# Battery pack watts help

1. Aug 30, 2010

### Travian

1. The problem statement, all variables and given/known data

A primary energy source is able to provide 0.7 Watts power in order to charge a mobile telephone battery pack. The telephone operates at 3.5 V, and the mobile telephone’s 3.5 V battery pack has a capacity of 1050mAh. In “talk” the mobile phone draws an average current from its battery of 100mA.

(i) How many minutes of talk time will this battery pack provide, at the average current for “talk”?

2. Relevant equations
P=0.7
I=100mA
V=3.5V

3. The attempt at a solution
Now i'm not sure what formula to use in order to find time. Can someone help me?

2. Aug 30, 2010

### nasu

Re: Battery

The capacity of the battery is given in mAh. This means units of current (amps) times units of time. So what would be the formula that defines the capacity of the battery?

3. Aug 30, 2010

### Travian

Re: Battery

This formula.
(Cb) = I x t

then
time = C / I
?
I need to find time

Last edited: Aug 30, 2010
4. Aug 30, 2010

### nasu

Re: Battery

Yes, this is it. It is actually electric charge (Q) but in battery industry they call it capacity.

No, this is not correct. Watch your algebra.
You need to find time and you can do it from the formula above (Cb=I x t). Solve for t.

5. Aug 30, 2010

### Travian

Re: Battery

Sorry i changed it to time=Cb/I

How can i use this formula (Cb=I x t) if i don't know time?
Shouldn't i use this? t=Cb/I (t = 1.05Ah / 0.1A = 10.5)

Last edited: Aug 30, 2010
6. Aug 30, 2010

### nasu

Re: Battery

Yes, now it's OK. You use Cb=I*t to calculate the time, t= Cb/I, as you wrote above.
First time you wrote t=I/Cb which is not correct.

7. Aug 31, 2010

### Travian

Re: Battery

so the answer i suppose is 10.5 hour = 650 minutes.

Last edited: Aug 31, 2010
8. Aug 31, 2010

### Travian

Re: Battery

There are more to this exercise.

(ii)How much energy (in Joule units) is stored in the battery pack, when fully charged?

I used this formula:

E = Cb x V
E = 1.05Ah x 3.5V
E = 3.675 (what units?)

9. Aug 31, 2010

### nasu

Re: Battery

If you convert your inputs in SI units (something you did not do here) then the answer should be in SI units of energy, as required in the question.

10. Aug 31, 2010

### Travian

Re: Battery

SI units.. Ah to mAh and V to mV?