# Watt-hour to Amp-hr conversion

#### anony

Joined Apr 3, 2010
12
hello,
i have a system that consumes 95.76 Wh of energy. I want to calculate how many AA batteries will be required to run this system for 14 hrs
Thankyou

#### retched

Joined Dec 5, 2009
5,208
Do you know the voltage of the system?

#### anony

Joined Apr 3, 2010
12
not exactly as its not constant. But the current is constant and its value is 40 mA

#### Jony130

Joined Feb 17, 2009
5,241
40mA *14 = 560mAh so in real life 1200mAh will be good

#### anony

Joined Apr 3, 2010
12
okie....sorr but now i m bit confuse about it. what about the resistance ? If the current is constant the resistance value will not matter for calcuting the battery usage?
and i read that AA batteries have arround 400-900 mamp-hr capacity so for my problem two AA batteries will be enough?
Thanks

#### Jony130

Joined Feb 17, 2009
5,241
Well in real situation resistance do matter. Because when cell is discharge the voltage on the cell drop so current will me drop to.
And this is so we need more mAh cell then in theory.

#### anony

Joined Apr 3, 2010
12
the system i need to run with AA batteries is giving constant dc current of 40 mA in form of pulses ( frequency of 200 hz with high time of 120 microsec). So the On time for pulse in one hour is= .024 hr ( is it right ??)

To calculate the AA batteries that can run the system . Can i use this equation
(40 mA) * (.024 hr) = 0.96 mAmp-hr

It means that one AA battery is enough ??
and also can i calcualte the battery usuage like this?
Thankyou

#### Jony130

Joined Feb 17, 2009
5,241
For F=200Hz an Ton = 120uA the average current will be equal:
Iavr = 40mA * 0.12ms/5ms ≈ 1mA
So you don't have to be worry about battery capacity, 400-900mAh will be enough.