CaptainBen Posted February 20 Report Posted February 20 Hi folks – I'm having trouble getting my external power (GPU) to work with my Ovation. I purchased a White Lightning 27-amp, 28-V unit (model M2827). When I hook it up to the external power plug on the airplane and turn it on, I hear a "clunk" sound, which I think indicates it's made a connection with the electrical system. After I power on the Master / Avionics, the unit display shows 10.8 amps, and the Ovation ammeter shows a discharge of -11 amps. That discharge shown on the ammeter is the same even if I shut down and disconnect the GPU. I talked to Gemini and ChatGPT about it and got conflicting suggestions, none of which worked. One suggestion was to find a knob or screw on the GPU that allows you to increase the voltage to 29V, but there is no such knob, dial, or screw on the unit. Another suggestion was to get a more powerful unit (50 amp), which I have not tried yet. I also tried turning the landing and taxi lights on to see if the increased load might help push the system to cut over from the battery to the GPU, but that didn't work either. It seems like the unit isn't having any effect at all and I'm just draining the battery. I just did an avionics upgrade (GTN 750xi, 650xi, and dual GI-275s), so these instruments are probably drawing more power than the original panel. But, I had not tried to use the GPU prior to the upgrade. Appreciate any thoughts or suggestions!
Fritz1 Posted February 20 Report Posted February 20 first thing I would do it connect the GPU to the battery that you are using with jumper cables, just be very careful, this will tell you whether the GPU is working, others more knowledgeable will chime in regarding the ground power plug 1
LANCECASPER Posted February 21 Report Posted February 21 Hi folks – I'm having trouble getting my external power (GPU) to work with my Ovation. I purchased a White Lightning 27-amp, 28-V unit (model M2827). When I hook it up to the external power plug on the airplane and turn it on, I hear a "clunk" sound, which I think indicates it's made a connection with the electrical system. After I power on the Master / Avionics, the unit display shows 10.8 amps, and the Ovation ammeter shows a discharge of -11 amps. That discharge shown on the ammeter is the same even if I shut down and disconnect the GPU. I talked to Gemini and ChatGPT about it and got conflicting suggestions, none of which worked. One suggestion was to find a knob or screw on the GPU that allows you to increase the voltage to 29V, but there is no such knob, dial, or screw on the unit. Another suggestion was to get a more powerful unit (50 amp), which I have not tried yet. I also tried turning the landing and taxi lights on to see if the increased load might help push the system to cut over from the battery to the GPU, but that didn't work either. It seems like the unit isn't having any effect at all and I'm just draining the battery. I just did an avionics upgrade (GTN 750xi, 650xi, and dual GI-275s), so these instruments are probably drawing more power than the original panel. But, I had not tried to use the GPU prior to the upgrade. Appreciate any thoughts or suggestions! Is your battery switch on Battery #1? It has to be when you plug into a GPU on a Bravo, Ovation or Acclaim. 1 1
dkkim73 Posted February 21 Report Posted February 21 Lance is correct and this is likely the answer here, since the clunk sound is the third pin engaging the relay which connects the positive side to the #1 battery. It's possible the relay or connection are bad but that would be rare esp. if you heard it operating. Fritz (electrical engineer showing through) is correct about systematic troubleshooting but you probably won't need to. BTW it's somewhat easy to accidentally short something back there in the tailcone once you expose the positive side of the battery (cramped and everything is metal) so use cables that fit if you try that. I would *not* connect an arbitrarily higher voltage to the bus. Maybe the LLM is confabulating or conflating a battery deep discharge recovery procedure. Let us know how it goes. And how you end up liking the GPU, I'd like to get one some day. 1
CaptainBen Posted February 21 Author Report Posted February 21 Thanks for the responses thus far. The behavior I described above was all with BAT-1 selected. I can tell the GPU unit is working and sending amps to the airplane (when I turned on all the lights the unit display showed about 24 amps). But the airplane ammeter still shows a significant discharge (-11 with the avionics on and -42 when I added the landing / taxi lights). So the battery is still being drained even though the GPU unit is functioning and sending current to the airplane. Very puzzling. I was hoping there was some kind of switch that would allow me to bypass the battery and power the avionics directly with the GPU, but that doesn't appear to be the case.
MatthiasArnold Posted February 21 Report Posted February 21 Depends on where the shunt of the ammeter sits… Since/if the APU plug (I did not check the schematics) feed into the same wire as battery one (via the relay actuated by the small pin, of course) you will see a negative current (independently whether it’s provided from battery 1 or the APU). This holds true, at least for my GX2. I see negative currents when powered via APU. However, as long as the corresponding voltage is >>24V (e.g. 26V) I’m not concerned at all. The electrical current is obviously provided by the APU, not by draining the battery (it won’t be drained at this voltage). Just my 2 cents (as yet another electrical engineer), Best, Matthias 2
dkkim73 Posted February 21 Report Posted February 21 2 hours ago, MatthiasArnold said: Depends on where the shunt of the ammeter sits… @MatthiasArnold Doh. That makes a lot more sense, esp. given what the OP just posted as well. Looks like it's physically up front by the circuit breaker panel and rectifiers in the M20TN at least (prob. similar to the M20R) Page EH420G - 029 of the MAN180 maintenance manual (914 out of 1169 in the PDF). Looks like the only thing between the battery and aux power port electrically-speaking is maybe the battery relay (hard to tell, I hate navigating PDF schematics). So "looking into the battery" you'd see a discharge if power is coming from the GPU port. @CaptainBen having the battery in parallel with the GPU should also help buffer voltage spikes. So I think running it off the GPU alone isn't something you'd want, so long as the bus voltage stays above the resting potential of a fully-charged battery. Which I am guessing this would do if it's a GPU set for a 28V system. 1
Slick Nick Posted February 22 Report Posted February 22 Does your ALT VOLTS light stop flashing when the GPU is plugged in? I have mine set to 28.0V and it stops flashing when the power is applied. Haven’t paid much attention to the ammmeter, but I have an MSE might be different anyways. 1
Fritz1 Posted February 22 Report Posted February 22 as stated supra, the ammeter will show a discharge no matter if the GPU is connected directly to the #1 battery or to the GPU connection, the number to watch is the battery voltage. If your GPU puts out 15A and the aircraft draw is 10A the battery voltage should not go down because the battery is not being discharged but rather being charged. Run 10A draw for 45 min, which would deplete 3/4 of the battery capacity, if the voltage does not drop at all the GPU is working fine 1
CaptainBen Posted February 23 Author Report Posted February 23 Hey all – circling back on this... thanks to everyone for your insights and suggestions. Turns out that the GPU is indeed working. I verified it by looking at the battery voltage on the EDM-700 and the battery power level on the GI-275. I was able to conclude that with the GPU on, the power stays stable (28.4 to 28.6 volts and 96% full), and The ALT VOLTS light stops flashing. Also, the GPU display shows increased amps flowing out as I increase the airplane electrical load. But when I turn the GPU off the battery voltage drops to 24V. The ammeter (still showing -11 with the avionics on) is just inaccurate for whatever reason (maybe it's wired incorrectly?). Appreciate all the good input! Now... time to go hangar fly my new avionics ;-)
dkkim73 Posted February 23 Report Posted February 23 Ammeter is accurate in that there is current flowing from where the battery is (in parallel with the GPU) through the remainder of the system. It's just not isolating the battery from the GPU connection. Think of the battery and the GPU as a yoked pair of oxen and the ammeter is telling you how much work they are doing together (from the wagon's viewpoint). Further, since the GPU "pulls harder", in fact as hard as needed , the battery ox can just loaf along without pulling at all.
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