14v vs 28v

npowell · #1 12-09-2007, 04:19 PM

I would be interested in learning more about the pros/cons of going either 14v or 28v with my system. I know one advantage iof 28 volts is you have less amps in a typical circuit, therefore smaller and lighter wires needed in general. But, you need two batteries, which adds space.

Any thoughts?

gmcjetpilot · #2 12-09-2007, 05:11 PM

Quote:

Originally Posted by npowell

I would be interested in learning more about the pros/cons of going either 14v or 28v with my system. I know one advantage iof 28 volts is you have less amps in a typical circuit, therefore smaller and lighter wires needed in general. But, you need two batteries, which adds space.

Any thoughts?

Great question; there are some threads in the archive. I looked one up for you:
http://www.vansairforce.com/communit...highlight=volt

Keep in mind 12v components are more plentiful and often cheaper to purchase. For example 12 volt AGM batteries are cheap and avaiable. 28 volts not so cheap or as avaiable.

On large jets they use 115 volts AC and 48 volts DC. However on a jet wire runs are much longer. Using smaller gauge wire saves weight. We have a little weight savings to gain, if any. By all means research it and learn as much as you can. After all that is what building is about.

asav8tor · #3 12-09-2007, 05:30 PM

Sometimes you will find 28v stuff on ebay or barnstormers for less than the 14 volt items since most of the homebuilders bid up the 14v stuff.

2 small batteries in series could give you the ability to have a 28v DC bus and a 14v DC bus, it would open up many possibilities. There really isn't enough long high current runs to make the case for weight savings by using smaller wire. A great project of an EE major though.

gmcjetpilot · #4 12-09-2007, 05:42 PM

Quote:

Originally Posted by asav8tor

Sometimes you will find 28v stuff on ebay or barnstormers for less than the 14 volt items since most of the homebuilders bid up the 14v stuff.

2 small batteries in series could give you the ability to have a 28v DC bus and a 14v DC bus, it would open up many possibilities. There really isn't enough long high current runs to make the case for weight savings by using smaller wire. A great project of an EE major though.

Both are true statements but did not want to muddy the water.

Cheap 28 volt stuff. Well take a KING KY196 24 volt/16 watt transceiver, yep its cheaper used than a similar 12 volt version. However those 24 volt units have 10-16 watt transmitting output. Although having more a powerful radio transmitter sounds great, they also draw big current, despite being 24 volts. If you are trying to get by with a little 40 amp alternator a 5 watt transmitter (2.6 amp draw) fine. Unless you plan on being at FL350, you don't need big boy transmitters which draw 6-10 amps! OK you got me, one cheaper radio.

Strange enough 28 volt Bendix/King stuff cost 3 times as much new. I'll guess it will cost more to repair 28 volt stuff.

I can't make an argument about avionics since most avionics now is dual volts or works from 10-30 volts but not all. There is still a lot of cool avionics that's 12 volt only. Flap motor? Yes you can always cut down 24 volt to 12 volts with a power supply, but that is more cost & weight. Most guys I know who go 24 volt, end up with 12 volt components somewhere. Some times a drop down resistor will work, sometimes you need a drop down power supply.

Two batteries in series would do it. There goes any glimmer of weight or space savings.

No conclusion but to each his own. Until the car, truck, motorcycle & recreational vehicles go to 24 volt, the economy of volume goes to 12 volt.

Bottom line 12 volt is technically adequate.

N395V · #5 12-09-2007, 05:49 PM

For what I thought were many good reasons I built my plane with a 28V system. Huge mistake.

First off the wire savings were less than 2 pounds, more than offset by several more pounds in battery weight.

Secondly there were some things it was just too difficult to find that are 28 volt. So now I have a 28 volt bus and a 14 volt bus.

Initially I thought of (2) 14v batterys in series tapping off the center for 14V. This is problematic however if you do not exactly balance the load on both batteries. This leads to a situation of one battery always discharging into the other and premature failure of both. So I got one big 28v battery, more weight, more cost. Then I had to add a 14 v equalizer(converter) equals more cost more weight. Then you have to switch, fuse, and distribute 2 different voltages adding complexity.

The 28V battery doesn't last longer when the alternator quits as equipment consumes Volts-x-Amps which is a constant for either system. In fact depending on how the units drop to operating voltage internally those items that can be run on 10-35 volts may consume power more quickly on a 28V system converting it to detrimental heat.

About the only benefit I get from 28V is it spins the starter quicker and I can turn on my efis before I engage the starter and it wont "brown out".

The only other benefit is my wiring is color coded as are the busses and they really look cool when you take everything apart.

Go with 14V.

Norman CYYJ · #6 12-09-2007, 07:13 PM

Another advantage of the 14volt system over the 28volt system is if you are caught out in the boonies with a dead battery esp. in the winter you will not be able to get a boost to get the plane going. Not too many trucks or cars operating on 28 volts.

Kyle Boatright · #7 12-09-2007, 07:49 PM

14V is the way to go for most people. A 28V system is technically superior, but from a maintainability standpoint (think alternator or battery when you're out in the boonies) and a cost standpoint (all that automotive stuff works on 14v), 14v is the right choice.

It reminds me of the old Beta vs VHS argument. Beta was a superior product, but VHS captured an overwhelming market share, and BETA died because it was virtually impossible to find Beta tapes for rent.

npowell · #8 12-13-2007, 11:52 AM

I appreciate the thoughtful responses and good ideas. The manufacuturer of my EFIS recommended 28 volts, but given all the good reasons above, I think I will stick to 12 volts.
Thanks again for responding.

Auburntsts · #9 12-13-2007, 01:59 PM

Which EFIS manufacturer recommended 28v and did they provide any rationale (other than what's been mentioned here)? I don't have a dog in the fight for EFIS customers, just curious since it seems most avionics manufacturers don't seem to advocate a particular voltage other than the range (say 10-30 VDC) so I'm a little surpised by them coming out with a preference.

w1curtis · #10 12-13-2007, 02:14 PM

Quote:

Originally Posted by Auburntsts

Which EFIS manufacturer recommended 28v and did they provide any rationale (other than what's been mentioned here)? I don't have a dog in the fight for EFIS customers, just curious since it seems most avionics manufacturers don't seem to advocate a particular voltage other than the range (say 10-30 VDC) so I'm a little surpised by them coming out with a preference.

I think those of use who have seen this topic come up a number of times can guess that one. That would probably be Greg Richter of Blue Mountain fame. He should spend his time improving his product and not recommending system voltages. I recommend doing a search on Blue Mountain to see comments from Blue Mountain users.