I had posted a video on youtube.com a few weeks ago showing my first very crude and simple battery charger/desulfator. Someone commented on how well it worked for repairing nicads. I'd like to explain that a bit. The nicads don't get sulfated like lead acid, but they do degrade. Little microscopic filaments that look like hair build up and eventually short out the plates. When you hook up a regular charger and it is trying to charge at half an amp, it does that by putting out a fixed voltage. The battery plates are so "fuzzy" that it never takes a charge.
But the desulfator / charger is a constant current device. If you set it for half an amp by using a 12 microfarad "Run" capacitor, then it will push half an amp at whatever voltage it needs. It can go as high as 170 volts DC. And then it pulses it 120 times a second. Believe me, that fries those little hairs pretty quickly. Then the voltage drops down to normal and you can charge for a few hours after that. I normally put a timer on it so it doesn't overcharge.
If you have a nicad pack that is charged at half an amp, don't set up the charger to push 3 amps into it. (or if you do, don't do it for too long) That is bad for the battery and could be dangerous. You change the amps by putting in a different capacitor. A 24 MFD capacitor will charge at 1 amp. A 50 MFD would be just over 2 amps.
You can see the post here that talks about making one. http://poormanguides.blogspot.com/2009/05/updated-chargerdesulfator.html
There are so many old nicad battery packs out there just waiting to be fixed.
Richard
