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Old Apr 23, 2006, 12:09 PM   #4
Junior Member
Join Date: Apr 2006
Posts: 2

You can dicharge a single NiMH cell down to about 0.5 or 0.6 V by connecting a silicon rectifier diode in series with your load resistor. A 1N4001 diode is suitable, but dont push the maximum current much above 0.25A, otherwise self heating will lower the forward voltage drop a little. Discharging to less than the usual 1.0V will not harm the cell according to manufacturers, but there is very little point in trying to use the very small amount of energy available in the 1V to 0.5V region.

A load resistor of about 3.9 or 4.7 ohms will do the job if you have one, otherwise use a small 1.5 or 3V flashlight bulb, but check the current when everything is connected if you can (remember that your ammeter will likely have a significant effect on the measured current so use a resistor if you can).

Dont be tempted to connect cells in series when discharging, as one will always discharge first, and the other will attempt to reverse charge it - THIS IS BAD NEWS, AN WILL IRREPERABLY DAMAGE THE CELL!!.

It seems that NiMH cells DO suffer from memory effects after all, but not nearly as badly as NiCd cells. Apparently a complete discharge every 30 or so charge cycles will restore them to original condition.

I use a Vanson BC1HU charger which has a seperate control channel for each cell, and also has the means of discharging cells should you so desire. 7dayshop in Guernsey sell these for around £14. These chargers will run from a 12V car battery as well as from mains, so are useful if a souce of AC mains is not available.
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