Alternators Putting Out Strange Amperage

[mestrezat]

I left port a month ago with rebuilt alternators(one for starter batteries and one for house). When I start the engine, typically the volts go up to 14.1 and the batteries start to charge. It slowly goes down to 13.10 as they charge.

Today when I started the engine,(my Webasto heater had just started up on the house set of batteries) I only showed 12.2 to 12.5 tops. Running at 1500rpms on our Ford Lehman 80hp engine. As we motored where we needed to go(no wind of course) it would slowly go up to 12.5 for a short time. If we reduced the RPMs to 1200, the voltage would go up(strange huh?) to 12.8 or 12.9. and still not make much amps. The "start" battery alternator is cranking at 14.1 volts. I did have my inverter on so I could run my computer as I was going through some tricky areas that I needed AIS info on. I'm sure it took some of my battery power while it was running but to knock us down to 12.2 to 12.5 seems a bit big a drop. I've checked the belts and they appear to be nice and tight.

The boatyard that I had been in had the alternators rebuilt while I was in dry dock since they looked quite blackened by belt dust(did I get conned?). There seems to be a good bit of the same black dust on the alternator again.

Any suggestions on what to do to check the alternators to see if something is wrong? I'm still new to this boating thing.

[DeepFrz]

The black dust is a sure sign that your belt is slipping or misaligned. How big is the alternator (amp rating)? Your belt may be to small for the alternator load.

You haven't given us much info on what type of regulator you have (external/internal) and the hot rating (amperage) of your alternators. Also how many amps was it putting out. Try starting the engine with no load on the house battery and then bringing the load on after the engine has run for a while. Some external regulators have a "soft start" feature that prevents the alternator from putting out current for a minute or so and then gently ramping up the output.

[GordMay]

Originally Posted by DeepFrz

The black dust is a sure sign that your belt is slipping or misaligned...

Which is (counterintuitively) confirmed by the ouput voltage increasing with reduced RPM. The belt slips less at slower speed.