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Battery Life

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fingaz

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Hi All,

Can anyone help me out?

I have a circuit that draws approx 3.6mA @3V. If I power this from 2 x AA alkaline batteries, how long can I expect them to last? :confused:

Is there some kind of calulator on the net that would work it out for me?

Thanks for any help or suggestions
 
Look at the datasheet for the battery cells at www.energizer.com .

The battery voltage drops as it runs down.
So if the current is constant and the circuit still works when the voltage is only 2V then the battery will last for (the lower limit on the graph is 10mA) maybe 800 hours.
 
The ampere-hour rating on a battery tells how much current it can deliver for one hour. So a 2000mAH rating will mean that it will deliver 2 amps for 1 hour, 1 amp for 2 hours, 500mA for 4 hours, etc.
 
Salgat, bear in mind that at higher currents the battery will drain a lot faster so your 2000mAh rating may last only 10 or 15 minutes at 2 Amps.

For currents at less than ± 20% of the Ah rating it is pretty accurate. or go by the manufacturer specs.
 
You just need to look at the datasheet for the battery cells.
They are not huge car batteries so their capacity drops at high currents.
Their voltage also drops.
 

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Thanks for all the replies. It's helped a lot.
 
dont forget that regular cheap rechargeable batteries lost there charge within one month with no load connected. I know duracell make precharge nimh battery that dont lost there charge before one year!
 
fingaz said:
Hi All,

Can anyone help me out?

............................

Is there some kind of calculator on the net that would work it out for me?

Thanks for any help or suggestions

you have access to your PC. perhaps windows. if so see under accessories section there is a calculator that doubles as scientific calculator even.
 
The ampere-hour rating on a battery tells how much current it can deliver for one hour.
This is not usually true. The ampere-hour rating normally given by manufacturers, is the current that it can deliver for 10 hours, times 10. It may appear to be the same, but the battery knows the difference.
 
mneary said:
This is not usually true. The ampere-hour rating normally given by manufacturers, is the current that it can deliver for 10 hours, times 10. It may appear to be the same, but the battery knows the difference.

It does not even appear same. the curve is not linear. as one tries to draw more current the total ampere hour capacity falls stteply. It only defines if at 20hour rate of discharge, 2000mAH is the capacity.
 
Ni-Cad and Ni-MH cells can provide more current than alkaline cells and their mAh capacity doesn't drop much at high current.

The 1.25V Ni-Cad or Ni-MH cell's voltage begins lower but is higher than a "1.5V" alkaline cell for most of a discharge.
 

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Audioguru is exactly right about the Ni-Cad and the Ni-MH batteries. The chemistry of the battery determines the discharge. Sealed Lead-acid batteries also known as gel-cells have a 20 hour discharge rate. If you discharge them in about 1/2 hour there capacity is near half the normal.
 
mvs sarma said:
you have access to your PC. perhaps windows. if so see under accessories section there is a calculator that doubles as scientific calculator even.
The only problem is, it's crap; a normal scientific calculator is much better.
 
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