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Thread: 12v, 14v, 24v, 28v system. Which will I have?

  1. #1

    Default 12v, 14v, 24v, 28v system. Which will I have?

    Iím looking at avionics, engines & batteries Some of Garminís items ask for 14/28 volts. Rotaxís website & the 915ISC discusses an electric starter of either 12 or 24 volts. Then at the bottom the same engine states it has a 24v power supply delivering up to 800w. Iíd like to go with the EarthX ETX680C battery which is rated at 13.2v. So my question is, do I choose 12/14 or 24/28v? Probably a rookie question but I know enough about electricity to get a good shock.

    My best guess is that I will have a 12v system. But what dictates this?

    Thanks in advance,

    Matt Koeniguer building the KingFox
    https://youtube.com/channel/UCghEv8ArOvf3qPesuz34EcA
    KitFox Model 7STi
    Monster Shocks - 29Ē Alaskan Bushwheels & TW
    Engine - Want the Rotax 915is
    Propeller - Not sure yet
    Stits Poly Fiber covering when I get that far

  2. #2
    patrick.hvac's Avatar
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    Default Re: 12v, 14v, 24v, 28v system. Which will I have?

    If you purchased a 915 with a 24V generator. (915ISC24) then you could design around that but there's a much higher chance you don't have that.
    The vast majority of these builds are 12V nominal systems.
    Your question of what decides this? It would be the charging system on the engine.
    🇨🇦CANADA
    Building | SS7 | G3X | 912
    Flying | Tin Can

  3. #3
    Senior Member Eric Page's Avatar
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    Default Re: 12v, 14v, 24v, 28v system. Which will I have?

    12V and 14V -or- 24V and 28V are used interchangeably to refer to the same systems, so there are really only two options. Traditional lead-acid batteries have a nominal voltage of 12V, but vehicle charging systems operate at nominal 14V (in both cases, those numbers are rounded by a few tenths). Likewise, the other option is a 24V battery in a 28V charging system.

    The advantage of higher voltage systems is that the builder can use lighter gauge wire to deliver the same energy to a device. For example, a 50W halogen landing light will draw ~4.2A at 12V, but only 2.1A at 24V. In a large aircraft the savings in copper wire weight can be significant, but in something the size of a Kitfox it won't even offset the added weight of a 24V battery.

    Battery choice is the other issue. There are lots of options for small, lightweight 12V-ish batteries, but I'm not aware of anything suitable for a Kitfox at 24V. You could put two EarthX batteries in series, but they might not charge properly in that configuration; I would ask EarthX before doing it that way.

    Finally, take a look at the electrical section of the 915iS installation manual before you choose a battery. Depending on the mission you're building the plane for, you may want more capacity than the ETX680C offers. See my longer post on this subject, here (there are several other useful posts in that thread as well).
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  4. #4

    Default Re: 12v, 14v, 24v, 28v system. Which will I have?

    [QUOTE=It would be the charging system on the engine.[/QUOTE]

    Hello Patrick, thanks for the explanation. I should have looked back in here later in the day but I must have seen a squirrel as it has been a minute.

    Matt
    Matt Koeniguer building the KingFox
    https://youtube.com/channel/UCghEv8ArOvf3qPesuz34EcA
    KitFox Model 7STi
    Monster Shocks - 29Ē Alaskan Bushwheels & TW
    Engine - Want the Rotax 915is
    Propeller - Not sure yet
    Stits Poly Fiber covering when I get that far

  5. #5

    Default Re: 12v, 14v, 24v, 28v system. Which will I have?

    Quote Originally Posted by Eric Page View Post
    12V and 14V -or- 24V and 28V are used interchangeably to refer to the same systems, so there are really only two options. Traditional lead-acid batteries have a nominal voltage of 12V, but vehicle charging systems operate at nominal 14V (in both cases, thosthe mission you're building the plane for, you may want more capacity than the ETX680C offers. See my longer post on this subject, here (there are several other useful posts in that thread as well).
    Thanks Eric, Iím giong to check out the longer post in a moment. Since I already have a 12v battery Iíll be sure to ask that my engine is also compatible with that charging system as I have seen on the Rotax page they are offering a 24v system.

    Have a good day,

    Matt
    Matt Koeniguer building the KingFox
    https://youtube.com/channel/UCghEv8ArOvf3qPesuz34EcA
    KitFox Model 7STi
    Monster Shocks - 29Ē Alaskan Bushwheels & TW
    Engine - Want the Rotax 915is
    Propeller - Not sure yet
    Stits Poly Fiber covering when I get that far

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