Confirming ground avionics power for 1977 182Q

P

PBinPA

Filing Flight Plan
Joined
Jan 22, 2025
Messages
2
Display Name
Display name:
PBinPA
Hello,

I just bought a 1977 182Q with a full glass garmin panel. I've read posts on using ground power to be able to play with the avionics without running the engine. My read of the POH is that when you use the ground power receptacle, a relay cuts off all power to the avionics bus. Is that accurate?

I saw another post (can't post links yet) that talked about using a 12V supply with a cessna ground power plug to power avionics - and all the reviews of that unit (on Aircraft Spruce, search for part 11-05084) talk about folks using it to power avionics.

Am I missing something? Appreciate any clarificaition!
 
Its possible that it works the way as described in the POH, and its possible that relay doesn't even exist anymore after several radio mods since it was built.
 
PBinPA said:
My read of the POH is that when you use the ground power receptacle, a relay cuts off all power to the avionics bus. Is that accurate?
Click to expand...
Yes. Some Cessna's use a split main buss bar with one half powering the electrical equipment and the other half powering the avionics equipment. There is a relay connecting both sides that allows battery voltage to power all equipment. However, when external power in plugged in or the starter is activated, the buss relay will disconnect the avionics portion to protect the equipment during start loads. In these aircraft models the ext power receptacle is there for start assist only. You have several options for ground or aux power depending on how your mechanic looks at things.

One option is to add a separate switch to the buss relay which will allow you to power the avionics side with ext power as a minor alteration. Another option is to remove the buss relay but that is usually considered a major alteration. A 3rd option as mentioned above is to connect a properly sized battery charger to the main battery and use the existing system to power your avionics. However, I would ensure the charger is the correct one as recommended by the battery OEM.
 
As @bnt83 mentions, if may or may not work like that anymore. _If_ it is still wired per the factory, your avionics bus is connected to the main bus through a normally closed relay that is opened by ground power being connected, or when the starter is engaged. With a full Garmin glass panel, I'd be willing to bet it is _not_ wired like that anymore. Even prior to glass panel upgrades a lot of those were re-wired to add an avionics master switch which eliminated the interconnection with the GPU plug and starter circuit.
 
Thanks! This makes sense. I hadn't considered that during the original panel upgrade, the relay might have been removed in favor of an avionics switch.

Regardless, I have a concord battery, and purchased an 8A concord-flavored battery minder, which I'm going to get wired in. I know 8A isn't enough to charge the battery and run the avionics, but even if I can get an hour or two of avionics runtime in the hangar, that's more than enough for database updates, playing around, etc.
 
You may be surprised at how little a modern panel draws, as long as you do not transmit.

Also, if you need 9 amps and the Battery Minder gives you 8, you have some 20 hours to play with. :D
 
Can you run the battery minder and the avionics at the same time?
 
mandm said:
Can you run the battery minder and the avionics at the same time?
Click to expand...
I find that my minder wire gets warm quickly. I disconnect it when I’m futzing with avionics, then reconnect it before I leave the hangar.
 
PBinPA said:
Thanks! This makes sense. I hadn't considered that during the original panel upgrade, the relay might have been removed in favor of an avionics switch.

Regardless, I have a concord battery, and purchased an 8A concord-flavored battery minder, which I'm going to get wired in. I know 8A isn't enough to charge the battery and run the avionics, but even if I can get an hour or two of avionics runtime in the hangar, that's more than enough for database updates, playing around, etc.
Click to expand...

I doubt the relay was removed even if you have an avionics switch. Relay will be located on the engine side firewall, pilot side down low. You may be able to see it from either the dipstick door or underneath from the cowl flap. If you install ground power at the receptacle you will hear a click if it’s working.
 
Yours is a 28V model, right?
Get a 24V 15-20A power supply (MeanWell is a decent brand for $40 or so) and connect it to the battery. There is usually a trimpot on those power supplies to adjust the voltage, I'd set it to about 26-27V so the battery is kept in the sweet spot.
If you have a cigarette lighter socket, you could make an adapter to feed power in that way, bypassing the master switch.
 
If it is a 28v system, then he would have a factory avionics master switch installed as the split buss went away with 28v systems. But he mentions 12v in the OP.
 
I think OP mentions someone else talking in a post about using 12V. But I recall reading somewhere that the 182Q was the first one to have a 28V system.

Either way, if it's a 12V system then he should just get a 12V 30A power supply.
 
Ground power supply was the first thing I purchased to support my plane. 2018, now the prices have went up a lot. Stinks. You can built one cheaper I am told.
I used one 3-4 times at the flight school to learn the different panels that I trained in. Best part it was free! I could sit in the plane and fiddle with the panel on my own with no pressure. Helped me to learn a new to me panel in each trainer they had.
And they always emphasized to me not to run the battery during down pre flight, etc.
So I wanted ground power, it has been so worth it for my own plane.


5e6cae8a-618e-46c5-8d23-08fced7765f6.jpg

a9659ee6-3c38-48fc-a40b-1e2d9efa281e.jpg

Crazy you would think that I would use it to update data bases each month but that happens during taxi through my I pad.
714e0345-7130-4888-b89a-3f898759cd90.jpg

I use it sometimes to power mood lighting in my hangar.
9a9f68e3-869d-4f61-9e56-77899408d68a.jpg
 
AV8R_87 said:
I think OP mentions someone else talking in a post about using 12V. But I recall reading somewhere that the 182Q was the first one to have a 28V system.

Either way, if it's a 12V system then he should just get a 12V 30A power supply.
Click to expand...
The 77 Q model was 12v. 78 on were 24v
 
You must log in or register to reply here.
Back
Top