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I suspected a charging system failure immediately and checked the
battery voltage in the SDS monitor. This read 9.5 volts so I knew then that
we had a charging system failure. At the time, I was not worried about
making it to an airport, assuming that the alternator had failed just in the
last few minutes and believing that I had 20-25 minutes of battery power
remaining. In fact, it is likely that the alternator failed soon after
takeoff from Springbank but I did not notice signs of the failure until the
battery was well over half dead.
At 10.5 volts, the battery is officially 100% discharged. At 9.5 volts, the battery was not "half dead" but completely dead. He was running on the battery "fumes".
The battery voltage drops very, very rapidly under 11 volts.
If the battery voltage drops much below 13 volts while in flight, an idiot light should come on, and you should head for the closest airport. You should turn off anything electrical that you don't really need to fly the airplane, communicate, and navigate. A full-throttle climb might be a good idea if you are at low altitude and far from the nearest airport.
If the voltage drops below 12 volts, you should turn off everything electrical but the systems absolutely essential to keep the engine running.
At 11 volts, you need to pick a place to land and do it.
During and very shortly after starting, it would not be unusual for the battery voltage to dip below 12 volts. However, I would not take off with the system below 13 volts. Checking the battery voltage should be on the run-up checklist. For an EFI or EI engine, the battery voltage indicator should be as prominent and visible on the instrument panel as the oil pressure indicator.
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