Last weekend, on a three hour flight, my Garmin Aera 796 discharged its battery completely even though plugged in to the 12 Volt system using the adapter that is supplied with the Garmin unit. I'm going to try an inverter and use the AC docking unit that comes with the 796. As suggested in one of the above posts, I tried in in a car, and it does work.
The inverter I am using also has a 5 VDC USB charging outlet. I plan on using that for my iPad and ForeFlight as I have experienced the same charging issues with the iPad.
Has anyone else noticed a short battery life when using the 796?
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Edit:
After posting this, I found another thread discussing voltage issues where it was suggested that a low voltage on the buss (as low as 13.6) could cause the unit not to charge. I'm thinking that an adapter reducing 24 volts to 12 might actually be too low a voltage to charge the iPad (original post) or a Garmin 796. In my case, the adjustment of the voltage regulator would be a much more elegant fix than the suggested inverter - and perhaps cheaper. For the 24 volt system of the OP, the adapter might still be in question.