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Electronics => Beginners => Topic started by: ThisDoesNotCompute on September 13, 2014, 09:32:48 pm

Title: Calculating life of alkaline batteries in a circuit
Post by: ThisDoesNotCompute on September 13, 2014, 09:32:48 pm

Hello all,

Sorry if this is covered elsewhere -- I searched here and also elsewhere on the Web and never found a clear answer.

Long story short, I modified a Game Boy Advance and want to figure out (and shoot a YouTube video on) how long the batteries will last, compared to a stock unit. I found the following calculator:

http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life (http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life)

...which is, basically, the formula

Code: [Select]

[battery capacity in mAh] / [current draw in mA] x .7 = battery life

The GBA uses a pair of AA batteries in series to supply 3V. The normal Duracells I'm using are rated for 2200mAh, and I measured 100mA of current while the GBA was on and playing a game. Thus, the calculator offers a value of 15.4 hours as the battery life, which jives with what Nintendo itself says it should be ("about 15 hours").

Here's what I can't find an explanation for: what is that multiply by .7 all about? That Digi-Key calculator is vague, and I'm just a hobbyist. I'd appreciate any advice -- thanks!

Title: Re: Calculating life of alkaline batteries in a circuit
Post by: IanB on September 13, 2014, 09:55:50 pm

I think that formula with the 0.7 factor is only correct occasionally, "by accident". The real picture is much more complicated than that.

Firstly, alkaline batteries do not have a single mAh capacity that you can use for all loads. The available capacity of an AA cell varies dramatically with the load, from a large value like 3000 mAh for tiny loads to a small value like 1000 mAh or less for heavy loads. The battery will also last a bit longer if you load it intermittently and give it some recovery time between uses.

Secondly, the voltage profile of an alkaline cell is not very flat. So it starts out at about 1.6 V when fresh and drops down to about 0.9 V when fully discharged. If you have a resistive load like a flashlight bulb the current draw will therefore drop as the bulb grows dimmer. But if you have an electronic load that aims for constant power the current draw will go up as the voltage declines, hastening the end of life and reducing the available capacity.

The only reliable way to estimate battery life is to look at the manufacturer's data sheet where there are discharge graphs and find the discharge curve that most closely corresponds to your usage.

Title: Re: Calculating life of alkaline batteries in a circuit
Post by: Flenser on September 14, 2014, 06:10:55 am

As IanB says, start with the datasheet:
http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1500_US_CT.pdf (http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1500_US_CT.pdf)

The attached chart is the discharge curve for AA Duracell batteries from this datasheet indicating that provided you can operate down to 0.8V you can expect about 12 hours @ 200mA.

Title: Re: Calculating life of alkaline batteries in a circuit
Post by: Dave on September 14, 2014, 07:37:47 am

You also need to take into consideration the circuit you are powering. Can its power supply go down to 0.8V per cell? For example, if it can't run on less than 1.2V per cell, your usable charge becomes significantly smaller.

You might want to try powering your GBA from a lab power supply and slowly reduce the voltage to see where it cuts off.