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12 Volts vs. 24 Volts
By Tony Bingelis (originally published in EAA Sport Aviation, December 1989)
The letter posed this question ... "Which electrical system should I install in my Glasair? A 12 volt system or a 24 volt system?"
Which system do you think I recommended as being the better one to install in his homebuilt? (I'll give you a clue. I'm prejudiced in favor of the 12 volt system.)
Actually, either system will do a good job of operating all the accessories, lights, bells and buzzers he could cram into his airplane. Furthermore, either system will function equally as well under IFR conditions should the aspiration to operate in that realm appeal to him.
And, for good measure, I might have added that both the 12 volt and the 24 volt, system are reliable. The difference in reliability, if any, most often appears to be in the quality of the individual electrical components installed for that particular system.
If all this is true, what difference does it really make which system he chooses to install? I guess from a functional point of view there is no difference . . . certainly no visible difference. He couldn't, for example, look at an airplane and say, "That one looks like it has a 24 volt system." To be honest about it, I can't tell by looking at electrical accessories and components which of them must operate on 12 volts and which on 24 . . . not without peeking at the data plates, I can't. Having demonstrated my impartiality to this point, let's get down to the nitty gritty of the title subject. But, before going any further, we need to reach a common understanding of the electrical terms we will be evaluating.
WHEN ARE 12 VOLTS 24 VOLTS . . . AND 24 VOLTS 28 VOLTS?
Confused? Some builders are. However, the explanation is quite simple.
An aircraft's electrical system has its origin in the storage battery installed primarily for starting the engine and for serving as an initial power source for the aircraft's electrical system. This chemically stored power source (the battery) can be either a 12 volt battery or a 24 volt battery.
After the engine is started, its companion alternator (or generator), installed to supplement and replenish the battery voltage expended in starting the engine, normally cuts in and provides all the electrical power needed in flight.
The power generated by the alternator/generator is controlled and limited by a voltage regulator. This regulator permits the excess voltage produced by the alternator to go to and charge the battery, thereby replenishing its stored capacity.
The voltage regulator is set at approximately 14 volts for the 12 volt battery installation, and for 28 volts when a 24 volt battery is installed. Because of this, most of us sometimes call it a12 volt system, and at other times a 14volt system. The same is true of the 24/28 volt system.
SELECTING AN ELECTRICAL SYSTEM
As I see it, the choice is simple to make.
- 1. If you already have an engine, look at its alternator (or generator) and starter. If these are 12 volt units, go with the 12 volt system . . . unless you are willing to replace them. They would be very expensive to replace with 24 volt units.
- 2. If you don't have an engine as yet, look over the instruments, radios and other accessories you may have already accumulated - and let them influence your choice.
But, what if you have both 12 volt and 24 volt stuff on hand that you would like to install? What then?
I think in almost every instance the 12 volt system would be your best option.
One thing is certain. Do not even entertain the thought that you will solve anything simply by having both systems on board. You can do that, of course, but then you will have a hybrid electrical system that is going to be more complicated, more expensive, and heavier than either of the two systems would have been alone.
This means you would have to start with a 24 volt battery, or two 12 volt batteries connected in series - this dual 12 volt battery arrangement may be found in some Cessna 31Os (see Figure 2). You would also have to find a way to step down the voltage for the 12 volt equipment.
Incidentally, the dual battery idea is not a good one unless you need ballast somewhere. You should realize that two 12 volt batteries will weigh approximately 42 pounds while a single 24 volter will only weigh about 21 pounds . . . and don't forget there is the added weight of another battery box to think about.
THE 28 VOLT SYSTEM . . . ITS GOOD POINTS
What are the touted advantages of a 28 volt system over those of a 14 volt system? There are not many, apparently.
Most of the local maintenance people I have talked to can only identify one generally acknowledged attribute. And that is that a 24 volt system can help you save weight by permitting the use of lighter wire gages for the aircraft's electrical system.
This is possible because the more powerful 24 volt system has the advantage of requiring less current to accomplish the same job, thereby allowing the use of the smaller and lighter wiring.
Had the weather been cooler at the time I asked them, some of these Texas acclimated maintenance types would probably have remembered to remind me of another advantage. Namely, that a 24 volt battery can produce a more spritely starting torque in the engine . . . especially during cold weather. This capability could conceivably ease the cold weather electrical problems most commonly associated with the 12 volt systems - low battery voltage.
WHY STAY WITH THE 12 VOLT SYSTEM?
The 12 volt system has been used in virtually all light aircraft since electrical What are the touted advantages of a systems first came into being.
Here's what bothers me. If the 12 volt system has been a suitable installation for most single engine and light twin aircraft for all these many years, why would anyone even consider changing to the so-called "new" 28 volt system?
Our familiar 12 volt status quo was jolted when Cessna, back in 1979 or thereabouts, decreed that, henceforth, all of its production aircraft would be equipped with the 28 volt system.
Yes, this change would also include their "bottom of the line" single engined Cessna 150s (152).
As the acknowledged leader in the production of single engine aircraft, Cessna exercised its considerable influence in making the switch and many experts freely predicted that all aircraft would ultimately be equipped with the "superior" 24/28 volt system.
DISADVANTAGES OF THE 24 VOLT SYSTEM
I'm sure the average homebuilder views the claimed advantages somewhat differently than do the manufacturers who embrace the 28 volt system as progress. For instance:
- For the homebuilder, one thing is immediately apparent. The 24 volt system is more expensive to install. It may not be more complicated, or much heavier, but it IS expensive to install. Very expensive.
