Alternator size

[VictorV]

I am starting an SS7 build soon which will include the EP915ti engine.
I'm looking at the power requirements for everything onboard and am
using the following numbers:
28667
My mission is day/night VFR only. I'm planning on using the Vertical Power VP-X and PPS.
I plan on a single power bus.

The 914 has an internal alternator that produces 18 amps so obviously not sufficient to run
everything at once. I've been looking at the Edge Performance 32a Alternator add-on but
that's still not quite enough.

Here are my questions:
1) Do I need an alternator that can handle the entire maximum load? (my guess is no since some
items are intermittent like radio xmit and trim motor)
2) How do I figure out exactly what size alternator I need?
3) Based on the load requirements will I need to make the add-on alternator the primary and use
the 914 internal alternator as the backup?
4) Do I even need a backup alternator since the battery can carry the load if the add-on alternator
fails? I'm assuming if the primary fails the system just runs on the battery and the engine will
continue running for some period of time based on what electrical components are still on and
the battery capacity. My goal would be 30 mins of flying + G3x PFD running and no radio.

Victor

[Shadowrider]

Get the 32a edge alternator and it will cover your needs. The stock 18 amp alternator will still be there also. I pretty much have the same avionics your planning other than I upgraded my heater fan and it draws 5amps, Bose 6 pin powered headset, underwing lighting and usb outlets to charge phone and the 32 amp is plenty. I have the same garmin suite you have. You are not going to be drawing max. You will not be trimming with heater on and radio transmitting and all lights on an and autopilot on draw max load, ect. Here is how I ran mine. I am single bus with 2 charging sources. So you will have two VR. They both run to my battery but first pass through a breaker switch that is a breaker and a switch to turn off or on that charging source. I use 32a alternator as primary. If my vr fails on my edge then I switch off it as a charging source and switch on the stock rotax that will get me home. I like having one bus for simplicity and two charging sources with two VR. If I am going to the back country I switch off the edge and turn on the rotax to check it before I head out. I just have two hidden switches under the dash to control charging sources.

[VictorV]

Thanks for the info Dustin. It's starting to make sense. Originally I thought that
there was a way for the switchover from the primary alternator to the backup
could be automated but it seems like a switch is much easier. I'm glad to hear
that the 32a Edge alternator is sufficient.

What do you use to keep the PFD power sufficient during engine start?

Victor

[PapuaPilot]

Most people are using the Intelligent Power Stabilizer from TCW:
https://www.tcwtech.com/product/intelligent-power-stabilizer/?v=7516fd43adaa

I'm pretty sure TCW had a 10% off Oshkosh special that goes till tomorrow (Aug. 31, 2021). I don't have the card I got from the TCW rep at OSH, but I think the discount code was "OSHKOSH21". Definitely worth trying if you are able to order one in the next 24 hours.

If you plan to have an autopilot you may also want their autopilot servo booster: Single axis servo booster, 12 volt, 10 amp (ST-1-12V-10A-BSTR)
https://www.tcwtech.com/product/safety-trim-intelligent-servo-controller/?v=7516fd43adaa

[VictorV]

Most people are using the Integrated Battery Back-up System from TCW.
https://www.tcwtech.com/product/integrated-battery-back-up-system-ibbs/?v=7516fd43adaa

I was hoping to use something like that but was not sure how it is connected. If I understand
correctly it's basically inline with the power going to the avionics devices you want to protect
from brown-out. Can this be integrated with the VP-X? Seems like it can't unless all the avionics
components you want to protect use the exact same power source from the VP-X. Am I understanding
this correctly?

Victor

[PapuaPilot]

Most of the modern avionics have two pins for power input. One way is to stop brown outs is to hook a device like this to the second pins of the components you want to power for the start.

Another way is to have a second battery that can power everything or an essential bus that is isolated from the start circuit and main battery. I designed my own backup battery system that uses this method.

