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Ambient temperature effect on voltage?

Apologies for the newbie question. I bought my RV-4, and I'm still getting familiar with the intricacies of the electrical system.

Is it normal for alternator output (or bus voltage) to vary with ambient temperature?

I went flying yesterday to warm it up before changing the oil. Everything electrical seemed normal. Resting voltage 13.2V (EarthX battery), with the alternator on it was charging at 14.something V.

Came back in and changed the oil. By the time I was getting ready to run it again to check for leaks the temperature had dropped to probably 2-3C. Running it with the cowling off, the alternator seemed to be outputting 15V! My EarthX warning light didn't come on but I shut it down pretty quickly. Actually, first I switched off the alternator master which brought the voltage down a bit, but not immediately to 13.2V, which I thought was strange. I would have thought de-energising the field would cause it to drop immediately.

Is the high voltage an expected effect due to no cowling and cold air? Or am I starting to see alternator issues?

It's a 35A IR automotive alternator with a Zener diode for OV protection.
 
A schematic would be helpful, but in general temperature is not going to change the voltage you observe.

13.2V is nominal for the EarthX line. 14.3 - 14.5 is what you should expect to see if the alternator is online. If you are seeing 15V then I would be concerned your alternator is going pear-shaped.
 
Depends if the regulator uses temp compensation.

A lead acid battery can take higher charge voltages when it's colder.
 
Thanks for the help guys. Here's the relevant schematic:

https://ibb.co/5cR8LFS

I stand corrected -- it's externally regulated but the regulator is attached to the back of the alternator so I didn't notice it.

The temperature compensation point is interesting. I'll be at the hangar later so will get the p/n of the regulator and look it up online. I'm also going to put the cowlings back on and run it again to see if there's any difference.
 
Warning - you may have a big issue, one that needs to be resolved before next flight.

The EarthX BMS will isolate the battery if buss voltage exceeds 15.5vdc. You got to 15vdc (which in itself is a fix it now issue) BUT depending on the accuracy of your voltage readout the alternator could well be above 15.5vdc.

The EarthX BMS does a fine job of protecting the battery on over voltage, but it does not protect your expensive avionics from the over voltage condition. Failed alternator regulators can (and have) put 90+Vdc onto aircraft avionics, resulting in immediate and permanent failures. Anyone running EarthX needs a reliable over voltage protection - perhaps even a primary and a backup.

The EarthX BMS is a much different animal than how we were raised on standard Lead Acid batteries. A Lead Acid battery would mitigate the over voltage condition by absorbing excess current to create heat. The battery will be destroyed over time but could buy the few minutes it takes to isolate the alternator and save the panel. I had one alternator go high on me, going above the crow bar over voltage protection set point. I had adequate time to turn off the alternator before anything bad happen.

So - do not start up the engine before you fix the alternator.

Carl
 
Uh ya

A zener diode is not overvoltage protection for a bad alternator. It will fry when 60 amps at 15 volts go thru it. 60x15=900 watts.
OV protection is normally a high current short that collapses the alternator voltage so that the circuit survives. 300 amps at 0.3v ( transistor fully on) is 90 watts, not 900 watts, which is survivable by electronic circuits for a short time (pun intended)

I would take Carl’s advice and go thru the electrical system since the plane is new to you.
 
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Thanks all. Unfortunately I didn't see some of the replies on this thread before I went back to the airport. With the cowling on, everything was normal again -- a ground run showed normal voltages, so I went flying.

I wasn't able to see any part numbers at all on the regulator, so my assumption is that it must not be temperature compensated, and the cold temperatures led to fluctuating voltage.

However, now I've seen the comments about the zener diode not being sufficient OV protection (I thought it was a case of 15V +/- would cause it to blow and shut everything down... didn't realise current came into play) , I'm going to go order a B&C alternator + regulator setup.

I'm not looking forward to reworking that whole system. Unless there's some way I can crowbar (no pun intended) suitable overvoltage protection into the existing system without replacing everything else?
 
here is my two, 02 cents ideas.....
..
Tim E.....per post #9......""" I'm going to go order a B&C alternator + regulator setup.""
....
FIRST recheck what you my have touched / bumped taking off the cowling,and changing the oil,
and recheck your wiring. and "pull" on the all wiring lugs, in the charging circuit.
SECOND changing the alternator and regulator, is a "DAM" good idea, it could be the fix a intermittent problem ,
and prevent big headaches, on a cross country.
../.. good day / rick

Q.O.T.D.
Tony Bingelis said it this way "machines are always talking to you, you have to learn to listen"
 
Just to close out this thread in case anybody is reading it in future, I haven't yet had any recurrence of high voltage, and I went ahead and ordered the Perihelion OVP module (https://www.periheliondesign.com/lovm.htm). He did me one with the trip voltage lowered from 16.2V to 15V, so that it will trip at no more than 15.2V which is the point at which the EarthX battery gets unhappy.

Have yet to install but it looks really simple and at $88 the price was right! Thanks all for the pointers.
 
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