Alternator charging intermittently

[StevenT]

I have a 1985 5.7 mercruiser alpha one. I replaced my suspected bad alternator last fall with a new marine alternator. Also a new battery. I ran the boat on muffs, volt meter said 12.7 in the driveway. Took the boat out Sunday and volt meter read 14 for a while. anchored and only had 12.5 volts. Motor would not run abouve 3500rpm. Any advise what to check?

Thanks
StevenT

--------------------
StevenT

 

[chargerboy]

Re: Alternator charging intermittently

Was the engine idling at all three tests with the voltmeter? If the engine is revving higher, it's going to make different readings...

 

[Icefire55]

Re: Alternator charging intermittently

normaly the voltage should be pretty constant as alternator are actually 3 phase with diodes, and only the frequency change with speed (the battery act like a capacitor to level the voltage)

I would guess a loose wire on the charging circuit (ground or positive), look for corroded connexions

 

[StevenT]

Re: Alternator charging intermittently

I tested voltage at 1500rpm each time

 

[chargerboy]

Re: Alternator charging intermittently

Then I agree with Icefire, make sure all your electrical connections on the alternator, engine ground, and battery cables are good and clean of corrosion. I like to sand them down till they are shiny and put a coat of dialectric grease, or you can use vaseline, to protect them from future corrosion.

After you do this, check again. Your problem doesn't sound big enough to worry about, you just need a little tune up maintenance to your electrical system.

 

[Glastron_V210]

Re: Alternator charging intermittently

The altenator/battery voltage WILL change with rpm, up to a maximum (Usually between 14 and 14.5 volts).

The voltage it charges at is much more a function of the state of the battery.

In other words, at the same rpm, if the battery is discharged the voltage will be at a lower voltage than with a more fully charged battery.

If the battery reads a higher voltage when you are running than when you are stopped it is charging...end of story. It could still be overcharging if your voltage exceeds 14.5 volts at any time.

As far as the 3500rpm deal, sounds like a totally different problem.

 

[chargerboy]

Re: Alternator charging intermittently

The altenator/battery voltage WILL change with rpm, up to a maximum (Usually between 14 and 14.5 volts).

The voltage it charges at is much more a function of the state of the battery.

In other words, at the same rpm, if the battery is discharged the voltage will be at a lower voltage than with a more fully charged battery.

If the battery reads a higher voltage when you are running than when you are stopped it is charging...end of story. It could still be overcharging if your voltage exceeds 14.5 volts at any time.

As far as the 3500rpm deal, sounds like a totally different problem.

Glad somebody else agrees with me, didn't think I'd lost my mind there. He does say that all 3 tests were taken at same rpm, so it's likely just bad connections. Thanks for verifying.

 

[Icefire55]

Re: Alternator charging intermittently

Sorry but car alternator are 3 phase, 6 diodes, voltage regulated AC alternator, same for boat but explosion proof.

An alternator runs 2-3 times the speed of the motor, so from 750rpm to 6000 rpm, the alternator runs between 1500-18000 rpm, so the voltage do not change with the speed.

One thing aftect the voltage is the load, at idle, there is almost no load, the more you go up in speed, the more you need "sparks", lights, etc.

Voltage should be 13.8 to 15 volts, +- 0.5 volts from rpm/load change.

If you have more load and the voltage drop, you might have burnt diodes (you can have only 2 diodes on 6 that are good, and still charge the battery, at 1/3 the power)

DC generator that were used it the old times, that costed a fortune, weighted a tons were DC and the voltage was changing with the RPM alone.

 

[chargerboy]

Re: Alternator charging intermittently

Even though the voltage stays the same, amperage goes up, charges the battery more, and gives you a different reading. In theory what you are saying is right, the alternator is designed to never overload anything voltwise. Hook a multimeter up to your battery sometime, rev the engine higher than idle for about 10 seconds and check your readings on the battery. They will go up.

 

[chargerboy]

Re: Alternator charging intermittently

If you're wondering, my friends and I have tested this theory when somebody called bluff on the idea of revving an engine to help jump start a car faster.

 

[Icefire55]

Re: Alternator charging intermittently

reving the engine is good when you jumpstart a car because the dead battery will ask for 100's of amps from the good one and alternator. plus if you try to start it, it will ask for about 250 amps at inrush.

By hooking the dead battery on the running car, you are maxing your alternator (load at max) 60-100A and you also have a dead 0-10volts battery in paralele to the good one so the voltage can be as low as 12 volts from the good one.

 

[StevenT]

Re: Alternator charging intermittently

Thanks for the info guys! I had a bad ground. That fixed the problem.