I am trying to modify some Black and Decker chargers originally made for an 18v nicad pack. One of the supplies is transformer based and the others are switching supplies. Now I know this is B&D we are talking about, but these supplies seem to have no regulation at all.
The no load voltage on the transformer based supply measures 21.7 using a cheap Harbor Freight multi-meter, but jumps to 33.8 volts if I put a 470uF cap of the output to smooth the ripple. A Tenma 72-7720 meter says 22.7 no load and 34.3 with the same cap.
Oddly enough the no load and capacitor smoothed voltages on the switching supplies is the same for both meters and with and without the capacitor, however the rated output for three is 24v, where the measured is 28.0, 28.8 and 29.6. The 4th switching supply is rated 26v but is measured at 24.7. Stranger still is that if I plug those chargers into a lithium pack they will severely overcharge the packs to about 2.8 volts above the respective measured outputs and do it quite quickly..
I want to charge a 6-cell 18650 3.7v each lithium pack, so the "real" voltage matters. It took 4 diodes facing forward to cut the voltage on one of the switching supplies down to a level that was safe for the lithium pack. I left it plugged in to 2 lithium packs for 8 hours and neither went overvoltage on any cell.
It shouldn't take an oscilloscope to figure the voltage a supply is putting out. As it is, I have to improvise with diodes to drop the finish voltage to something safe for my packs. Nicads won't explode and burn like lithium, but the still burn up and out prematurely.
Any suggestions?