Why is alternator slow to charge battery?

[Amphibiographer]

With no load on it, at a fast idle, my 35 amp Motorola alternator takes over 25 minutes to raise the voltage of my starter battery from its resting charge of 12.6V to 13.5V As soon as I lower the engine revs to a slow idle the voltage begins to drop 0.01V per minute. If I stay at fast idle and turn on the Bilge Blower the battery voltage drops more than 0.6V at about 0.05V per minute.

Is this happening because the voltage regulator is not working? Why does voltage keep dropping under load when I'm running the engine more than 1200rpm under load or with no load? Am I draining the battery by running with the bilge pump on? Can I run the Atomic 4 without the bilge pump on after I use it to clear any fumes before starting the engine?

The bilge blower seems to draw a lot of amperage. What would an average amp draw for a blower in a 4" diameter hose be?

I keep the starter battery charging on a 21W solar panel all the time just in case I'm draining it by running with the bilge pump on.

Thanks for any help with this.

[Dave Neptune]

The alt should be running positive easily at 1000 with the blower, GPS, radios and even a few lights. Either your belt is slipping, the alt is failing or you have some weak connections.

Look at the voltage the alt "sees" IE the exciting wire or the lead from the coil.
Look for weak connections on the battery leads and engine to battery ground. Poke around you may not be delivering enough voltage to truly get the alt's "field" to get excited.

Dave Neptune

[Amphibiographer]

Thanks a lot for this info. I have checked the connections. The field flashing lead from the alternator to the Pos coil was almost broken so I repaired it.

Don Moyer said in another post that the orange cable from "the large output terminal connects either to the ammeter in the cockpit or the big battery cable on the starter solenoid (preferred by most)"

In my old 1972 C & C that cable goes to the ammeter. The ammeter needle is broken so it doesn't work. Would that be causing problems? Should I disconnect the orange cable from the ammeter and connect it to the starter solenoid instead?

[Dave Neptune]

Where does the "other end" of the ammeter connect? You could connect the wires and see if the broken gage was the culprit, doubtful to me.

Check the connections as you go with the volt meter and jot down any voltage drops.

Dave Neptune

[tac]

An easy way to quickly bypass the ammeter (if you have easy access to the back of the ammeter) and not have to run a new wire, is to remove one of the orange wires from its stud on the ammeter and reconnect it to the other stud with the other orange wire. This connects the two orange wires into a single wire, bypassing the ammeter. It doesn’t matter which wire you move. Do this with the engine off.

If you’re curious, and have a voltmeter or multimeter, before doing the above run the engine and check the voltage drop across the two ammeter studs. You should see a voltage drop of no more than a few tenths of a volt. This would be the voltage drop across an internal shunt (copper or other metal of known resistance) - the greater the alternator current, the greater that voltage drop.

The bilge blower draws about 4.5 amps or more.

[jpian0923]

For me, my alternator doesn’t even turn on til I rev the engine to above 2000 RPM.

[ndutton]

A few observations:

1. As a battery's state of charge (SOC) comes up, the charge rate slows down. A resting battery voltage of 12.6V equates to roughly 80% SOC so the last 20% will by design be the slowest to charge by any alternator. The concerns you're reporting may be a case of expectations vs. reality.
   
   
   
   
   
2. Post #5 suggests eliminating the ammeter by connecting its wires together. The whole point in eliminating the ammeter is to eliminate the long, voltage drop inducing wiring back and forth to the engine panel from the alternator output to the point of use be it battery charging or any other loads.
3. I did not read that the voltage had been measured at the alternator output post and compared to the input voltage at the battery. That will tell us the health of the alternator and what effect the ammeter run is having on the charging system.

[Amphibiographer]

Why is alternator slow to charge battery?

Thank you all. Our boat is on a mooring in the local bay far from any electrical source. The back of the atomic 4 is accessible only through a lazerette where movement is constricted and tools can barely be manoeuvred so it's taking a while to work on all of this.

I hadn't considered that the last 20% of battery charge takes much longer than the first 80% so I am going to measure the voltage at the alternator output post and compare it to the input voltage at the battery as nduttton suggests.

I will also measure the voltage drop between ammeter studs. If there is a significant drop I will move the output wire coming from the alternator.

[Amphibiographer]

After all the tests it turns out that the old 35 amp alternator is not "getting enough excitement" from the wire between the field generators? and the coil.
The slip ring is worn and lower brush is not quite long enough. Rather than a rebuild I bought a new 60amp Wilson alternator from my local repair shop. The alternator now charges up to 14.7V at cruising revs ( 1200rpm - 1500rpm)

My partner had to grind 1/4 inch off the top bracket on the engine head to lift the alternator high enough to tension a 25" belt. But the alternator extends a bit further out and there is a whine that sounds like metal on metal. We replaced the alternator fan with one that has slightly shorter blades but it is still making the whine - like a dentist's high speed drill. The belt on the new alternator pulley is at a 5 - 10 degree angle. We think it is rubbing on the part of the distributor cap that sticks out.

Question:

1.Would a longer support arm to raise the alternator move the belt away from the distributor cap?

This Forum has been an invaluable resource. Everything about engines and electrical systems is a new learning experience for me and my partner. It probably takes 5 - 10x as long for us to problem solve and then figure out how to fix something. Thanks again all experienced old salts for your expertise and willingness to help.

[ndutton]

Rather than a rebuild I bought a new 60amp Wilson alternator from my local repair shop. The alternator now charges up to 14.7V at cruising revs ( 1200rpm - 1500rpm)

Two critical questions:

1. Is the new alternator ignition protected?
2. Do you have electronic ignition?

[Amphibiographer]

We don't have electronic ignition. There is a battery switch which isolates the starting battery from the house battery (or they can both be accessed at the ALL position.

I don't know whether the new alternatior is ignition protected. There is an internal ground and I also added a ground cable to the engine block. I will ask the shop when it opens this morning. Why would that matter?

Thanks for the questions.

[Peter]

Ignition protection prevents sparks getting out of the alternator. In an enclosed engine compartment in a boat with a gasoline engine, this is important to prevent potential explosions should you have an undetected gas leak (blowing your boat up is a poor way to detect such leaks). Starters and solenoids should also be ignition protected.

Best,

Peter

[Dave Neptune]

Fumes and flames

It matters because a spark from it could cause an explosion or start a fire around gas fumes. That is why you need a marine type instead of automotive!

Dave Neptune

[Amphibiographer]

Yes, it is ignition protected. We are going to try installing a longer 10" supporting arm to move the belt away from the distributor cap.

[ndutton]

Can we please see some pictures?