Mr Happy
Flashlight Enthusiast
Some of us may want to estimate the capacity of our batteries, and either we don't own a CBA III (or I or II), or even if we happen to own an MH-C9000 we might want to test other cell chemistries than NiMH or NiCd.
Well, there is an answer. If you can obtain a digital multimeter with a computer logging interface, then with a few simple components you can get some results. The nice thing about the DMM is that even when you are not using it for logging it still has a full set of DMM functions, so the purchase price does not have to be justified as a single use tool. It can sit on your tool bench and be used for other things too.
Here is how to go about it. The theory is to set up a discharge circuit with voltage logging like this:
You discharge the cell or battery under test through a resistor of known value. The voltage across the cell terminals then tells you everything you need to know. Given the voltage and the resistance value you can calculate the current using:
This is not quite the same as constant current discharge, but for many practical purposes this does not matter. For one thing many cells have a somewhat flat discharge profile, and for another many cells have a capacity that is relatively unaffected by the discharge current.
One thing you have to do manually is watch the progress near the end and stop the discharge when the voltage reaches the appropriate threshold. You can't go to sleep and forget about it.
Here is what my test configuration looks like:
I used a couple of 1 Ω 10 W resistors from RadioShack connected in series. I measured the total circuit resistance by simultaneously measuring the voltage across the measurement points and the current through the resistor. In this case I obtained a value of 2.17 Ω.
The circuit is attached to the cell under test using two strips of polished copper foil held against the cell terminals by strong magnets.
Finally, here is a graph of a test result. I discharged an Eneloop that was previously charged on the Duracell Power Gauge charger. This is an older Eneloop of 2006 vintage that has not received very much TLC. It is feeling a bit tired nowadays with a capacity measured at about 1830 mAh.
Well, there is an answer. If you can obtain a digital multimeter with a computer logging interface, then with a few simple components you can get some results. The nice thing about the DMM is that even when you are not using it for logging it still has a full set of DMM functions, so the purchase price does not have to be justified as a single use tool. It can sit on your tool bench and be used for other things too.
Here is how to go about it. The theory is to set up a discharge circuit with voltage logging like this:
You discharge the cell or battery under test through a resistor of known value. The voltage across the cell terminals then tells you everything you need to know. Given the voltage and the resistance value you can calculate the current using:
I = V/R
When you know the current you can calculate the capacity. For instance if a current of 0.500 amps flows for 1 minute, the accumulated capacity is:
∆Q = 0.500 x 1/60 x 1000 = 8.33 mAh
Using the recorded table of voltage measurements and time intervals, you can therefore use a spreadsheet to sum up the total capacity during the discharge.
This is not quite the same as constant current discharge, but for many practical purposes this does not matter. For one thing many cells have a somewhat flat discharge profile, and for another many cells have a capacity that is relatively unaffected by the discharge current.
One thing you have to do manually is watch the progress near the end and stop the discharge when the voltage reaches the appropriate threshold. You can't go to sleep and forget about it.
Here is what my test configuration looks like:
I used a couple of 1 Ω 10 W resistors from RadioShack connected in series. I measured the total circuit resistance by simultaneously measuring the voltage across the measurement points and the current through the resistor. In this case I obtained a value of 2.17 Ω.
The circuit is attached to the cell under test using two strips of polished copper foil held against the cell terminals by strong magnets.
Finally, here is a graph of a test result. I discharged an Eneloop that was previously charged on the Duracell Power Gauge charger. This is an older Eneloop of 2006 vintage that has not received very much TLC. It is feeling a bit tired nowadays with a capacity measured at about 1830 mAh.
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That's what I like about CPF. If someone forgets something as simple as this, someone else points it out.
