I know little about electronics and...

I forgot to mention: The circuit is being used to revive a NiCad battery (I DO NOT WANT TO DEBATE NICAD BATTERY THEORIES!). I am reviving a 9.6v battery. I am using about 10.2v from a variable bench power supply (because I have a diode between the power supply and the battery to prevent damage to the power supply should the electricity cut off). The power supply can only limit current down to 10 milliamps. I need an output of 1 milliamp. I was hoping to just add a resistor between the power supply and the battery to further limit the current.

john_force snipped-for-privacy@yahoo.com wrote:

"I know little about electronics and..."

You are right. The usual way to fix these if they can be fixed is to hammer a pulse through them to burn out any short.

However if you want to do this illogical thing, a FET circuit is the way to go.

You can use a 10K resistor to limit current to 1 mA for a totally dead cell, but it will drop to almost nothing when (and if) the battery ever recharges. You can make a 1 mA constant current supply by using a PNP transistor, a 600 ohm sampling resistor from the emitter to your 10 VDC supply, two diodes from there to the base, and maybe a 10k from base to (-).

I had some old (maybe 1972 vintage) NiCad D-cells that I rejuvenated by charging a 20,000 uF capacitor to about 10 VDC and zapping them with a really good wallop of current (sparks are good!). Some of them recharged OK, others never did, and I think all of them had some problems, but I got a little more use out of them.

A very light float charge is probably good to keep a good battery at full charge by compensating for internal discharge and resultant shorting whiskers.

Paul

If you are careful, you could put a resistor in parallel with the battery that draws 9 ma. 10.2/.009=1133 Ohms. P= 10.2**2/1133=0.1 W.

Tam