# Computing the Life of Batteries

Discussion in 'Homework Help' started by Lebe, Dec 20, 2008.

1. ### Lebe Thread Starter New Member

Sep 16, 2008
4
0
Hello guys,

I am trying to figure out how long my batteries would last for a certain project of mine. I am using 4 X 1.5Volts AAA energizer e2 Lithium Batteries.

I found these equations in your online books, but was just a little confused on the calculations.

Would the second equation "Charge/Discharge time(hours)" be what I am looking for (battery life)?

And also, the Amp-hour rating, is this the total current*time used by all of my components combined (my chips, LEDs, and speaker)?

Thanks

Last edited: Dec 20, 2008
2. ### Audioguru New Member

Dec 20, 2007
9,411
896
The datasheet for Energizer's lithium AAA cell is on its website. It shows a capacity of 1200mAh when the current is from 25mA to 400mA and the voltage is allowed to drop to 0.9V per cell.

The battery has a mAh rating. The battery can supply 25mA for (1200/25=) 48 hours or can supply 400mA for (1200/400=) 3 hours.

The circuit has a current.

You need to know:
1) What is the average current of your circuit?
2) Will the circuit still work when the battery has dropped to 4 x 0.9V= 3.6V?

Then use simple arithmatic to calculate how long the battery will last.

3. ### leftyretro Active Member

Nov 25, 2008
394
2
The bottom line is that it is not easy to calculate remaining capacity in time without knowing what the current draw will be in the future.

Batteries don't have a fixed linear amp/hour ratings that apply over a wide range of current draws. That is why you see charts with graphs showing the discharge time Vs different current draws.

Laptop computers that have built in remaining battery life indicators are notorious for being inaccurate and requiring frequent re-calibration.

Lefty

4. ### Audioguru New Member

Dec 20, 2007
9,411
896
No.
Energizer's new lithium battery cells do have a fixed amp/hour rating from 25mA to 400mA. The internal resistance is extremely low. The voltage stays close to 1.5V per cell for most of its life.

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