Mercruiser 260 Alternator overcharging?

[areoseek]

Getting the boat out of storage and prepped for the water this year, I was running her on the hose and noticed that after warming up I was looking at 14.7 volts according to the chartplotter. charger unplugged, engine running at 1500rpm. This was very curious to me, as I've never noticed it before. Unless a boat differs from a car, I shouldnt be looking at more than 14 volts correct?

If this Is too high, Is there an external regulator, or is it internal in the alternator?


Might have to replace it if its inside. if so, Can I use an automotive one from any 350?

Sorry. lot's of questions. i'm a car car not a boat guy.


It's at the mechanic's right now getting the shift cable and ujoints changed in the outdrive, but when it get's back, i'd like to make sure i'm not boiling my batteries.


The plot twist is at my radio deck (audio), I can't turn it up above 17 (medium loud) or it turns it self off and says "Low voltage".
Indicating I may have too much wiring and i'm loosing DC voltage over distance, or it may be too thin.


*bought this boat at the end of the summer last year, and am just now addressing problems I noticed*.

 

[Bondo]

Ayuh,.... A low, under charged or dead cell battery can cause that,..... I'd start there, testin' the batteries, 'n their connections,....

Can I use an automotive one from any 350?

Absolutely Not,... It has to be ignition protected, as in Marine rated,.....

 

[harleyman1975]

Alternators make sparks when they are running. Marine alternators have screens to contain the sparks automotive alternators do not.

 

[Scott Danforth]

your battery voltage display may much more accurate on your chart plotter than the $3 gauge in the dash, or the $1.25 gauge in your car. Or the calibration could simply be off. 14.7 volts is slightly high, however normally it is 14.2 or so.

the fact that turning your radio up causes a low battery alarm would indicate a bad battery or a bad connection as Bondo indicated, and your alternator is trying to bring the voltage its seeing on the excite line up.

under no circumstances ever install an alternator, starter, ignition system, or any electrical device that is not J1171 compliant (Marine rated for prevention of sparks)

 

[areoseek]

Ayuh,.... A low, under charged or dead cell battery can cause that,..... I'd start there, testin' the batteries, 'n their connections,....



Absolutely Not,... It has to be ignition protected, as in Marine rated,.....

Batteries are both good, brand spankin new and fully charged, both sitting at 12.8 volts when I installed them.

I'm curious to see if, when I get her in the lake, The voltage will go up even more when I get to WOT. Since running at idle it reads 12.9, at 1000rpm it reads 13.4, and at 1500 upwards of 14.7
wondering if it's proportional. I know on a car it is not SUPPOSTED to be.(to an extent).

When she comes back from mechanic i'll check out the wiring situation behind the radio and take some voltage readings.

Not much more I can do untill then.

 

[Scott Danforth]

the output curve on an alternator is ALWAYS RPM dependent for amperage, and feedback voltage/ RPM dependent on voltage. this is the case for all alternators, irregardless of automotive, marine, locomotive, or heavy truck. the only type of AVR that holds it dead steady on voltage is on gensets, however they are designed for one engine speed not designed to charge batteries

at idle, your alternator probably has the ability to only generate about 5-10 amps, and about 13 volts at most. most alternators dont come "on" until you get above 2500 RPM rotor speed, and most alternator pulley drives are about a 2.1:1 drive, so that would be over 1200 RPM (engine) before the alternator actually starts to charge/support loading.

you need to check all your connections. each battery cable connection (both battery and motor ends) needs to be shiny enough to eat with (ask yourself if you would you put your existing cables in your mouth). then when the connection is cleaned and completed, seal with marine electrical varnish. if the cable/terminal is green, buy new cables

 

[achris]

Make the measurement with a PROPER calibrated digital multimeter (such as a Fluke). Instrument voltage indications are just that, indications.

 

[Lou C]

Just went thru this with my OMC. Excessive resistance in the sense wire circuit was causing over charging. I re wired it as per the instructions that came with the new Arco alt I installed. I put a good digital meter on the batteries and got 14.2V. Before that I was getting 15.5V. But you could not tell from the dash gauge due to resistance in the old dash wiring.
Always put a good digital meter on the batts...

 

[JustJason]

14 to 14.9 is considered acceptable. 15 on up means the alternator is overcharging. Before changing the alternator, check for a bad battery first, as a bad battery can cause an alternator to want to over charge. If you have a maintenance type battery that you can check the cells with a hydrometer, check them. If you have a sealed/maintenance free battery, swap it with a known good battery and try it. Also make sure your battery cables are in good shape with clean end connectors with minimal voltage drop across the cables.

 

[alldodge]

The plot twist is at my radio deck (audio), I can't turn it up above 17 (medium loud) or it turns it self off and says "Low voltage".
Indicating I may have too much wiring and i'm loosing DC voltage over distance, or it may be too thin.

Batteries are both good, brand spankin new and fully charged, both sitting at 12.8 volts when I installed them.

I think you have a corrosion issue and need to start cleaning connections from the battery and throughout

 

[areoseek]

14 to 14.9 is considered acceptable. 15 on up means the alternator is overcharging. Before changing the alternator, check for a bad battery first, as a bad battery can cause an alternator to want to over charge. If you have a maintenance type battery that you can check the cells with a hydrometer, check them. If you have a sealed/maintenance free battery, swap it with a known good battery and try it. Also make sure your battery cables are in good shape with clean end connectors with minimal voltage drop across the cables.

took them to work today and load tested them. 997 cca out of the starting battery, and 866 out of the deep cycle.