Experiment about Discharging Batteries

[johnsmith12345]

I am trying to find the capacity of a AAA battery by discharging it. How would I be able to discharge the battery at a 1A current so that I don't have to wait for a long period of time?

 

[Simon Bridge]

How would I be able to discharge the battery at a 1A current so that I don't have to wait for a long period of time?

... pick a low capacity battery.

 

[johnsmith12345]

I was actually inquiring more about the constant 1A current discharge bit.

 

[Simon Bridge]

Ah - not the time part.
Use Ohm's law and pick an appropriate resistor.

 

[DrZoidberg]

You realize that the discharge current affects the capacity?
If it's discharged at just a few mA, a AAA alkaline battery can deliver around 1200mAh. However at a current of 400mA the capacity is only 400mAh and at 1A it will be even much less than that. That's why alkaline batteries are really bad for digital cameras.
http://data.energizer.com/PDFs/l92.pdf

 

[johnsmith12345]

You realize that the discharge current affects the capacity?
If it's discharged at just a few mA, a AAA alkaline battery can deliver around 1200mAh. However at a current of 400mA the capacity is only 400mAh and at 1A it will be even much less than that. That's why alkaline batteries are really bad for digital cameras.
http://data.energizer.com/PDFs/l92.pdf

Well I'm actually using a NiMH battery. It happens to be one of those eneloops so looking from the data sheets on their website, the capacity doesn't decrease a lot with an increase in the discharge current.

The only problem I am facing now is that I can't measure the true emf acrosss the battery at the same time I am discharging it. Does anyone have a solution?

 

[Simon Bridge]

What do you mean by "true" emf?

 

[johnsmith12345]

What do you mean by "true" emf?

The emf of the battery without connecting it to any resistors.

 

[Simon Bridge]

Well, there is no way to measure the open circuit emf without opening the circuit.
You could just throw a switch.

What do you need to know that for though?
You may want to see how the internal resistance varies as the battery loses energy.

 

[johnsmith12345]

Well I need it so that I can get enough data about the open circuit emf to plot a discharge curve.

 

[Simon Bridge]

http://www.powerstream.com/AA-tests.htm
http://www.mpoweruk.com/performance.htm
... I don't see anyone using open-circuit voltage for the discharge curves - but you can just open the circuit periodically to measure it. If the internal resistance is fairly constant, you can calculate the open-circuit voltage from the closed circuit one. You'll want to keep the battery at a reasonably constant temperature though.

 

[NascentOxygen]

Well I need it so that I can get enough data about the open circuit emf to plot a discharge curve.

I think you'll find that the open circuit emf is generally not of much interest. (Though maybe you have a niche application where it is.) You should check that you are required to plot the no-load voltage, as that would be unusual. Most common would be a plot of load voltage vs. time while the load current is maintained at some specified constant level.

I think for common types the no-load voltage changes very little, once past the initial drop.
Immediately after the cell has its load removed the voltage takes some time to rise and reach a steady value. Do you intend allowing your cell time to recover before measuring your no-load voltage?

 

[Simon Bridge]

Continued on:
https://www.physicsforums.com/showthread.php?t=673582