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40 or 60 amp alternator? I am at 38 amps.

AX-O

AX-O

Well Known Member
Guys I did my load analysis and I am at approx 38 amps draw during landing configuration at night while talking on the radio. That means pretty much everything ON. I will spend about 5 to 10% of the time flying at night (if that). During day operations I am about half that.

So I bought a 60 amp alternator from Vans and it showed up damaged. Now I am thinking that the 60 amp is way too much and a 40 amp will do just fine. The problem is that I am very close to 40 amp at max load and if I add any future avionics or heated seats or whatever, I would be over the 40 amps output of the smaller alternator.

Would the 40 amp alternator be sufficient or should I go with the 60? Thanks for any guidance or help.
 
I would go with the 60. You are gonna add something later on and you'll be glad you have the extra capacity. Overvoltage protection is a must.
 
Alternator choice

Hi Axel,

There is no choice! You should aim to install an alternator that has at least a 20% margin. So the total load you anticipate should be no more than around 80% of the alternator capacity.

I have the B&C L60 with the external Regulator and over-voltage protection. Beautifully made and has been totally reliable, a friend has the Planepower AL12-EI60 60A alternator and is similarly pleased.

Regards

Mark
 
Remember to add battery recharging into the equation as well. As if it's not fully charged it will take some Alt load.

Peter
 
Load Chart

A little thread drift. Sorry.

Sometime back someone posted a link to an EXCEL spreadsheet to do a load analysis with. I can't find it again. I would appreciate a link or a copy if someone has it.

Thanks
 
These day's, 60 amp alternators are much smaller than the old ones; so why not...

L.Adamson ---- RV6A/ 60 amp
 
As an electrical engineer, I would most certainly go with the 60 amp alternator. Not knowing what the design margin is on the 40 amp model, you could be running much hotter at nearly 95% of rated capacity, thus reducing the life of the alternator. Plus like others have said, you should always plan ahead for expansion.
 
It kind of depends on what you had on and how often you expect to have that kind of load on. If it's only very infrequent that you're expecting / seeing 38 amps and you already have an 40amp alternator installed, it might be Ok, unless you just want overkill or need to spend $$.

How many / what wattage are your landing light(s)? Do you have pitot heat? Conventional strobes and nav lights?

On the CT, the 100W landing light and other loads quickly overwhelm the Rotax's alternator when the engine is at low RPM, and voltage will drop quickly to and below 12V. Thankfully, the dual EIs are driven off separate coils on the alternator.

TODR
 
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Most 40 amp alternators put out well over 40 amps. I draw nearly 40 amps with everything on in my RV, and have a 40 amp alternator, plenty of night flying. No troubles.
 
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Where do those amp come from?

What I mean is, when you turn on your seat heater (I don't have one) and it increases the amp draw on the alt. How much HP does it take from the engine? Will your rate of climb decrease? Does fuel flow go up?

Just curious.

Kent
 
Back of the envelope calculations

kentb said:
What I mean is, when you turn on your seat heater (I don't have one) and it increases the amp draw on the alt. How much HP does it take from the engine? Will your rate of climb decrease? Does fuel flow go up?
Click to expand...

Worst case with alternator load maxed out:

60 amps at 14V = about 1 KW

1 KW = about 1.3 HP

Assume efficiency of say about 75% for alternator: therefore 1KW takes about 1.7 HP from the engine.

1.7 HP / 160 HP is about 1% of your engines total power output. This should not affect performance too much.

A seat heater draws about 6 amps, or say 100 watts. That's 0.1% of total engine power.
 
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AX-O

The fact that you mention "while talking on the radio" in describing your loads in your original post leads me to believe that you may not be analyzing your electrical loads correctly. This item alone may not make a difference, but talking on the radio is not a constant load and should not be considered in the analysis - your battery will easily take up the slack on such short duration loads.

If you care to post your list of potential Constant Load items and their current draw, the group could probably supply a more useful critique. I was amazed how LOW my loads were, and went with a B&C 40 amp alternator.

regards

erich
 
erich weaver said:
AX-O

The fact that you mention "while talking on the radio" in describing your loads in your original post leads me to believe that you may not be analyzing your electrical loads correctly. This item alone may not make a difference, but talking on the radio is not a constant load and should not be considered in the analysis - your battery will easily take up the slack on such short duration loads.

If you care to post your list of potential Constant Load items and their current draw, the group could probably supply a more useful critique. I was amazed how LOW my loads were, and went with a B&C 40 amp alternator.

regards

erich
Click to expand...

All I was trying to say was that the worst situation (everything working at the same time) would require about 38 amp. Over 90% of the time my loads will be around 20 amps. The strobes, nav lights and taxi/landing light are a significat draw on the system. Will try to type a list later. Thanks!
 
I dont know the specifics of the alternators in question, but this is somewhat timely as today at the shop we got in a load of standard chevrolet style Mando alternators. They are rated at 65 amps, but each comes with a computer print out from the load tests the factory does during QC. All put out over 85 amps at their fastest speed (6,000 rpm). I assume the 65amp rating is a comfortable, continous load, not technicaly what they can produce as clearly they can put out a fair margin more. With that said, just on general principal, I would go with the bigger alt and leave room for future expansion (bigger radios, heated seats, dvd player for the wife :)
 
Are you concerned about

Over voltage events?

If so the Plane power alt is 60Amps ans its a very small unit with OV protection...Seems to be a slam dunk choice to me if you have an expensive panel

Frank
 
I had same question

When I did my load analysis it came to just over 40 Amps in a night landing configuration. Over 10 Amps was for both HID landing lights and the fuel pump which only need to be on for the last few minutes before landing. Since I will rarely fly a night and my daytime max load is about 18 amps , I decided to go with the 40 Amp B&C alternator.
 
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