[dcbrown]

Updated circuit as follows:

. selected 0.1% resistor values available from mouser.com that yeilds 8.5v
. use germanium diode (or small power transitor w/Vce of 0.3v) to block reverse current and drop voltage to 8.2v through charging resistor which has a C/16 trickle charge current target (45ma).
. use 1N4xxx diodes to drop voltage to 7.1v (or less, say down to 6.8v min), load current through R1 provides up to ~100ma of helper current if battery backup voltage drops due to extra load or loss of backup battery capacity.
. dropped steering diode between battery and R1 - shouldn't be needed on normal battery operation at 7.2v, but this new circuit will add charging current via R1 as battery voltage drops below 7.1v
. changed .1uf cap to 100uf on input. this is quite large for most regulators, shouldn't have to go bigger -- would need to see the charger circuit input at regulator in use with oscilliscope to determine if there were additional problems that would be addressed by larger capacitor.

Still not happy with diode selection, I'd almost rather swap out the germanium diode with a small power switching transistor that had a typical Vce of 0.3v - it should do the same thing as the diode, namely block discharge current when 12v battery not present.

User could add slow blow fuse into circuit as optional component in front of regulator. Say 250ma fuse. Another optional component would be a switch to disable the whole thing. Another optional component set would be a LED and series resistor at point in front of D1 to ground to show when circuit was active.

Looking for help on D1/D2 selection to achieve design voltages.... or someone to redesign the whole thing. Its easy to get carried away, for example I thought of using a 2nd LM317 in load support branch. It would be in constant current mode at 7.1v. This would current limit that branch more effectively but changes all the resistors and output voltage target of the first LM317. This would replace the D1/D1/R1 components. But goal of the design is to just be a basic/simple on-board charging circuit to prevent loss of voltage from the backup battery due to camera loading. The backup batteries main purpose is simply to prevent a brown out interrupt on the PICs which would result in processor resets.

1N4xxx diodes list Vf as 1.0v, but this is at maximum load. 1N58xx schottky diodes list Vf as .55-.6v range. Just need to get to a 1.4-1.5v drop across a couple diodes or transistors but don't know which would be best choice.

I'm looking to order some components with another order something in next day or two so I can breadboard and try this circuit out/take some measurements.