After measuring the AA NiMHcells' internal resistance, and selecting those with low values, I decided to measure their capacity. I had never been sure whether the camera said the battery was low because of internal resistance or genuine low capacity.

This was a simple exercise. Using the same crocodile-clip leads to connect a cell to a resistor and to the USB25IO, I just measured the voltage over several hours while the cell discharged. I used a 1300 mAh cell that had been freshly charged in my new 'smart charger'. The only problem was deciding the resistor value. Conventionally, cell capacity is quoted at a discharge rate of "C/10", which is a bad way of saying that the current is chosen to discharge the cell in 10 hours. I did not want to wait 10 hours, so I used a higher current.

I started with a 4 ohm resistor, and as the cell voltage is about 1.2 volts (whilst discharging), that means 333 millamps, so for a capacity of 1300 mAh the discharge should take 4 hours. Here is the result:


4 ohm discharge curve

The x-scale is in mAh, obtained by calculating the current at every measurement point and integrating them over time. Without being too fussy, the capacity is about 900 mAh, and the discharge took just over three hours. Here's an even more aggressive discharge, using a 2 ohm resistor:

2 ohm discharge curve

(I must combine them on the same graph sometime). Now the capacity is less than 700 mAh, and the discharge took just over two hours. This clearly shows that the 'capacity' of a cell is not a fixed quantity but depends upon discharge rate. The rate of C/10 is fine for low-current applications but is not relevant when using a high-current device like a digital camera.



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