# Homework Help: Calculating battery usage?

1. Feb 26, 2012

### moonman239

I have a phone. Since it was last charged, I used it for 8 hours and it was on standby for 16. Now my battery's down to 50%. How much battery did I use when I used the phone?

I'm thinking it's as simple as x + 2x = 50%.

2. Feb 27, 2012

### Staff: Mentor

Maybe or maybe not. You're assuming that the battery draw is the same when the phone is in use versus when it's in standby mode. It's possible and maybe likely that the phone uses less power when it's in standby mode, since it might not need to use power to drive the screen. If so, your equation doesn't take this reduced current draw into account.

3. Feb 27, 2012

### moonman239

Yes, the screen shuts off in standby mode.

4. Feb 27, 2012

### HallsofIvy

So the phone uses less energy in standby mode. Unfortunately, you do not know how much less. If you knew, for example, that in standby mode you phone uses fraction m of the amount of energy it uses when in use, you could argue that, since it was in standby twice as long as in use, the total energy uses is E+ 2mE= 0.5. That gives (1+ 2m)E=0.5 so the amount of energy used while in use would be E= (0.5)/(1+ 2m).

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