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

AX-O · #1 10-21-2008, 10:39 PM

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.

Geico266 · #2 10-21-2008, 10:44 PM

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.

Mark Castle-Smith · #3 10-22-2008, 12:50 AM

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

kiwipete · #4 10-22-2008, 05:01 AM

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

Peter

PaulR · #5 10-22-2008, 06:41 AM

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

L.Adamson · #6 10-22-2008, 06:52 AM

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

L.Adamson ---- RV6A/ 60 amp

jayres · #7 10-22-2008, 06:58 AM

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.

the_other_dougreeves · #8 10-22-2008, 07:10 AM

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

rocketbob · #9 10-22-2008, 07:19 AM

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.

flytoboat · #10 10-22-2008, 07:23 AM

Quote:

Originally Posted by PaulR

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

Check out Vertical Power's website under the documents tab.
http://www.verticalpower.com/