External Power Connection - DA40NG

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AaronF
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External Power Connection - DA40NG

Post by AaronF »

Hey guys,

Newbie here with a brand new DA40NG.

Searched through the forums but couldn’t find any pertinent info. Apologies if duplicate post…

Recently purchased a 24V 25A External Power unit to familiarize with the Nxi Avionics without having to worry about draining the battery.
When power supply is connected to the receptacle, I hear a click sound from the baggage area, I assume this must be a relay engaging.

When I switch ON the Electric Master whilst power is connected, both PFD + MFD powers up as usual. On the MFD <Engine> page, voltmeter reads 24.2V, decreasing slowly so long as the Electric Master remains ON.
Am I correct to assume that a gradual voltage drop should not occur if external power is connected?

The only info about the matter that I could find on the POH:

Battery Bus 1:
The battery bus 1 is connected to the main-battery via the battery-relay which can be controlled by the ELECTRIC MASTER key switch. The battery bus 1 provides power to the battery bus 2 and heavy duty power to the starter.
The battery bus 1 is also connected to the power input line of the external power plug.

Battery Bus 2:
The battery bus 2 is connected to the battery bus 1 via a 100 A fuse and provides power to the ECU bus via a 80 A fuse. It also provides power to the main bus via the power relay which can be controlled by the ELECTRIC MASTER key switch and the ESSENTIAL BUS switch. The ELECTRIC MASTER key switch must be set to ON and the ESSENTIAL BUS switch must be set to OFF to connect the battery bus to the main bus.

and about the voltmeter/ammeter:

Voltmeter
The voltmeter shows the voltage of the essential bus. Under normal operating conditions the alternator voltage is shown, otherwise it is the voltage of the main battery.

Ammeter
The ammeter displays the intensity of current which is supplied to the electrical system by the alternator, including the current for battery charging.

Here are some screenshots from the Training Manual:
Image
Image
Image
Image
Image

and a picture of the circuit breaker panel:
Image

I studied these but still not sure I fully understand how the system is wired. :(
Your input would be greatly appreciated.

3 questions:
1. How does one make sure that the PFD + MFD is powered by external power when connected and that in fact main battery is not draining?
2. Is the system wired in a way that it's also possible to charge the main battery via external power?
3. When external power connected, what's the difference between Electrical Master switched ON and OFF?

Thanks in advance for your comments…
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jast
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Re: External Power Connection - DA40NG

Post by jast »

Since the battery bus 1 is connected to the epu the plane should power up with the master switch=off. However there is a relais which has to be activated from the EPU as well. Maybe check the documentation?

Then enabling the master switch also charges the battery from the EPU. A slowly constantly dropping voltage should not happen unless the EPU itself is a battery. This is how I understand the electrical diagram. However I have never tried to power my DA40NG with an EPU...
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Re: External Power Connection - DA40NG

Post by chili4way »

I use an EPU monthly, when I'm doing my 28-day database updates on my DA40NG. The click when you plug in and turn on the EPU is indeed the external power relays. My EPU appears to be different than yours because it provides 28V and is plugged into 120VAC so your situation may be a little different if your EPU is battery powered.

https://www.sportys.com/pilotshop/24v-p ... 99-00.html

In the DA40NG , you need to turn on the Electric Master key switch to turn on the avionics. The Hobbs does not run (this requires the Engine Master to be on, which is neither typically required nor recommended). If the Engine Page is showing 28V (i.e. more than 26V) and zero amps, then your panel is indeed running on the EPU. If you see 24V-25V, then you're running on battery so check the connection to the plane, the EPU power switch, and the EPU AC power connections/GFI. The amps indication shows what the alternator is producing, so zero amps makes sense when running from an EPU. (The DA40NG is wired a little differently than the Lycoming DA40s.)

I agree with Jan/jast that the EPU-powered configuration charges the main battery only when the Electric Master key switch is on. The third (small) + terminal on the EPU port powers the external power relay, connecting the larger + terminal to Battery Bus 1. The Electric Master key switch allows current to flow from the Battery Bus 1 through the battery relay, connecting the battery to Battery Bus 1. Since the voltage provided by the EPU is higher than the battery, it charges the battery (so be sure your EPU is set for the proper battery type if that is selectable on your EPU). It's not exactly clear from the schematic for planes without air conditioning/RACC, but the ECU backup batteries are also charged. (It is very clear from the schematic for planes with the 2nd alternator that runs the RACC that the ECU backup battery is charged. The ECU backup batteries are used to excite the alternator field when starting.

