Hi,

I bought 4x 1.2v rechargable batteries (2500mAh) connected in series to power my circuit. The voltage to circuit must be greater than 4.4v for it to work properly. The circuit drains about 20mA.
To calculate how long the battery will last I did the following:
tolerance = max volt - min volt = 4.8 - 4.4 = 0.4v

time needed to drain the entire battery (from 4.8V to 0v) = 2500mAh/20mA = 125 hours

therefore, time needed to drain 0.4v from the battery = (125hours/4.8v)*0.4v = 10.4 hours

The thing is that the batteries I bought give 1.35v when they are fully charges , even though 1.2v is written on them.
So, how do I do the calculate the consumption time now?
does the time needed to drain the entire battery changes ?

 

Unfortunately with battery life, a lot of it is guesswork. Batteries discharge in rather odd wats, although they are understood, it involves a lot of math...

The easiest way is to set up your circuit and measure the voltage under load until it drops to 4.4V. However, as the current drain is low, it is probably not feasible to measure it yourself. Instead, it might be a task for a data logger, if you can get access to one.

 

The voltage / amp-hour discharge curve of a battery is not linear, so you cannot really make the simple calculation which you have attempted. The nominal battery voltage of 1.2V is an approximation, and will only apply roughly around the middle of the discharge, assuming a moderate current drain relative to the battery capacity. See this link:
http://www.dcordes.freeuk.com/cells.htm

Depending on the type of battery technology you are using (NiMh?), you might get quite a good fraction of the rated capacity if you are discharging a 2500mAH battery down to 1.1V per cell at only 20mA. It is likely that the 2500mAH value would be calculated for a considerably higher discharge rate, although admittedly down to a lower voltage per cell. If you can get hold of discharge curves for the cells you are using, you might get a better estimate, but my guess is that the battery will last far longer than you have assumed.

 

Actually, small cells usually rated at a 20-hour discharge; so for 2500mAh, the constant current drain would be 125mA until it fell below the threshold. For an AA rechargeable battery, that would be roughly 1.1v.

At a constant 20mA rate of discharge, you should get more than 125 hours of operation.

However, battery manufacturers aren't always honest in their mAh ratings on the packaging; a number of them are significantly overrated. Also, as rechargeable batteries age, their mAh capacity decreases.

 

thanks a lot

so if the ratings on the packages are not accurate, any of you guys know which is better, sony or energizer ?

 

Have a look here:
http://www.rechargeable-battery-rev...attery-tests/aa-nimh-battery-performance.html
I don't know how exhaustive their testing was (you have to create a loginID to see their test methodology), but the Energizer 2500mAh NiMH AA battery/batteries they tested measured 100.65% of their rating (2516.30), where the Sony 2300mAh NiMH AA battery scored 95.84% at 2204.4mAh.

How would you feel about paying for a 2300mAh battery, but getting one that only performed as a 2200mAh battery?