# Homework Help: What is the estimated battery life?

1. Aug 29, 2011

### Pete789

1. The problem statement, all variables and given/known data
Electric motor in put power rating is 17.5kw
Voltage is 48V
Battery is 7.5kw/h
Average W needed to maintain motor per sec is 1446.7W

2. Relevant equations
Motor: W=I*V > I = 364.58Amps

3. The attempt at a solution
My attempt: If 1kw = 3.6 x10^6J > 7.5 * 3.6 x 10^6 = 2.7 x10^7
Then the battery is 2.7 x10^7 Jouls

I can't go any further than this. Can anyone help please?

P789

2. Aug 29, 2011

### Staff: Mentor

Batteries are rated in Amp*hours, or in this case in kW*h (not kW/h). Power multiplied by time is energy, and the battery initially holds some amount of energy.

So to work problems like this, you are given the average power supplied to the load, and asked how long the battery will last....

3. Aug 30, 2011

### Pete789

Thanks Berkeman, However, the only other information my teacher gave me was
The average power required to maintain the battery = M*V^2*fr = W
M= 176kg
V=13.8ms
Fr=energy usage fraction= 12%

Any ideas?
P789

4. Aug 30, 2011