"If I lose my alternator while flying at night, what should I do to keep my battery running for as long as possible?" - Steve B.
“No alternator at night should be considered an emergency, and you should land as soon as practical. If you’re IMC on an IFR flight plan, it’s even more critical. Don’t hesitate to declare an emergency, if you’re talking with ATC, and get guidance to the nearest, well-lit airport.
How much battery time you have depends on the health and size of your battery as well as what you turn off. You can’t do anything about your battery in flight, so here are some suggestions for power savings.
Strobe lights draw a shocking amount of power. Turn them off and leave them off. You might consider turning off all lights, in fact. Pitot heat draws a lot of power, but don’t turn that off if you’re concerned about pitot tube icing. You have enough problems without losing your airspeed indications. You can likely turn off your transponder and one radio, or even the entire radio stack if you have other navigation, such as pilotage or an iPad. Presuming you have a flashlight you can fly by, the simplest thing may be to just turn off the master until you approach your destination. Then there is no power draw until the final 15 minutes when you can contact the Tower or CTAF, enter the pattern, and land. I would even consider turning off the master and flying by iPad in IMC to conserve the battery for all my avionics and the approach I need to land.
Instrument backlights for glass panels burn a lot of power, too, so turning them down to zero brightness can help presuming you can still fly the aircraft on backup instruments. Traffic systems, stormscopes, MFDs and anything else non-essential is a candidate for turning off or pulling the breaker.
If you are talking to ATC and plan to kill all your radios and transponder, let ATC know and tell them when and where you plan to turn them back on. They can give you the frequency to call when you come back online, and the controllers there will be expecting you. Yes, you could do this in IMC on an IFR flight plan. This is an emergency.
Remember that electric items like flaps might not work come landing time. You may need to extend the gear manually. Even electric flight instruments, such as an electric HSI, might stop working. All-electric aircraft with glass panels usually have multiple alternators and batteries, but even they can have failure modes leaving only one battery source.
Know your aircraft and do what it takes to get down on the ground while you still have some juice left.”