However I am using schottkys specifically for the lowest possible forward voltage. unless i was using very low-value resistors (even an ohm would be doubling my voltage drop at a half an amp per diode) it wouldn't work. The idea is that when the car is on, the voltage will be high enough to charge the secondary battery even with the diode drop. with the car off (but computer left on for a little while) the secondary battery will slowly drop until it is one diode drop below the main battery voltage. (then the diodes would switch on and allow current to come from the main battery, powering the computer) if the diode drop is large enough, the computer will start to screw up and/or reset before that point, and thus the system won't be much good.
Also, when the car and computer are off, there is still a small current drawn by the computer, so the secondary battery would, as before, drop to a diode drop below the main battery voltage. If that voltage is 0.7v or even 1v, then the battery will be forced to recharge that 1v or so when the car is turned on, which seems like extra wear and tear on the battery, and also would make it so i had to have the car running for a certain amount of time before the backup battery was charged enough to run the computer when the car was turned off.
What I was thinking was that I could put a power resistor in parallel with the diode(s)... say, 50 or 100 ohms. That wouldn't make much difference when the computer was actually running, but in the case where the car and computer were both off, where current draw is only a couple dozen milliamps, it would keep the secondary battery charged so at least help with one of the problems. And even when the starter motor was cranking, and main battery voltage dipped down to 6 or 7 volts (the entire reason for using this backup system) it shouldn't allow significant enough current to flow back and reduce the secondary battery voltage...
My other option is to drop $85 on a commercial DC-DC converter specifically intended for this purpose, that can handle engine cranking without losing output... but being a poor college student makes that a little unreasonable until a few months from now.
Any thoughts?