(Earlier version of) Backup onboard charger not correct

[Al Skierkiewicz]

Ralph,
That is the schematic from last year and as I have said before, I don't like the design of this circuit. In analyzing what takes place, the 317 puts out 8.26 volts. V out= 1.25(1+R3/R2) With this in mind, the drop across R4 is about 1 volt with no load on the battery so R4 limits current to under 100ma which is a good charging current for a 700maHr battery pack. However, the 43 ohm resistor R1 will provide some current as well, perhaps more than the battery should be charged at. The zener diode is obviously there for protection should the backup battery be removed and will clamp at 8.7 volts.
If the circuit is intended to only charge the battery, there should be no path for current from the main battery except through the charger. If the intent is to operate the 7.2 volt input from the main battery, then I would use either the zener changed to a 7.2 volt device (and recalculate the value of R1) or set the regulator to 7.2 volts and eliminate the backup battery. If the intent is to perform both tasks then a more complex circuit is required.
The only limit on the backup supplying power to the RC is where this circuit is wired and R4. Since the RC goes to sleep at under 8 volts, there is likely very little current being drawn except by the fans.