Testing battery capacity

[Twinkle-Plank]

I have this science assignment and need to test the capacity of different batteries e.g alkaline,lithium,Ni-hm Carbon-Zinc. So far I've come up with connecting a clock in parallel to a light but the clock keep going after the light goes off so the results are way off. Does any one know how i could make the clock stop at the same time as the light goes off.

 

[HKJ]

I have this science assignment and need to test the capacity of different batteries e.g alkaline,lithium,Ni-hm Carbon-Zinc. So far I've come up with connecting a clock in parallel to a light but the clock keep going after the light goes off so the results are way off. Does any one know how i could make the clock stop at the same time as the light goes off.

Some other ideas:
A computer connected voltmeter, where you log the voltage each second.
Use a camera that can be set to take a photo each minute (Remember to lock exposure).
Use a webcam and record a video on the computer (Place a lamp in the picture to keep fairly constant exposure).

The light will not draw the same power for different battery chemisties, i.e. when testing with a light, you are not testing the capacity of the batteries, but the runtime. There is a significant difference.

 

[Battery Guy]

Some other ideas:
The light will not draw the same power for different battery chemisties, i.e. when testing with a light, you are not testing the capacity of the batteries, but the runtime. There is a significant difference.

Agreed. A lightbulb is a constant resistance load. Therefore, the amount of current the battery will provide depends on the voltage of the battery. For example, if you compare a 1.2V 2500 mAh NiMH cell to a 1.5V 3000 mAh Li-FeS2 cell (e.g. Energizer e2 lithium), the lithium cell will push 25% more current through the bulb. The lower capacity NiMH cell will likely last longer than the higher capacity lithium cell in this test, although the bulb will be brighter with the lithium battery.

I have helped a lot of people develop tests for comparing batteries, and most are generally surprised when they learn that results are usually dependent upon the test. In other words, you can take almost any battery and design a test that will make it look great (even carbon-zinc if you take cost into consideration). I think that a clever science experiment would be to take three different AA battery chemistries (alkaline, lithium and NiMH would be good choices) and design three tests that "prove" each is the best.

Cheers,
Battery Guy

 

[HKJ]

A lightbulb is a constant resistance load. Therefore, the amount of current the battery will provide depends on the voltage of the battery.

Sorry, but no. The resistance of a (old fashioned) light bulb will change with voltage.
A led with driver is another story and much more complex.

 

[Battery Guy]

Sorry, but no. The resistance of a (old fashioned) light bulb will change with voltage.
A led with driver is another story and much more complex.

That's why they call me "Battery Guy", not "Bulb Guy".

Ok, the resistance of the filament does indeed change with voltage because the higher the voltage, the higher the filament temperature and the higher the resistance. So the load is not technically "constant resistance", but it is closer to a constant resistance than a constant current or constant power load. I guess that my point was to reinforce your comment that the light would draw different amounts of current for different cell chemistries.

Cheers,
Battery Guy

 

[HKJ]

That's why they call me "Battery Guy", not "Bulb Guy".

I guess that my point was to reinforce your comment that the light would draw different amounts of current for different cell chemistries.

I guessed that, but I could not let you comment pass, especially not when OP is working on a science project.

 

[Battery Guy]

Twinkle-Plank

You might consider a low cost, USB data acquisition system like this:

http://www.dataq.com/products/startkit/di148.htm?source=googlebase

I am sure that there are cheeper ones. This would give you multiple channels for monitoring cell voltage on several experiments at once. Then go to Radio Shack and get yourself a range of resistors (be sure to get ones with sufficient power ratings) and AA battery holders. You could even measure the voltage drop across the resistor to get a real-time measurement of the current. By integrating the area under the current vs time plot you can get the discharge capacity.

Have fun!

Cheers,
Battery Guy

 

[45/70]

I have helped a lot of people develop tests for comparing batteries, and most are generally surprised when they learn that results are usually dependent upon the test. In other words, you can take almost any battery and design a test that will make it look great (even carbon-zinc if you take cost into consideration). I think that a clever science experiment would be to take three different AA battery chemistries (alkaline, lithium and NiMH would be good choices) and design three tests that "prove" each is the best.

Ah, a classic example of the "university study", wherein the result is actually the goal, and is known before the test is executed. Recently I've brought this up in a couple posts. They are very useful for educational purposes concerning those involved, but often lack credibility. This is often how ten different companies each end up having the "best" product, and makes this type of test very popular with marketers.

Dave

 

[Battery Guy]

Ah, a classic example of the "university study", wherein the result is actually the goal, and is known before the test is executed. Recently I've brought this up in a couple posts. They are very useful for educational purposes concerning those involved, but often lack credibility.

Precisely! That's why it makes a good science project because it explores the nuances of battery testing and teaches why it is important to understand how the test is conducted, not just the results of the test. You learn a lot by designing performance tests for three completely different batteries that show how each is "the best". And at the same time you hopefully learn to be skeptical of marketing claims.

This is often how ten different companies each end up having the "best" product, and makes this type of test very popular with marketers.

You hit the nail right square on the head with that one!

Cheers,
Battery Guy