Test circuit for power capacity

[Pengwuino]

Hey guys, I need to figure out how to make a simple test circuit so I can test how much energy is stored inside various power sources (probably 1-10V, ~10mA range). I figured all I need to do to get a rough estimate of how much power these sources (they're like batteries, not continuous supplies) can provide is to take down readings of the current for a while and extrapolate down to current=0 and tell excel to make a graph and best fit equation and do an integration * voltage.

Then again i was wondering if the voltage dropping over time would play a big role in this. Can anyone suggest a circuit or other information that could help?

 

[berkeman]

You're on the right track, but keep in mind that the extractable energy will in general depend on the current drain, especially for chemical power sources like batteries. Check out the main battery websites (Eveready, etc.) for tutorials on how they test and specify battery capacity.

 

[Pengwuino]

Ok neat. These aren't going to be conventional batteries per say... I am testing out this devices electrical generator that makes electricity out of electrolytes.

 

[dlgoff]

Wouldn't this be like, say a lead acid battery? The kind of test/data that you should make and record would be voltage of each cell, temperature of the cell (terminal temp), and power under load (maybe some minimum current on a short time).

 

[Ouabache]

Ok neat. These aren't going to be conventional batteries per say... I am testing out this devices electrical generator that makes electricity out of electrolytes.

It sounds like you have made some batteries, generating electricity from electrolytes.

If you want to understand batteries, I would start with http://www.batteryuniversity.com/index.htm of testing battery capacity: DC load, AC conductance and a third nondestructive method that the website's sponsor developed called EIS (multi-frequency electro-chemical impedance spectroscopy)

They go into an http://www.batteryuniversity.com/parttwo-42A.htm of the benefits and shortcomings of each method. And of course go on to point out how site sponsor's method is the best one to use. Overlooking their plug for sponsor's product, this reference lends some insight on the test you want to perform.

 

[Pengwuino]

Well basically what I have is this device that runs on water. It has 4 little pipes that you dump around 100ml of water in and that water will run the clock for 3 months or so. All I have is the 2 leads from the + and - so for me, its basically a battery where I have a way of changing the chemicals. Thanks for the links guys.

 

[Pengwuino]

Ugh, this thing doesn't help at all. I need to figure out a way to determine the total capacity of this puppy :).

 

[berkeman]

What's the issue? Just drain it at some rate and monitor the voltage and current over time. The main disadvantage is that you will be doing a shorter experiment than the actual drain time, so that's why it's good to do it several times with different durations to start to see what effects come into play so that you can try to extrapolate to the longer real-life discharge time and capacity. Battery guys have to do that all the time.