Charging question, while we're at it

[MikeZ]

A couple of years ago I upgraded my 79 P337 alternators to 65 amp to accomodate electric AC.... ended up putting in a Riley engine driven AC and I'm COOL now..... but the alternators don't start charging till well over 1500 RPM..... this has occasionally been a REAL problem......

anecdote: The other hot day on the ramp at TEB, after a long hot taxi, waiting among the jets for IFR departure..... rear engine starts to cook, I'm try keeping the RPM down, so no charging.... hmmm....temps coming up....shut down radios..... still temp is too high on rear engine.....shut down rear engine..... OK, cleared for takeoff.... uh oh.... try to fire up rear engine.... no voltage left in battery, so radios lose frequencies.... there I am rolling onto the active with no rear engine and no radios..... oh shit......

I 180 and taxi off with no radio contact, lucky it was clear.... jets all around me.....somehow I start my rear engine, rev the front, get my frequencies back on the radios (had to reboot) call tower, and tell them what happened.... they say OK, GO..... so I did ....

SO, back at the shop, put a new battery in..... it was two years old anyway....

but really, if the alternators charged at lower RPM it would not have put such a load on the battery. The old standard alternators kicked in earlier. The regulators on the front firewall don't seem to be adjustable. Or is there a way?

Any ideas or comments?

Next time I'll shut down sooner, keep the temps down.....but it would be nice to generate some charge at lower RPM....

[Kevin McDole]

Mike,

I question the value/benefit of converting to the 65 AMP alternators. I have air-conditioning and a single 38 AMP alternator has no problem (in flight) producing the 20+ amps the entire electrical system plus air-conditioning is requiring. I would think the bigger alternators would just put more stress on the direct drive coupling and presumably reducing it's reliability. Or, did you covert to a belt driven system? (I think the O2's had those - and it would seem like changing the pulley ratios would be a simple way to solve the problem).

Even with the 38 AMP alternators, the power output at idle is inadequate. The mechanics tell me that this is symptomatic of IO360's, not just the Skymaster installation. Seems to me that the gear ratio that drives the alternators is wrong.

I recently converted to the 400 series regulators because I could not get the alternators to share the load. When idling, one would do all the work and the other would sit at 0 AMPS - even though the battery was discharging. Simply turning off the working alternator would cause the idle one to pick up the load, but they would not share the work if they were both online (on the ground, in flight they shared a little bit). So, under these circumstances, I'd have to run the engine with the "working" alternator at 1500 RPM on the ground.

Now after the conversion, I can run my air-conditioning and avionics with both engines running at 1200 RPM. Between the two alternators, there are enough Amps to run everything.

Can you see that both alternators are outputting Amps equally? Do you have the EI ammeter?

If I read your post correctly, it sounds like you shut down your radios manually to reduce the rear engine temp? I doubt you'd be able to see any temp benefit from reducing the electrical load (if that was your intent). And besides, if that were what you were trying to do, then shutting off the rear alternator would have been the way to do it (thus reducing 100% of the alternator load on the rear engine). Regardless, I think your only recourse in that case is shutting down the rear engine. And when trying to start the rear, you probably want the front engine at a high RPM (and therefore lots of amps available) so you don't experience that shutdown glitch which scrambles your radios frequencies and the brains of all the electronics on board. I had the problem previously and I had to one by one pull and reset various circuit breakers to get things like the JPI, Shadin, and autopilot out of their brain-dead states.

[Mitch Taylor]

Kevin:

The O-2 alternators are coupled, not belt driven. They are 60 amp alternators, and don't seem to strain things too much. Mine come on line right away, I think I get positive charging at about 1000 rpm.

[Bob Cook]

re alternators. Mine is a 74P with air Mike. I can sit in Flordia at 90+ degrees without a problem and with the air running. Both alternators are putting out about 10 amps each on the ground at low idle.

Re overheat. Make sure you do not lean the engines when this hot. USe fuel cooling to assist in keeping the CHTs down.

A trick I have at TEB and other airports is using the handheld to get atis and clearance. I then switch to ground an ask if an immediate departure is available due to potential heating problem with rear engine. I usually get their cooperation, otherwise I just ask to remain on the ramp until a slot is available. This saves sitting around at idle waiting for hell to freeze over.

HATE TEB. They jockey you all over the east coast at 4k ft. !!!!

Bob

[MikeZ]

Thank you all for replying.

I shut the radios to lower the load and save the battery. In hindsight I could have revved the front engine to generate some juice and maybe re-starting the rear wouldn't have imploded the radios.....

Please, Kevin can you provide more details on the 400 series regulators? Cost? Part #? I understand they are adjustable, whereas the standard regs on my firewall are not.

I would not have changed out to 65 amp alternators if I knew I'd go with engine driven air conditioning.....

No, Kevin, don't have any sophisticated ammeter. Next on my list. Any recommendations?

Thank you all.

[GMAs]

I haven't seen one yet that was not adjustable... and even if you go to the 400 series alternator regulator.. your going to have the same effect... again... they alternators won't balance... its a physical impossability.... because you have 12 feet of cable on ones output and 1 foot... on the other.. now can you calculate the IR loss .. if not look in the 43.13.. under voltage drop of cable... again its not the same for each... as they have to be offset to make them have the same output at the battery...

I think you all should make a reservation for the meeting on the charging system... then we can all sit down and see what is happeing... smile...

As to the higher output ... we are seeing the 85 amp 28 volts belt drive alternator that can either come off the prop flange or off the right angle drive at the rear behind the vacuum pump area...

will keep you all informed... as to when the higher ones are aviable.. and they are quite smaller...

GMAs.

[Damon Banks]

Another technique on the (rear) engine-overheating-on-the-ground-waiting-for-a-clearance thing...

#1 Whenever possible park facing into the breeze! This alone will almost guarantee the temps will behave.

#2 If there's not much breeze, run the front engine at 1500RPM or so, with the rear at low idle. The front benefits from flow generated by the propeller (although it will heat up somewhat), and the rear gets a breeze too!

Regards,
Damon

[Bob Cook]

you can also "nozzy" up close to a "47". they generate a good "breeze".

make sure you keep your nose pointed toward the breeze and step on the brakes. <GGG>

Bob