# Thread: how much charge and energy

1. ## how much charge and energy

1. A rechargeable flashlight battery is capable of delivering 85mA for about 12h.
How much charge can it release at that rate?

well i think I did this right.... what ?? me was the time given as h so looks this has to be converted to s (seconds) to do this. is this always the case?

$\displaystyle Q=\int_0^{12x60x60} (85x10^{-3})dt = 3672\rightarrow3.672kC$

2. If its terminal voltage is 1.2V, how much energy can the battery deliver.

so on this one I just multiplied the voltage to the current

$\displaystyle W=\int_0^{12x60x60} ((1.2)85x10^{-3})dt 4406\rightarrow4.406kJ$

2. Originally Posted by bigwave
1. A rechargeable flashlight battery is capable of delivering 85mA for about 12h.
How much charge can it release at that rate?

well i think I did this right.... what ?? me was the time given as h so looks this has to be converted to s (seconds) to do this. is this always the case?

$\displaystyle Q=\int_0^{12x60x60} (85x10^{-3})dt = 3672\rightarrow3.672kC$

2. If its terminal voltage is 1.2V, how much energy can the battery deliver.

so on this one I just multiplied the voltage to the current

$\displaystyle W=\int_0^{12x60x60} ((1.2)85x10^{-3})dt 4406\rightarrow4.406kJ$

there is no need to use integration.

I= Q/t
V=W/Q

use the above equations can give you the answers
.