In reply to:
I have a 24 vojlt Aero-tow (tug for moving my 22 in and out of the hangar), and keep the batteries charged with a solar panel and controller since the hangar does not have power. This works fine. Following the advice of this forum, I have connected the 24 volt battery to a plug that fits the 22’s external power socket. When I connect this external power source and turn bat 1 and avionics on, the Garmins, Sandel etc turn on as expected. I’d like to be able to power the instruments on the ground using only the external power source. Page 7-41 of the POH shows the external power source and bat 1 in parallel. I don’t fully understand what happens in this situation but I assume that if both batteries have the same voltage they both contribute to the load. Hence, bat 1 is discharged at half the rate if external power is connected - not what I had in mind. Raising the voltage of the external power source by a couple of volts would probably spare bat 1 completely but I have no practical way of doing this with the Aero-Tow battery. Any suggestions?
You’re quite correct - doing it the way you describe, you would reduce the contribution of Bat 1. The reduction isn’t necessarily 50% - but that’s a whole other can of worms.
There is a relatively straightforward way for you to raise the voltage; it isn’t pretty, but it will work. The PRETTY way would be to find yourself a DC-DC converter that will take say 18-30 volts in, and provide you with say 27 volts (a good number for this) at the output. They do exist, but for a couple of reasons, I suggest you go with this ugly method:
Get a power inverter – the kind that will give you 110 VAC from your DC source. [They are a dime-a-dozen for 12V input, so hopefully you can find a 12V point to “tap” from on your Aero-tow; otherwise you’ll have to find one that takes 24V]. From that point on, you can run an AC powered 24V charger into your airplane. Of course, you’ll need to be sure that your inverter can handle the charger you’re using; but unless you go for a monster charger, that will not be difficult.
Even though Rube Goldberg might want a personal visit to see the creation, it will have the distinct advantage of giving you 110 VAC in your hangar for those times you might want it for other purposes.