Judging actual alternator need

[jwilbur]

I'm trying to judge what size alternator I actually need. For example, if all internal cabin lights are on, all external lights are on, pitot heat is on, both AP servos at full torque, flaps moving, fuel pump on, all transmitters transmitting, all USB plugs drawing ~3 amps, etc, in other words all possible electrical current being drawn at the same time, I'd need an alternator to supply over 80 amps. Obviously this just isn't realistic.

How have others judged your actual alternator needs? Any lessons learned by way of bad assumptions?

Thanks for the help,

[Mark Dickens]

I'm no electrical engineer, but the way I figured it, first, the odds of all that happening at the same time are fairly low, second, remember you're getting power from the battery which is more than capable of providing power for that short period of time you'd need that much. I wouldn't worry about it too much and build on. I think I remember all my loads adding up to more than the 60A that my alternator would provide, but when am I going to run every last electrical device in the plane?

[Bill Boyd]

How many of those loads will be on when you are in night IMC trying to get home without draining the battery? THAT's the load target. Everything else is hypothetical/unrealistic/what battery reserve capacity is for.

-Stormy

[Carl Froehlich]

Unless you are running 747 landing lights, a 60 amp alternator is fine.

Now - don't forget that you have responsibility on how you run your electrical system. The first job of your alternator is to recharge the battery. If you jump a dead battery to get the engine started and launch into IMC, even with an alternator putting out more than needed to keep up with what you have on you will have no battery reserve if the alternator fails.

Determine your reserve electrical capacity need (in flight time and then into amp/hrs) and then design and operate to always have at least that much available.

Carl

[Taildrgr]

I did my planning based on continuous load only, ignoring transient loads such as keying a mic, boost pump, etc. The assumption is as previously stated, that the transient load runs off the battery and the alternator will replace the used amps. So, if you fly IFR, the the design point is a night IFR landing - lights, pitot heat, EFIS power, gps, etc. Mine came to 48 amps. I used 80% design factor and came up with a 60 amp alternator as adequate. To finish the design, you would then have to decide how much battery capacity in amp-hrs you want in case your alternator failed. For that I used an emergency buss amp load for one hour. I found on actual flights I am running about 40 amps under the high load scenario so the calculations were a bit conservative. Day VFR I run 13 amps. Landing with boost pump and landing lights is 25 amps. I have dual Dynon 10 inch screens, GTN-650, and a second comm radio.

[jwilbur]

All makes good sense. Thank you for your insight.

[Canadian_JOY]

Originally Posted by jwilbur

I'm trying to judge what size alternator I actually need. For example, if all internal cabin lights are on, all external lights are on, pitot heat is on, both AP servos at full torque, flaps moving, fuel pump on, all transmitters transmitting, all USB plugs drawing ~3 amps, etc, in other words all possible electrical current being drawn at the same time, I'd need an alternator to supply over 80 amps. Obviously this just isn't realistic.

How have others judged your actual alternator needs? Any lessons learned by way of bad assumptions?

Thanks for the help,

Two thoughts...
Firstly, from experience, electrical load analysis almost always produces some interesting surprises. Most folks don't take into consideration that many items use a constant number of watts, so as voltage decreases (alternator failure) amps consumed increases. This can catch a person by surprise.

Having said that, every ELA that I've done has proven so conservative that I have stopped adding "fudge factor" to the equation. It seems almost every equipment manufacturer has already added their "fudge factor".

My second point is around the price/performance tradeoff point. If you are trying to get away with an alternator driven from the vacuum pump pad, be prepared to spend some bucks on low-power devices. LED nav/strobe lights are a great place to start (the typical incandescent position light is a real power hog!). Likewise, incandescent or halogen landing lights are really hungry for juice. There's a surprising amount of power consumed by traditional incandescent lighting in the cockpit. LEDs can really cut your power consumption and are likely the best "bang for the buck" in terms of reducing power consumption. Next on the list is the careful choice of avionics. As an example, a Trig TT22 transponder consumes less than half the power of a GTX330ES while providing the same functionality. Newer EFIS systems which use LED backlights are more efficient than older ones using cold cathode fluorescent backlit displays.

All that to say that sizing your alternator properly can be a function of more than just doing the math - if you don't like the answer you get, play with some of the choices of other devices you're installing!

[MarkW]

I have a full glass/electric panel and installed a 60a alternator. I fly IFR and almost never draw more than 20 amps.
Only to replenish the battery after start-up for a few minutes.
I do have LED strobes/Nav and landing lights and no heated pitot.
I should have gone with a 40 amp.

[rocketman1988]

You fly IFR with no pitot heat?

Gutsy move, Mav...

[rzbill]

I believe Aeroelectric gave examples of analysis.

It is a chart with every (really every) electrical part in your craft listed down the left and the selection of operating conditions across the top. Example operating conditions might be taxi day, taxi night, taxi ifr, takeoff day, ifr cruise night etc.

The amperage loads are entered into each cell that is operating in that flight condition. Total each column at the bottom. Decide whether you want the alternator to support every condition. Buy.

For mine ( a full on night and IFR craft) a 40 amp belt driven alternator was plenty.