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Old Apr 19, 2006, 3:15 PM   #1
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Can anyone tell me how to completely discharge my AA rechargable batteries before charging them again. My powershot A510 shuts down itself when there is low battery. But the battery has still some life in it. How can I take the remaining life out of the batteries.
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Old Apr 19, 2006, 3:30 PM   #2
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Unless you have charger which can discharge them it's very hard...
Because if you use resistor, or for example small light bulp to do it you can kill battery if load drains it completely empty.

NiMH's don't have much memory effect so unless you have charger for that don't try to do it yourself.
(1.2V idle voltage measured by digital multimeter means practically empty battery)
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Old Apr 19, 2006, 3:52 PM   #3
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thanks E.T.

I was going to discharge my battery using a small bulb, but as you said it would kill the battery I won't do it.

thanks again.
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Old Apr 23, 2006, 12:09 PM   #4
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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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