no need to calculate (measure) it.

Or do you mean the capacity (current rating) of the supply?  If so, much more information is required.

Furthermore, the r.m.s. value of a d.c. is a nonsensical concept.

And whatever made you think that "RMS (of a switched d.c.) gives a higher value (of course)" ?

If the d.c. is being switched on and off then the average value is proportional to the on time.  In your case the average output voltage is 5.4.