What is the estimated battery life?

[Pete789]

Homework Statement

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

Homework Equations

Motor: W=I*V > I = 364.58Amps

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

 

[berkeman]

Homework Statement

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

Homework Equations

Motor: W=I*V > I = 364.58Amps

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

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...

 

[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

 

[Pete789]

Ok this is my answer.
Aver E = 170kg*8.3^2*0.12=1405.35w or 1.405kw
The Battery has 7.5kwh so 1.405/7.5= 5.3 hrs of use

Thanks Peter