New Backup Battery Circuit

[Al Skierkiewicz]

Cjohnson,
I thought I had seen that circuit before. This charger is meant for a lead acid cell which requires a constant voltage but current limited source. In operation, excessive current (through the battery and R3) is prevented from flowing as the transistor reduces the value of R2 and thereby reduces the output voltage until the current is stable at the design peak.

NiCad cells require a constant current for charging. As Steve has pointed out above, R3 is shorted by the sneak around ground through the RC, the backup battery connector and the robot common wiring. With R3 shorted this circuit produces an output voltage of 7.61 volts up to the cutout current of the LM317. C, sorry I didn't catch this before. I feel I mislead you with my earlier post.