# Calculate a phone battery capacity by percentage charged with a given input

I wanted to see if I can calculate an Android's phone battery capacity by plugging into a powerbank that displays the output in Watts. I see that the output is shown steadily at 17.2W and the battery charged from 25% to 51% in 20 min. The battery is Li-Po rated at 5000 mAh, but I wanted to see how much it degraded.

Is it possible to estimate the capacity of the battery (in mAh) given this information? If so, how? If not, what other information do I need?

Assume the powerbank is 5V (You didn't say... If it's not, you need to re-do the math below)

Watts = Volts * Amps

If you read 17.2W, it also tells us the current

17.2 = 5 * i

i = 3.45

You've charged about 25% of the battery with 3.45A in 20 minutes, which is 1/3 of an hour so it's 3.45 / 3 = 1.15 Ah i.e. 1150 mAh. Thats 1/4 of the battery capacity

That would imply 1150 * 4 = 4600 mA-h capacity for the battery

It's a reasonable approximation.

HOWEVER this assumes that % rating reported by the phone is linear and the reported watts measurement is accurate. I doubt either really are. "Pretty close" maybe

You can use an electronic load. Usually it can register in time domain power/voltage/current.

You could set a reasonably/realistic constant current draw and stop at power collapse, minimum voltage or when a protection circuit cuts the withdrawal.