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Critique my G3X VFR load analysis

Draker

Draker

Well Known Member
Recently working on a load analysis for my build's future electrical system, for the purpose of correctly sizing my alternator. This is a pretty simple single alternator, single main battery, small backup battery system. But it has a pretty much fully loaded Garmin G3X setup. Was hoping I could get away with a 40A alternator, but unless I estimated something wrong here, that doesn't look achievable. I cannot squeeze myself under 40A (minus 10% safety margin) in either takeoff or approach configurations.

Can anyone see a flaw in my calculations or reasoning here? If not, it looks like I'd be better off with a 60A alternator than a 40A and will need to pay the weight penalty.

 
Flap motor is used for 5 seconds. That really doesn’t belong in sustained loading calcs for capacity. Remove that and you fit in a 40 amp alt. Nicely. Several other numbers seem high. Lemo headsets draw less than .1 amps. Alt fiel is an amp or two once the battery is topped off and running a 15 amp load. Will you really be running a virb during a night approach?I have a similar setup and draw only 13 amps in day cruise, and that is with two EI's drawing a couple of amps.
 
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Ryan,

I went with the 60 amp unit.

I think you could probably assume that all of your devices will not be at max current at the same time. It’s unlikely that you would charging your phone and backup batteries at max current and running trim and adjusting your flaps at the same time. However, how much derating do you want to do. Personally I’d want my expected max load to be around 30 amps for a 40 amp alternator.

I think your USB ports are 4A at 5 volts. So that’s 20 watts or about 2 amps with some charger inefficiency. That is unless your charger is a linear charger which I doubt.
 
Hey, I never really gave it much thought but what is the draw backs of just going with 50 or 60 amp and having a huge surplus of power available? I admire the spreadsheet and the amount of work that goes into making this.
 
Considering your worst case scenario being your NIGHT column, some of your loads listed will actually be turned off. Don’t include short term momentary loads as those will be supplied by the battery such as flaps, transmit and servo motors which will get recharged immediately after. I doubt you will be charging your phone during takeoff as others have said.

If I recall, my B & C 40 amp alternator is rated at 40 amps MINIMUM. I seem to recall it actually being closer to 50 amps but is de-rated so you may want to call them and ask.

My panel is similar to yours and I rarely see more than 30 amp draw except when recharging the battery after engine start which is pretty much complete before takeoff.

Bevan
 
Disadvantages of too big of an alternator are weight, cost, and the greater
ability to fry the battery and other loads in case of an over-voltage situation
(assuming no over-voltage protection).
 
We just went with the B&C pad mount alternator. We wanted to take some weight out of the nose of the Rocket to allow for a EarthX in the tail. The pad mounted alternator is the same actual unit as the belt driven one however the pad spins it slower at a given RPM so less amps. I see 37 amps available at 2700 RPM and 33 amps at 2500. So far it seems to handle all my loads with ease. I have a single screen Dynon HDX, Garmin 175, old school lights/strobes, D10a, Dynon radio and intercom. Load in day cruise with strobes on and pitot off is 14 amps. 20 to 22 amps with Dynon pitot on but if in cloud needing pitot heat will secure the strobes. Next mod will be new lights and strobes(fly leds perhaps) to bring the loads down more. So far very happy with the setup.
 
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lr172 said:
Flap motor is used for 5 seconds. That really doesn’t belong in sustained loading calcs for capacity. Remove that and you fit in a 40 amp alt. Nicely. Several other numbers seem high. Lemo headsets draw less than .1 amps. Alt fiel is an amp or two once the battery is topped off and running a 15 amp load. Will you really be running a virb during a night approach?I have a similar setup and draw only 13 amps in day cruise.
Click to expand...

Good points. I can ensure camera is not operating during night ops to save some juice. So these get updated to "OFF".

Bevan said:
Considering your worst case scenario being your NIGHT column, some of your loads listed will actually be turned off. Don’t include short term momentary loads as those will be supplied by the battery such as flaps, transmit and servo motors which will get recharged immediately after. I doubt you will be charging your phone during takeoff as others have said.
Click to expand...

Yea, I have quite a few things marked as OFF at night and I'll add a few more. Good point on the momentary loads. This analysis assumes a worst case, but as someone else pointed out, a momentary load above the alternator's output will just stop the battery from charging momentarily.
 
Here's a look at my load analysis spreadsheet. It hasn't been 100% finalized but it will give you another data point.

load.pdf


Open the image in a new tab or window and you'll be able to read it.
 
The thing that caught my eye was the 8 amps from the pitot heat, along with 6 amps for landing lights. (Flaps have already been mentioned). IIRC Cessna put landing lights into the ‘intermittent use’ category (e.g., it was acceptable to let the battery carry that load for the 5 minutes they were in use). And of course, landing lights are not even required, so you could leave them off if you so choose. For pitot heat, I assume you have one of the more modern, temperature regulated units. Unless it’s very cold and you are at high speed (when you won’t require landing lights) the pitot will draw less. The only time you’ll need both pitot heat and landing lights is if there’s visible moisture (and it’s cold) all the way to the ground. In that case just hold off on the lights until you need them, let the battery carry the load for a few minutes.
 
Thanks to everyone who pointed out that rarely or intermittently used devices' loads can be safely carried by the battery. This is something I did not think of.

I guess one other argument for going with a larger-than-required alternator would be future proofing. If I ever one day decide to put in a big ol current sucking IFR navigator or another comm radio, I'll need the extra juice. But I suppose at that time I could swap out to a bigger alternator.
 
Hallo gent's,
my approach to the load analysis is quite different, I started from an assumption:
I will not take off with battery not enough loaded considering that I will need to land without alternator charging or without battery ( failure ).
So ( FAR ) if my alternator gives up in flight I must have 30' of partial load to land safely. The battery does not have the nominal capacity ( age, temp, efficiency) the alternator suffer from temp and low RPM.... so I calculated how much time is neaded after start ( energy absorbed from starter ) to recharge my battery with a stated load. My spreadsheet is developed in time( so it is easy to avoid to consider to use the flap motor all the time you flight ) and efficiency and not only in load. ( for example remember that you need 14,2V to charge a battery , the current of a resistive load at 14V will not be the same as at nominal 12V, the battery will give you the 80% of the electrons charged at 12V or less ).
Here the link at my simplified spread sheet :https://www.dropbox.com/s/bxdxwe301zs490u/analisi dei carichi rev 19:08:18.xls?dl=0
It is written in Italian but I hope it will be easy to translate the few words.

Claudio
 
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