What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Alternator Load Analysis

BOHICA

Well Known Member
Anyone done/have a load analysis for their plane? My analysis looks ok for a 60A alternator for continuous flight.
60A x80% = 48A continuous load target. Stay less that this all good.

Kicker is Night flight landing with Pitot heat on puts me at or above 60A with what I am calculating for continuous load. I am looking at using FlyLED Seven Stars for lights @ 7.5A ea, 15A Total. Garmin GAP 26 Pitot heat is 12A Max (varies with temp). So up to a 27A night landing adder. Lights don't fall under my continuous load but exceeding max current draw for alternator would be an issue.

General System Spec
Dual G3X
650xi or 750xi
GMC 507 Auto Pilot dual axis
GMA 245 Audio Panel
GTR 20 Comm
+Supporting bits.

Just trying to figure out what people are seeing for loads on similar setups and what calculations where done beforehand.
 
My real-world results are much lower

I have similar Garmin equipment, with 3-axis AP and unregulated pitot heater. LED lights all around. With everything operating I see 16-18 amps. I don’t recall seeing higher than that except right after starting with a less-than-optimal battery charge. Hope that helps.
 
This is what I came up when planning, Actual ended up being 15 - 18 amps in cruise with wig-wag lights on, strobes, all avionics, and without pitot heat on.
 

Attachments

  • Screen Shot 2022-01-11 at 7.04.42 PM.png
    Screen Shot 2022-01-11 at 7.04.42 PM.png
    64 KB · Views: 299
Last edited:
Here's mine for a RV-7 with a 60A alternator.
 

Attachments

  • load_analysis.jpg
    load_analysis.jpg
    430 KB · Views: 345
Anyone done/have a load analysis for their plane? My analysis looks ok for a 60A alternator for continuous flight.
60A x80% = 48A continuous load target. Stay less that this all good.

Kicker is Night flight landing with Pitot heat on puts me at or above 60A with what I am calculating for continuous load. I am looking at using FlyLED Seven Stars for lights @ 7.5A ea, 15A Total. Garmin GAP 26 Pitot heat is 12A Max (varies with temp). So up to a 27A night landing adder. Lights don't fall under my continuous load but exceeding max current draw for alternator would be an issue.

General System Spec
Dual G3X
650xi or 750xi
GMC 507 Auto Pilot dual axis
GMA 245 Audio Panel
GTR 20 Comm
+Supporting bits.

Just trying to figure out what people are seeing for loads on similar setups and what calculations where done beforehand.

Dual G3X
G5
GTN650
GTX45R
GMC507
GDL51R
GMA245
Regulated GAP26
2- Baja Design Squadron Pro @ 5 amps each
AeroLED Pulsar 180s +Suntail
2-FlyLeds singles @ 1 amp each
2-FlyLeds duals @ 2 amps each

With everything ON I see about 27 amps continuous; when the strobes flash, add +15 for a total of 42 amps momentarily...
 
Some good info here on this topic. I will work into my spreadsheet then might become a good item for WIKI. If on the WIKI could be updated with more items and make a selectable tool for determining your load. Give me something to do while I wait for QB Fuse/Wings.
 
Here's mine for a RV-7 with a 60A alternator.

Michael,
Nice Work.
It really is a nicely detailed spreadsheet I wouldn't mind using as well. It is electrical and avionics for the winter.
Any chance to get a copy?
Thank you in advance!
Ken
 
If you would like a excel version of this spreadsheet just email me at the address in my signature.

EDIT: Unless someone knows of a way to share an Excel doc on this forum....
 

Attachments

  • load_analysis.jpg
    load_analysis.jpg
    430 KB · Views: 161
Last edited:
Great care is needed with this subject - I've done a bunch, most supplied by builder that I've looked at are inaccurate!

The bottom line is that the real world consumption is usually way less than the theoretical number.

The first step is to write down all items in your aircraft that consume electricity. Every individual item with the current or power consumed - from manufacturers' figures, from measurements or from estimates. The things most usually missed are master solenoid (uses about an amp) and the alternator field (can be up to 4 amps). USB socket (perhaps 2 amps) and aux socket (unknown power draw, say 5 amps max).

Next identify what is an intermittent load and what is continuous, flaps, radio transmit, trim, starter button/contactor are intermittent and can be discounted from power requirements calculations. Most other stuff is continuous. Take into account the voltage of the system when calculating current requirements from power consumption. Group equipment by busbar. If one busbar is powered through a diode deduct a volt from the bus voltage and increase current values accordingly.

Figure out the total consumption with all continuous loads on. Allow 20% of alternator capacity to charge the battery.

The second part of the calculation is to figure out how long the battery will last when the alternator quits. Re-calculate current requirements based on 12.5 or 12.2 volts. Use the 1 hour rate from the battery - for example a PC680 1 hour rate is something like 14Ah. It is worth de-rating the battery capacity to take account of that recent engine start and for a battery that has been in service for a year or two - so take 80% of the 1 hour rate. BTW the certification requirements use 72% of the 1 hour rate. Most owners are surprised how short a time the battery will last for with everything switched on! Figure out what must be switched off to get at least 45 minutes endurance and write that list in the POH.

This whole question is more complex than it first appears. It only bites your butt when something goes wrong, which is when you wish you paid more attention when the airplane was built!

Peter
 
My -14A is similarly G3X equipped and I run on average about 20 amps and a few more if the GAP26 pitot heat is on. The 60 amp stock alternator is way plenty of head room for the proposed system.
 
This is a good exercise for every builder to do. I suggest however that the last step of the exercise is the most important; “Theory to Practice”.

You will need a nice instrumented ~30amp regulated power supply (available for ~$100 and prefect as a battery charger as well). Connect the power supply to your main buss via a fuse or breaker. Have most if not all loads off. Apply power and bring up the voltage on the power supply starting around 12vdc and note current as you bring up the voltage. When you get to 14.1vdc or so the current should stay about the same, if so this is one indication of a healthy battery.

Once stable, start bringing up your loads, logging power supply current as you do.

The last step is to have all the panel up and turn off the power supply. Log the time it takes for buss voltage to fall below 12vdc. This is your battery reserve IF you have a fully charged and healthy battery.

For IFR I design for at least 2 hours of battery reserve (regardless of number of alternators).

Carl
 
Back
Top