# Calculating Battery Drainage Times

Discussion in 'Power Electronics' started by Dustin Smith, Jul 10, 2012.

1. ### Dustin Smith

50
0
Jun 27, 2012
The website posted below is one that I found to try and determine how long I could run two - three double A batteries in a series to light a white LED with a resister. I've got them hooked up to a voltage source of 3.66V and they are drawing 0.09 - 0.10 amps last I checked with my fluke meter.

So..
The battery life of AA batteries (alkaline) seems to be around 1800-2600 mAh.
Using the mid range of 2200mAh and my constant amp draw of 0.10 amperes the calculator tells me I'll get about 27.8 hours before it's 80% drained. I suppose I'd have to use at least 3 AA batteries in order to get closer to full drainage because two AA at 1.5V =3V, the minimum I'd need.
I also used 1.2 for Peukert's number because that website said that is the average.

http://www.csgnetwork.com/batterylifecalc.html

Please help me check what I've done and verify if that information I got is within an acceptable range for approximation.

Thanks to anyone taking the time to read this.

2. ### CocaCola

3,635
5
Apr 7, 2012
Numbers only get you so far in this case, doing the math for this is just an estimate and in most cases a very generous one... When I attempt to figure out battery run times, I will generally take the lower side of the average mAh ratings, work the math and then skim off an additional 25% or so... If I want real numbers I do a real world test, as that is really the only reliable method to get solid numbers...

So in your case, I would expect 'real world' times based on my experience to be closer to 1800/100 to get 18 hours - 25% or so and end up with about 14-15 hours run time... Yes, my numbers are generally on the conservative side but IMO best to consider a potential real world case vs laboratory perfect case...

But, in this case you have other considerations, the LED will not pull a constant current as the battery voltage changes, and the LED will get to a point where it simply cuts off... If your LED needs 3 volts, it won't take long before the two batteries fall below that level and regardless of them still having a decent charge, it's no longer usable to you in your circuit... If you use 3 batteries and then use a resistor based on 4.5 volts to limit current you face the same type of issues as the battery voltage diminishes.... BTW a 1.5 volt battery is not 1.5 volts, that voltage will vary by type of battery and even brand... NiCads will be about 1.2 fully charged and alkalines will be about 1.6... This has to be taken into consideration as well as the voltage curve for each battery as it drains...

In the end take any mathematical estimate as just that, don't count on it being accurate unless you factor in a ton of variables based on your circuit and battery specifically... Let it guide you only...

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,397
2,777
Jan 21, 2010
You also need to remember that the LED current will fall as the battery voltage falls (actually it falls faster than the battery voltage).

The effect of this is that the batteries will last significantly longer than you predict because the load you calculate is only at 100% charge.

However, the light output will also fall as the batteries discharge. You might decide that the light is too low after a smaller (or larger) number of hours.

4. ### Dustin Smith

50
0
Jun 27, 2012
Ok, so basically, I could safely say that using 3 alkaline double A batteries (even of a good brand) would not be able to light my circuit for more than 48 hours. (I gave it a huge boost on the giving side of things because if it can't even go 48 hours I think I'll stick with making my own batteries.)

Anyways, I'll put it to the test since it probably won't take more than a day to find out. I've been running my homemade battery for over 5 days straight now and it's going strong. I'm worried though that my LED keeps getting brighter and drawing more current!

(off to find 3 new AA or AAA batteries for my new test)

5. ### CocaCola

3,635
5
Apr 7, 2012

AAA will only last about half the time of AA... If you want longevity you need to go to a bigger cell, like C or D size or whatever...