Jer Posted January 2, 2023 Report Share Posted January 2, 2023 Folks: I recently removed a good wet 24V battery from another aircraft (replacing it with a gel) and decided to use it as an APU to allow for powering the avionics without engine start to let me gain proficiency on my antique Garmin GPS unit at leisure. My POH seems to be silent on the use of the external power plug, I have a set of jumper cables with clamps on one end and the big connector plug on the other. I plugged it in and expected to be able to leave the master off (thus not connecting the on-board battery) while being able to turn the avionics on through the radio master. This was not the case. The avionics did not power up. I assume I have to plug in to the APU and turn on the Master then the radio master. I did not do this as I thought it would be prudent to understand the situation prior to doing so. I am guessing that doing it as such, the two batteries would be essentially connected in parallel, and then turning on the master and radio master would still drain the on-board battery (although likely at a slower rate than unsupported by the APU battery). This is just my supposition, but I would like to hear from someone with knowledge of the situation. Thank you all for the help. Jerry Quote Link to comment Share on other sites More sharing options...
EricJ Posted January 2, 2023 Report Share Posted January 2, 2023 Yes, you need to turn on the master, which enables power to the airplane systems. The external plug just allows the electrical power to connect to an external source rather than just the on-board battery. The third pin in the plug closes a relay on the airplane that connects the external source to the system in parallel with the on-boad battery. So if you plug in an external battery it'll help supply current but so will the on-board battery, e.g., they'll both draw down at about half the rate that one of them would by itself. Usually the external source is plugged into hangar power or a big enough source that it can maintain charging voltage to your on-board battery, typically 28V for a 24V system. An external battery by itself can't do that. 1 Quote Link to comment Share on other sites More sharing options...
Jer Posted January 2, 2023 Author Report Share Posted January 2, 2023 Eric: Thank you for your thoughtful and knowledgeable reply. I guess my suppositions were correct. The extra battery will help, but not eliminate the problem of battery drain during GPS exploration. Jer 1 Quote Link to comment Share on other sites More sharing options...
Greg Ellis Posted January 2, 2023 Report Share Posted January 2, 2023 9 hours ago, Jer said: Folks: I recently removed a good wet 24V battery from another aircraft (replacing it with a gel) and decided to use it as an APU to allow for powering the avionics without engine start to let me gain proficiency on my antique Garmin GPS unit at leisure. My POH seems to be silent on the use of the external power plug, I have a set of jumper cables with clamps on one end and the big connector plug on the other. I plugged it in and expected to be able to leave the master off (thus not connecting the on-board battery) while being able to turn the avionics on through the radio master. This was not the case. The avionics did not power up. I assume I have to plug in to the APU and turn on the Master then the radio master. I did not do this as I thought it would be prudent to understand the situation prior to doing so. I am guessing that doing it as such, the two batteries would be essentially connected in parallel, and then turning on the master and radio master would still drain the on-board battery (although likely at a slower rate than unsupported by the APU battery). This is just my supposition, but I would like to hear from someone with knowledge of the situation. Thank you all for the help. Jerry I know it may not be the same but Garmin makes online trainers and Apps for some of their GPS's. There are one's for the GTN's, the GNC 355, GNC 355A, GPS 175, GNX 375. They also have an outdated but still useable trainer for the GNS 430/530. You can learn from the comfort of your home and not have to run down your airplane battery. Quote Link to comment Share on other sites More sharing options...
A64Pilot Posted January 2, 2023 Report Share Posted January 2, 2023 10 hours ago, Jer said: I assume I have to plug in to the APU and turn on the Master then the radio master. I did not do this as I thought it would be prudent to understand the situation prior to doing so. I am guessing that doing it as such, the two batteries would be essentially connected in parallel, and then turning on the master and radio master would still drain the on-board battery (although likely at a slower rate than unsupported by the APU battery). This is just my supposition, but I would like to hear from someone with knowledge of the situation. Thank you all for the help. Jerry I believe this is exactly correct for my 14V J model anyway. Ideal use for the APU plug is to use a power supply that provides the same voltage the alternator does, that way the ships battery isn’t depleted at all. Different manufacturers wire the APU plug differently, Cessna does it like you initially supposed and bypasses the ships battery, but if you use it for starting then the supply has to be a big one to supply enough power, where the way Mooney does it you can get away with a smaller one as the ships battery is in the circuit. Or maybe turning on the Master on a Cessna connects the ship and power supply? Thrush we wired the plug directly to the battery, no solenoid. Did it that way so owners could easily plug in a float charger Quote Link to comment Share on other sites More sharing options...
Jer Posted January 4, 2023 Author Report Share Posted January 4, 2023 To answer a few folks: Mr Greg Ellis, thank you for the suggestion for training tools aside from the actual unit. When I said "antique", i meant it. It is a Garmin GPS155XL. It has a very different OS than one I am a bit more familiar with, a 530W. I had been recently flying a Bonanza with a 530W, and am now back to flying my Mooney, and I need to twist my mind back into the 155XL mindset. The Bonanza is the machine from which I removed the working 24V wet cell. I just didn't like the slimy condition inside the battery box. To answer member, 1980Mooney, I do not have a hangar. I am tied down outside, far from power. While operating the Bonanza, I had the need to start it once using an APU. The Bonanza POH specifically states to leave the "battery" switch off when starting using the APU. Note this is not noted as a "master" switch. I only did it once, and my recollection may not be correct, but I believe power was supplied to those normal things as turn coordinator, boost pump, etc. as well as the starter motor. After start, APU was removed, then battery switch turned on. This is where I got the idea that power might be available with the APU attached and the master off. I can only conclude that the Bonanza and the Mooney are wired differently and the "Master" switch is named differently than the "Battery" switch for a reason. Jerry 1 Quote Link to comment Share on other sites More sharing options...
