# mAh - battery capacity question

Discussion in 'Homework Help' started by Morpho, Dec 16, 2013.

1. ### Morpho Thread Starter New Member

Dec 13, 2013
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If I connect 2 - 1.5V AA batteries in parallel, I know the voltage is cumulative. I now have a 3V power source, but what happens to the capacity of the two batteries together? The technical sheet gives me a capacity of about 2800 mAh at a discharge of 25mA for 1 of the batteries alone. In parallel, will I still have the same capacity of 2800 mAh even though I have 2 batteries? Thanks for the help!

2. ### inwo Well-Known Member

Nov 7, 2013
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If by parallel you mean series.

3. ### Morpho Thread Starter New Member

Dec 13, 2013
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Sorry. I had the LED's on my mind. Yes the batteries are in series. The assumption of still only having a capacity of 2800 mAh with both batteries in series is correct? Can I assume the 40 mA load will exhaust my 3V power source in approximately 70 hours? Thanks again!

4. ### #12 Expert

Nov 30, 2010
18,076
9,677
You have the math right. The uncertainty is about whether real batteries always have the advertised capacity, and they don't when they get old. However, this being in the homework category, you can use the mathematical answer.

5. ### MikeML AAC Fanatic!

Oct 2, 2009
5,444
1,073
With one caveat. If the the specified discharge rate to 2800mAh is 25mA, that would be 112hrs to reach some specifed final voltage. If you discharge them at 40mA, you will reach said final voltage sooner than 70h

6. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
Correct. That's what I thought you meant.

I don't know how it is with the type of batteries that you are using, but my experience with ni cad and sla is about half the rated power until voltage drops to unusable values.

Could be your batteries will live up to full specs. I have no experience with carbon zinc or alkaline types.

7. ### Morpho Thread Starter New Member

Dec 13, 2013
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Got it. So better to stick with 1 LED at 20 mA to more closely follow the advertised discharge profile of the batteries.