# How long does it take for the batter to lose its charge?

## Homework Statement

The two headlights of a car sonsume a total power of 120 W. A driver parks the car but leaves the lights on. The 12-V battery is rated at 95 A * h.

P=IV
V=IR

## The Attempt at a Solution

I don't know any equation that associates this with time. [/B]

gneill
Mentor
Hi pandamonium, Welcome to Physics Forums.

Can you determine the amount of current drawn by the lights?

Since power and voltage is given, we use P=IV to solve for the current I, correct ?

Hi pandamonium, Welcome to Physics Forums.

Can you determine the amount of current drawn by the lights?
Since power and voltage is given, we use P=IV to solve for the current I, correct ? And thanks!

gneill
Mentor
Since power and voltage is given, we use P=IV to solve for the current I, correct ? And thanks!
That's right. What value do you get for the current?

That's right. What value do you get for the current?
I got I=10.

gneill
Mentor
I got I=10.
Okay, now take a closer look at the units of the battery capacity. How long will you have to draw 10 A to equal that capacity?

Okay, now take a closer look at the units of the battery capacity. How long will you have to draw 10 A to equal that capacity?
Oh, since 95 is the total charge that the battery can produce before failing, I can divide 95/10 to get the time before it fails, correct ?

gneill
Mentor
Oh, since 95 is the total charge that the battery can produce before failing, I can divide 95/10 to get the time before it fails, correct ?
Yes. You can check your logic by working with the units.

Yes. You can check your logic by working with the units.
Thanks for all the help ! I really appreciate it c: