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Alternator Not Charging (Enough)?

S

ssokol

Well Known Member
Sponsor
Greetings Folks,

I seem to have some sort of charging issue with my RV-6A. It has a 35a Denso alternator which appears to be making power as the voltage on the Dynon SkyView reads 14 - 14.1. However, the Dynon also indicates a drain of -1 to -3 amps. I thought perhaps it was the battery so I replaced it (new Odyssey 680) but it did not appear to help.

This morning I did a test. Charged up the Odyssey to full using a 2 amp charger. Flew for ~40 minutes, with the Dynon indicating -3 throughout the flight. After landing I put the battery back on the charger and waited. After 30 minutes it was still not showing full.

My first guess would be the alternator, but it's putting out the right voltage. It has an integrated regulator so I can't point the finger at the reg. Anyone have a guess what this might be? Is it possible for an alternator to fail in some weird way that it can make rated voltage but not push enough amperage to both run the systems and charge the battery?

Thanks!

Steve
 
I wouldn't be surprised with the amperage being off just a little bit in reading. If it's sitting at 14+ volts the battery is staying close to full. My experience with trickle chargers is that they charge for quite a while before sensing the battery is full even if it is full to start with.
 
Oops, should have mentioned...

I should have mentioned that I noticed the problem when I was unable to re-start the airplane after a short flight. The battery wouldn't spin the prop / mags fast enough to light it off. A buddy put a jump pack on it and it fired right up, so it wasn't a hot start issue.

This happened a couple of times. So I ordered the new battery...
 
Multiple thoughts:

Make yourself a test harness. Clip directly on the battery + & - terminals, & run the leads into the cockpit. Check voltage with a known-good, accurate digital voltmeter. Then check at the same point, in flight, with all electrical loads off, then with all normally used electronics on and operating. With a 35A alternator, you might have enough electrical load in the plane to pull charge voltage *at the battery terminals* below what's needed for full charging.

You need close to 14.3 volts to reliably/fully charge an SLA battery (Odyssey, etc). Any resistance losses between the alternator's B lead and the battery's + terminal, *OR* between the alternator's case and the battery's - terminal, will reduce the charge voltage at the battery, where it matters. I've had corroded battery terminals prevent charging, even when the circuit would start the plane. With SLA batteries, you may not see the nasty white foamy powder common on automotive battery terminals, but if you unbolt the connection, the copper/brass terminals can have a layer of corrosion that has high resistance. Same can apply to the wire/terminal interface, which is almost impossible to inspect directly.

Amp readings are tough to interpret from a distance, without knowing exactly how the current sensor is installed (there are at least three ways to configure a current reading system).
 
Odyssey publishes a minimum charging Amperage of 6A. I can not find a minimum voltage but I would assume anything above 12.8v, the published full charge voltage.
My B&C SD8 charges the PC680 just fine at 13.8v.
 
If a diode or two burned out the ALT will still put out voltage and little amperage. Easy to take it to a local autoparts store that tests ALT's. The machine will put a load on the output and check the AMP's out.
 
Battery

I just went down this road and the alternator was the problem. I replaced the plane power with a Nippon Denso and everything is happy now. 12 hour trip and everything is working. If you are reading 12 volts at the battery with engine and alternator running, it is basically battery voltage. I'm not an expert but it worked for me. You can spend money replacing parts but check the voltage.
 
I agree with Fasglass, they can lose a diode, still produce good voltage but not have rated capacity.
Other thoughts,
Odyssey recommends 14.7 volts for charging system.
The battery should be found at 12.8 after resting several hours if it is fully charged.
These batteries take an extended "saturation" phase of several hours while charging to bring them up to full charge, if you are not using an approved charger, or not going on long flights occasionally you likely are not getting a full charge on it.
As others have mentioned, the amperage indication is meaningless unless you understand where it is located in the electrical system so you know exactly what it is sensing, and depending on how current is sensed, the reading may not be especially accurate.
HTH, Tim
 
Last edited:
Check ground

Check your ground wires. I found a broken terminal end at the engine side of my ground wire and my battery was not charging properly.
 
bhester said:
Check your ground wires. I found a broken terminal end at the engine side of my ground wire and my battery was not charging properly.
Click to expand...

+1 on what Bobby said.

The worst case is you have a ground via some small diameter wire (as in avionics) providing a ground for your voltage reading. This is a great way to fry a panel.

I ground the engine on the starter ground lug with the same gauge wire as that feeding the starter (the highest load gets the best ground). That goes to the firewall ground lug - the same one both battery grounds go to.

Carl
 
Follow up on this...

After reading through the responses on this thread I took the alternator to the local O'Reilly auto parts store and they tested it out. Sure enough, it was making 14v but the current was significantly below spec. Not good - this was Friday afternoon and I was planning on leaving for OSH on Sunday.

Fortunately, they were able to track down a replacement and get it to me the next morning (at a rather healthy mark-up). With some help from Paul - resident A&P / IA, Charles Taylor award winner, and owner of the airport - I was able to get it adapted and installed in about an hour. Fired up the airplane and low and behold the battery charged as expected.

I shut it down, let the heat soak for a few minutes, and then tried a hot start. No problem - fired up after 5 blades or so. The next day I packed up and flew it to Oshkosh without any problems.

Thanks, everyone, for the suggestions and advice!
 
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