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.