Carl,
A friend of mine has an RV-10 with a dual battery system--one is labeled "auxiliary" and can be held in reserve or tied to the main bus simultaneously with the main battery. I believe it is tied to the charging circuit through a diode so it can be kept charged, but can only feed the main bus when it's tied in manually. Is this the kind of thing you're talking about?
Without details I suspect holding a battery in reserve is the same as a backup battery.
For an IFR install consider:
- Two identical batteries (I use PC-625 as they are a little lighter and have more AH and CCA than PC-680). I replace one every three years so that neither is more than six years old. This provides some confidence on maintaining my reserve capacity design objectives.
- For IFR most people have two EFIS, two Nav GPS systems and two Comms. Divide these into Left and Right avionics. For single loads like XPDR, AutoPilot servos and such, pick a side.
- Power the left from the left battery, the right from the right battery. Battery connections is between the battery and that batteries master solenoid. I use 30amp 12vdc relays for this (they draw 100ma when on),
- Now add two more relays so that the left avionics and be powered from th right battery as a backup mode - the same for the other side.
- For normal operations, both master solenoids are shut connecting the batteries (in parallel) to the alternator and for engine start. On this same buss are the non-IFR loads (lights, pitot heat, etc.). For any electrical fault both masters go open as the immediate action (mitigates a fat wire fault). This leaves the panel in the most reliable state.
This provides over an hour (one battery on line) and over two hours (two batteries on line) of continued IFR flight. For the very simplistic loss of alternator risk, adding the nice B&C 20 amp standby alternator provides for the “get gas and continue home” option. I caution that while the loss of an alternator is the most probable electrical risk we have, it has the least impact on safety of flight (assumes the rest of the electrical system has at lease some reserve capacity).
For those with electrical power dependent engines, adding one of those tiny backup batteries to keep the engine going would not be my choice. Here there is option to run half of the engine in each battery (e.g. Left Ignition on the left battery, etc.). These two engine busses would be connected directly to the associated battery, breaker then on/off switch. Not on the output side of any solenoid.
Note - incorporating a VPX or such would not be practical.
This design is on five RVs, soon to be six. The first flying for 20+ years.
Carl
