This circuit provides up to 20V output from a regular 12-V automotive battery, to enable constant current charging of nicad battery packs up to 15 cells @ 1.2V (18V total). IC1 is a Linear, Audio Power Amp (10W) and was orginally designed for car radios. Several types can be used like the ECG1288, NTE1288, the TDA2003, and other substitutes will work fine. In this circuit, with S1 set in the 'boost' position, IC1 is used to form a squarewave oscillator, and by coupling this square wave to the 12-V battery supply via D1 and D2 to obtain over 20 vdc. If this is not needed, S1 is left open (normal). T1 acts as a current regulator to determine the charge rate of the battery pack. R4 is selected from the table below, or it can be adapted and switched with a rotary selector switch. R4 can easily be adjusted for any value to suit needs other than listed here.
fred1144