Since my EPU doesn't provide the amperage to turn the starter, I pull the starter circuit breaker as an extra precaution, particularly when there is a special reason (unusual) for me to have the Engine Master on while running from the EPU. My plane has the RACC option and it is energized via the EPU (independent of the Electric Master), however the current capability of my EPU doesn't support running it.

The only way you can accidentally discharge the main battery in a DA40NG when the Electric Master key switch is off is through the "cigarette lighter" power port that is connected to the hot battery bus. Take care not to leave anything (like a USB charger) plugged in there when the plane is parked. Of course it's always wise to run your checklist to make sure everything is "off" before parking the plane.

Hope this helps, and I welcome feedback on what other DA40NG owners have experienced.
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Re: External Power Connection - DA40NG

Post by Pehu »

To my understanding you need 28V power, anything below can damage the plane's systems.

Don't plug it in unless you are 100% certain the unit is suitable for DA40NG. My shop said 28V is a must.
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Re: External Power Connection - DA40NG

Post by Boatguy »

I use the same unit, though purchased for $55 less at Aircraft Spruce. :)

I always attach the EPU to the plane, then turn it on, and then wait for the click. If I don't hear the click, then start the process over again. Then Master Electric on, verify 28v, then Avionics On.

Reverse the process when disconnecting (i.e., Master power off, EPU power off, wait for click, disconnect).

I've found the connection to be fussy and sometimes even when I've heard the click, I don't get 28v. Sometimes I have to remove and reconnect the big red paddle connector several times before it makes a good connection. When connected I usually see about 28.2v.

I wasn't aware that it was also charging the main battery as chili4way describes so that's a bonus!
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Re: External Power Connection - DA40NG

Post by Rick »

chili4way wrote: Sat Jan 25, 2020 4:08 pmThe only way you can accidentally discharge the main battery in a DA40NG when the Electric Master key switch is off is through the "cigarette lighter" power port that is connected to the hot battery bus.
Did Diamond change the map light in the NG to require the main to be on? In the Lycoming DA40's, the pilot-side map light bypassed the master switch and would drain the battery if left on overnight, but the cigarette lighter port is only energized when the master is on. Interesting...
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Re: External Power Connection - DA40NG

Post by astaib »

Change the bulb by a led and then you will need many nights to drain all the energy from the battery
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Re: External Power Connection - DA40NG

Post by AaronF »

Rick wrote: Mon Jan 27, 2020 11:51 am Did Diamond change the map light in the NG to require the main to be on? In the Lycoming DA40's, the pilot-side map light bypassed the master switch and would drain the battery if left on overnight, but the cigarette lighter port is only energized when the master is on. Interesting...
On my DA40NG (YOM 2019), map lights require Electric Master to be ON. Only the cigarette lighter is connected to hot battery bus.
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Re: External Power Connection - DA40NG

Post by Rich »

Rick wrote: Mon Jan 27, 2020 11:51 am
chili4way wrote: Sat Jan 25, 2020 4:08 pmThe only way you can accidentally discharge the main battery in a DA40NG when the Electric Master key switch is off is through the "cigarette lighter" power port that is connected to the hot battery bus.
Did Diamond change the map light in the NG to require the main to be on? In the Lycoming DA40's, the pilot-side map light bypassed the master switch and would drain the battery if left on overnight, but the cigarette lighter port is only energized when the master is on. Interesting...
The original setup was that the map light, Hobbs and accessory socket were all powered off the hot bus, with fuses in the engine compartment. We installed a Stratus USB in place of the accessory port. The USB adapter is powered through a circuit breaker on the avionics bus. The clearly-marked fuse for the accessory port was removed and the lead tied back. My (feeble) map light bulb burned out years ago.
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Re: External Power Connection - DA40NG

Post by chili4way »

Rick wrote: Mon Jan 27, 2020 11:51 am
chili4way wrote: Sat Jan 25, 2020 4:08 pmThe only way you can accidentally discharge the main battery in a DA40NG when the Electric Master key switch is off is through the "cigarette lighter" power port that is connected to the hot battery bus.
Did Diamond change the map light in the NG to require the main to be on? In the Lycoming DA40's, the pilot-side map light bypassed the master switch and would drain the battery if left on overnight, but the cigarette lighter port is only energized when the master is on. Interesting...
Yes, it's different wiring in the DA40NG. The Map/Reading lights are connected to the Main Bus (via the TAXI/MAP circuit breaker) and the Main Bus is connected to the battery via the battery relay (which requires the Electric Master key switch to be ON), Battery Bus 1, a 100A fuse, Battery Bus 2, the PWR relay (which requires the Essential Bus switch to be OFF), and the PWR circuit breaker.
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