- I'm sure that a change to the 28 system will not result in any significant weight saving by using smaller gage wiring in our homebuilts . . . after all, how many feet of wiring is involved?
For that matter, I wonder if the manufacturers themselves will be able to save additional weight by switching to even lighter wire gages. Hasn't their wiring already been engineered to the minimum acceptable sizes (see Figure 3 - wire selection chart)?
To use the chart (Figure 3) for selecting the correct wire size for a particular circuit, you need to know:
- The length the wire has to be.
- The number of amps of current it is to carry.
- Whether the current carried will be intermittent or continuous and whether the wire is single or bundled.
You will find that the wire lengths in your homebuilt are quite short and the current loads generally light . . . almost all of them less then 10 amps.
Using the chart and the values for the 14 volt system, let's select the correct size for a single wire that will be 15 feet long and carry a continuous current of 6 amps.
- 1. Find the diagonal line for 6 amps and follow it down to where it intersects the horizontal line number 15 in the 14 volt column (wire length).
- 2. From that intersection drop downward to the bottom of the chart to find the correct conductor size. The wire size falls between 18 and 16. In such split decisions, always select the larger gage wire size . . .16 gage in this example.
Using the same example, check to see what size wire would be recommended if this were a 24 volt system. Checking where the 6 amp diagonal intersects the number 15 in the 24 volt column, drop down and read the recommended wire size. Notice that it runs off the chart around the 20 gage point.
Obviously, there wouldn't be much difference, weight wise, between a 16 gage and a 20 gage wire in such short lengths. The weight saving with a 24 volt system is more significant for long lengths because, generally, the wire size can be about 2 gages smaller than that required for a 12 volt installation.
Technically, wiring sizes can be reduced to a point where they become more difficult to fabricate and too fragile ness by tying more money up in slow to withstand handling in day to day service.
Most certainly, the average light single engined aircraft doesn't have too much wiring anyway . . . not like a 747 with its miles and miles of wiring and cable. Now, that's where the total weight reduction could be very important.
As for easier starting? . . . that is a subjective call. If an aircraft engine starts easily enough with a 14 volt system, how much easier would it have started with a 28 volt battery? In severe cold weather operations, the aircraft engines are preheated anyway.
I don't think the switch to a 28 volt system was very popular, or welcomed, by many of the avionics and parts departments around the country because it increased their inventory requirements. They would, henceforth, have to supplement their 14 volt stocks by adding duplicate 28 volt accessories . . . or, at least, more of them. This certainly would increase their cost of doing business by tying more money up in slow moving inventories.
Will the 24 volt system become the inevitable future standard for light aircraft, and homebuilts as well?
In my opinion, no. Not during the next decade, at any rate.
The virtual collapse of single engine light plane production was a dismal development that really, really threw a monkey wrench into the grand idea of converting the aviation community to the 28 volt system.
As production declined, the anticipated thousands of 24 volt equipped single engined aircraft never materialized.
As we all know, costs are generally determined by quantities produced. And since large numbers of 24 volt accessories would not be required or be manufactured, guess what's been happening to their prices?
Here is another observation you can think about. With a smaller 28 volt aircraft fleet, fewer of the aircraft will ultimately end up in aircraft salvage yards. This means that even fewer used 24 volt parts will become available for the economy minded homebuilders and aircraft owners alike. These scarce components and accessories are already overpriced and the future outlook is even less encouraging.
WHAT ABOUT RADIOS AND AVIONICS EQUIPMENT?
Most radios and other avionics equipment advertised in Trade-A-Plane and elsewhere were manufactured for use in 14 volt installations. This means that when you order a radio or some accessory, a 12 volt unit will probably be shipped unless you specifically ordered a 24 volt version of the unit.
Fortunately, the prices are, in most instances, the same for 24 volt radios as they are for the 12 volters . . . but you must always' remember that you may have to specify your 24 volt requirement, if you have one.
Here is something else to think about. Transponders are made to operate on 12 volt systems. Should you want to use one of them in an aircraft having a 24 volt system installed, you would have to install a dropping resistor. This can be done with transponders without experiencing any trouble because they have a constant current draw.
On the other hand, using a dropping resistor is not possible with aircraft radios because they have such a wide current operating range that you have to regulate the current at its source. This means you would have to install a power convertor to change the 24 volts to 14 volts. More costs and more weight. What does a power converter cost? About a couple of hundred bucks I am told.
Automobiles are equipped with 12 volt electrical systems and utilize a number of very reliable components and accessories that have been used by homebuilders in their aircraft for years. These include alternators, voltage regulators, fuel pumps, air filters and starters, to name a few. This valuable, economically priced source would no longer be usable by any builder who elects to install a 24 volt system.
Even in everyday operations, the 24 volt system could be a handicap. For example, what do you do when you have a dead battery? That's right . . . get a jump start.
This is easy enough to obtain when you have a 12 volt system because almost any vehicle can provide the 12 volt boost needed.
But, on the other hand, with that 24 volt battery installed, you would have to find a 24 volt power source somewhere. Maybe somebody will have a 28 volt APU . . . if you're lucky.
Pretty much the same situation arises when you want to charge your 24 volt battery in the hangar. Of course, you'd buy a more expensive charger that could handle all kinds of voltages. But, chances are, if you're like most of us, you already own an inexpensive 12 volt battery charger.
Finally, maybe only the folks flying aerobatic type homebuilts or those who have to have a battery located in the cockpit area really care - but have you ever seen a 24 volt Gel Cell battery?
I don't know about you, but I've convinced myself that the 12 volt system is the only practical one for my RV-6 project.