Small alternator

[Greg Arehart]

A general question for the electrical folks on here. I notice that I'm using somewhere around 5-7A of power (avionics, lights) on a regular basis just flying around. Obviously larger power draw when using the flap motor or starter or such, but those aren't used very much or very long. So, I've been wondering whether a small alternator would be sufficient to keep the battery charged for a modest panel and other electrical needs. I'm thinking here of the vacuum-pad-mounted B&C 20A model or something similar. Seems it would allow for some weight savings compared to the larger 50- or 60A belt-driven models that are more typical. I'm not going to be changing things out any time soon, but just idly wondering about weight savings on a potential future project.

So, any reasons why this wouldn't work reasonably well, if the power draw is not too much?

Greg

 

[1bigdog]

Deeply discharged battery

Greg

Nothing wrong with your thought process but the alternator should be sized for worst case scenario, which is usually when you have a battery deeply discharged after long start attempts. It will that take 1/2 or more of the capacity of the alternator just to replenish the battery.

Then add everything else on top and make sure you are not discharging. You will probably be back to the size of your current alternator. The light weight b&c on the vacuum pad is usually for backup purposes. There are how ever lightweight full size alternators and starters.

Cheers

Michael B
C-FWMB reserved
College fund paid up

 

[rzbill]

The vacuum pad turns too slowly to create a "weight efficient" power generation solution.
As an example of that, please know that the BC-20 vac pad alternator uses the 40 amp belt driven alternator parts with a different casting to allow vac pad mounting. Because of the slow vac pad speed, you only get 20 amps and that is at rated speed. On the ground, your electrical system would be sagging at idle. The belt driven units spin much faster and will generally supply plenty of power at 1000 engine rpm.

with that, please understand I do support vac pad use (I have a BC-8) as long as one understands the limitations.

 

[AllThumbs]

Hi Greg,

I can attest that the 20AMP (or even the 8AMP) B&C pad mount will do the job. I lost my primary belt driven on the Toga the other day - the B&C kicked in and got me home just fine. I did not have to shed ANY load on our over-stuffed panel. I did, of course, because I wanted to make sure the battery was topped up for gear, flaps, and landing lights.

NOTE: RZBill is correct that, below about 1500 RPM, our pad driven B&C was not producing full amperage.

Call me if you want details. Also, I called to thank B&C and they mentioned they're considering a higher capacity version of the 8 AMP in the same form factor using a new regulator.