Fly Boomer Posted January 5, 2023 Report Share Posted January 5, 2023 On 1/1/2023 at 10:34 PM, EricJ said: Yes, you need to turn on the master, which enables power to the airplane systems. The external plug just allows the electrical power to connect to an external source rather than just the on-board battery. The third pin in the plug closes a relay on the airplane that connects the external source to the system in parallel with the on-boad battery. So if you plug in an external battery it'll help supply current but so will the on-board battery, e.g., they'll both draw down at about half the rate that one of them would by itself. Usually the external source is plugged into hangar power or a big enough source that it can maintain charging voltage to your on-board battery, typically 28V for a 24V system. An external battery by itself can't do that. I assume (I know, bad idea) that there is no voltage regulation being done by the ship's electrical system, so the GPU voltage is a critical number? 1 Quote Link to comment Share on other sites More sharing options...
EricJ Posted January 6, 2023 Report Share Posted January 6, 2023 25 minutes ago, Fly Boomer said: I assume (I know, bad idea) that there is no voltage regulation being done by the ship's electrical system, so the GPU voltage is a critical number? Yes, it's the same as connecting a battery, you get what you get. If you plug 28V into it, which uses the SAME PLUG, and turn on the master and avionics, it'll toast a lot of stuff. If you have a 12V system it is a good idea to put a placard or sticker of some kind on both sides of the little access door to the plug, and/or above it or near, indicating that it is a 12V system. There have been many systems fried by an FBO rampie plugging the wrong GPU voltage into the external jumper socket. Some cart GPUs have one cable and connector and a switch that determines how much voltage gets applied. If you're getting a jump from somebody using a cart, triple check that it's the right voltage. Or if you're just configuring a home-brew GPU/supply, verifying the output voltage is what you want is a good idea before connecting it to the aircraft. Switching supplies sometimes won't regulate properly without a load, so an open-circuit voltage test may not be sufficient. Quote Link to comment Share on other sites More sharing options...
Fly Boomer Posted January 6, 2023 Report Share Posted January 6, 2023 28 minutes ago, EricJ said: Yes, it's the same as connecting a battery, you get what you get. If you plug 28V into it, which uses the SAME PLUG, and turn on the master and avionics, it'll toast a lot of stuff. Your admonition is that it will toast a 14-volt airplane? Mine is 28v. I was just considering the voltage being pushed into ship's batteries. For example, if I have a weak battery, a 28-volt push might not be a good plan. Quote Link to comment Share on other sites More sharing options...
bradp Posted January 6, 2023 Report Share Posted January 6, 2023 Also… some smart battery chargers do not have sufficient output when in trickle mode to charge through the NATO plug. They won’t keep the relay open… thus they should be connected to the battery directly. I use an old school analog 30 lb HAM radio power supply (about $100 on eBay) that plugs into the nato supply with + terminal connected to the relay pin inside the plug. It works great and is very reliable and safe. I can dial in a calibrated voltage to match my VR charge and I can current limit the system. Quote Link to comment Share on other sites More sharing options...
EricJ Posted January 6, 2023 Report Share Posted January 6, 2023 4 hours ago, Fly Boomer said: Your admonition is that it will toast a 14-volt airplane? Mine is 28v. I was just considering the voltage being pushed into ship's batteries. For example, if I have a weak battery, a 28-volt push might not be a good plan. 28V applied to a 14V airplane will toast anything that gets turned on that can't handle the excessive voltage. That can be a lot of or most of the electronics, especially if they're older. In a 28V system the alternator and regulator together put out 28V, so even the weakest battery will get charged at that voltage in the system when running. A typical constant-voltage external charger will do that same. Some smart chargers may start with constant-current charging at a lower voltage, but it's generally not going to hurt a 24V battery to charge it at 28V. It will hurt a 12V battery to do that, though. 1 Quote Link to comment Share on other sites More sharing options...
A64Pilot Posted January 6, 2023 Report Share Posted January 6, 2023 Depending on the amperage of the power supply, if you connect a 24V supply to a 12V battery, you won’t get 24V in the electrical system. The reason is the battery acts as a huge capacitor, you have to raise the voltage of the battery to 24V to get 24V into the system, and I doubt that’s possible with out lots of power. I’m not saying it’s safe, it’s not and over time will cook the battery at least, but if your supply isn’t that big, your avionics etc may survive a short time oops. Now if the battery is bypassed like I think it is in a Cessna then yes you would put whatever the voltage being supplied is into the system Correct system voltage for a 24V battery is 28V, so it definitely won’t hurt a 24V battery. That coincidentally was one of my oral question from the FAA guy for my Commercial ride, he asked what voltage was the aircraft, I said 28, he said no it had a 24V battery and I explained that it took 28V to charge a 24V battery but I don’t think he believed me. But it got real good when he asked how the elevator trim worked and I told him there was a jack screw that changed the entire empennage angle of incidence. He looked at me liked I had horns growing out of my head, I had to take him to the aircraft to get him to believe me Quote Link to comment Share on other sites More sharing options...
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