# How can you measure battery output to see if the wattage is accurate? [closed]

When you have a battery and it shows you the milliamperehours and volts. You can use those to calculate the wattage. How can you test the battery to see if its wattage is accurate or if the company that is selling the battery is displaying inaccurate information?

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 More on batteries and energy: physics.stackexchange.com/q/19165/2451 – Qmechanic♦ Apr 17 '12 at 15:10 The question formulation (v1) mixes up the notions of power (wattage) and energy. – Qmechanic♦ Apr 17 '12 at 18:28

## closed as too localized by Qmechanic♦Jan 23 at 23:56

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mAh $\times$ V = 0.001 A $\times$ 3600 s $\times$ V = 3.6 J (joule) -> energy, or using formula

$$E = Q \Delta V.$$

You can check data on battery by emptying it. You put resistor on its ends, battery should be working for at least the following time

$$t = \frac{E}{P} = \frac{E R}{(\Delta V)^2} = \frac{Q R}{\Delta V}.$$

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 This assumes ΔV is constant over time, which isn't precisely true. (It depends mostly on battery chemistry) – MSalters Jul 17 '12 at 12:06 More precisely--- the voltage will shrink so that the actual voltage is correct for the resistor you are using at the maximum ampage of the battery, so you need to calculate the draining time using I and R, not V and R. – Ron Maimon Sep 14 '12 at 20:21

The running time totally depends on your usage. If you have bought a 100mAh,12V battery. You are using it run a motor(RC Cars) of 50mA,12V it operates for 2 hours. If you are using it to power another motor of 100mA,12V it operates for 1 hour. If you are using it in a digital clock, 100uA(pretty huge for a digital circuit),12V, it takes around 1000 hours to discharge.

Also, as you go on using the battery, it's discharge time does not vary linearly.

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