I don't know anything about Vertical Power, maybe someone who does can chime in. Maybe VP already has this feature built in.

Check out the document tab for the TCW device, it shows how to hook it up to multiple systems.

[Shadowrider]

I was worried about power cycling the g3x when I start but learned the g3x is designed against this. You put the G3x and the gea-24 (because you want engine gauges working for start) on the master circuit. The g3x can go down to extremely low voltage momentarily to keep it alive on power starts. Then once you start have an avionics master that turns on the rest of avionics so no backup battery needed.

[Delta Whisky]

As an aside, for planning purposes, don't forget the power draw of your master solenoid. Some are as much as 1.5 Amps. The one that came with my kit measured right at 1 Amp.

[VictorV]

As an aside, for planning purposes, don't forget the power draw of your master solenoid. Some are as much as 1.5 Amps. The one that came with my kit measured right at 1 Amp.

Good point. In my case I am using the Vertical Power PPS which draws about 1/3 of an amp which is
better than a mechanical solenoid.

[efwd]

Hi Victor
I hesitate to reply, in fact I have written this reply three times. The reason, I don't have the calculations to refer too and as with many of us, electrical was not my favorite. Anyhow, for what it's worth I have everything on your list except the tail light but I do have aft facing white lights on both wing tip position lights. Some of our numbers don't match such as the trim motor. I calculated my amps (but I don't have the data here at home) then, while at the Kitfox Factory, I spoke with Judd. On his advise, I am not running any addtl alternators. My 912iS electrical system is operating all those items on your list. I do have the backup battery as Justin mentioned but I have no reason to believe it has ever kicked in to keep the PFD or MFD screens on. Im running the VPS as well and I have a mechanical Master switch. I run a lot of stuff on startup too. Landing lights, strobe, position lights, Radio, Transponder, both PFD and MFD etc etc.

[VictorV]

Hi Victor
I hesitate to reply, in fact I have written this reply three times. The reason, I don't have the calculations to refer too and as with many of us, electrical was not my favorite. Anyhow, for what it's worth I have everything on your list except the tail light but I do have aft facing white lights on both wing tip position lights. Some of our numbers don't match such as the trim motor. I calculated my amps (but I don't have the data here at home) then, while at the Kitfox Factory, I spoke with Judd. On his advise, I am not running any addtl alternators. My 912iS electrical system is operating all those items on your list. I do have the backup battery as Justin mentioned but I have no reason to believe it has ever kicked in to keep the PFD or MFD screens on. Im running the VPS as well and I have a mechanical Master switch. I run a lot of stuff on startup too. Landing lights, strobe, position lights, Radio, Transponder, both PFD and MFD etc etc.

Thanks for the info. So it looks like you turn everything on before you hit the start button? I think it's
not obvious when the battery backup kicks in to keep your avionics alive during start. You could verify
by disabling the battery backup and measuring the voltage at the avionics power terminal during
start to see what it does and then turn the backup battery on and see if it's different. Not sure that will
work though since there are 2 power inputs to each avionics device.

One important difference between the 912is you have and the 914 I am planning to use is that the 912is has
2 internal alternators. One is 16 amps and the other is 30 amps. The 914 only has a single 18 amp alternator
which is pretty wimpy considering the engine needs 8 amps to run and the radio can use up to 7.5 amps
during transmit.

Victor

[PapuaPilot]

Another thing to consider is that you only need to account for continuous loads to determine the electrical load, and the alternator size required. The trim motor, transmitting radios and fuel boost pump do not need to be counted. Larger intermittent loads can use the ship battery. I counted my nav and strobes as a continuous load, but not my landing lights because I consider my plane a day/VFR aircraft. For radios like COMs or transponder that transmit you need to find out what the current draw is in standby (continuous), not when transmitting (intermittent).

[Shadowrider]

Based on what you said you are going to have, you will not regret adding the 32 amp alternator but very possible you will regret not getting it.