alternator high output of 16 volts?

[Nauti Cat]

My port alternator is putting out 16 volts (as read from port engine battery), I have twin engine with house bank, alternator on each engine and an ACR between eng battery and house bank on each. If I'm unplugged from shore power and only start port engine-I read 13.5 volts to all batteries, but when I start stbd engine-voltage goes up to 16 volts on all batteries. If before I disconnect shore power, if I start port engine- all batteries read 16 volts, if leave port off and start stbd engine w shore power on-batt read 13.5 volts. over all port alternator puts out 13.5 volts unless any other charging source is on , then port alternator puts out 16 volts. Can anyone explain this or suggest anything?

[smac999]

Sounds like you need a new alternator. Assuming the wiring is all clean and good. Are these internal or external regulator?

Do not run like that you will damage batteries and electronics at 16v

Disconnect the acr ground wires for trouble shootingif you are confused which one is causing the issue. So only one battery will go to 16v. Otherwise all batteries will be at 16v like you are seeing.

[GrowleyMonster]

Is this a so-called "one wire" alternator? Or are there two or three small wores coming from it? Or even just one? Likely your alternator is putting out max voltage because the regulator does not sense that the battery voltage is above what should be the cutoff point and is not throttling back. Or a bad regulator. In fact one trick for boosting charge voltage is to put a diode inline with the sense wire so that it can be switched in or bypassed, to manually raise or lower voltage by a set amount. Internal regulators can be replaced, usually, but if you are not familiar with the process a good alternator/starter shop can do this for you pretty cheap and pretty quick. While you are there, consider ordering a spare alternator. Maybe even a starter. Those are very important spares to have aboard, IMHO.

[Lepke]

Most auto parts stores can test alternators. Regulator is probably bad. Alternator probably ok if it can put out 16v.
Internal parts for most alternators can be found online. If you decide to open up the alternator, it's a good time to change the bearings. I rebuild my alternators. It's not brain surgery.

[GrowleyMonster]

Originally Posted by Lepke

Most auto parts stores can test alternators. Regulator is probably bad. Alternator probably ok if it can put out 16v.
Internal parts for most alternators can be found online. If you decide to open up the alternator, it's a good time to change the bearings. I rebuild my alternators. It's not brain surgery.

That's smart thinking. Any problem spec-ing and sourcing the bearings and regulator? That looks like the only real speed bump for me. I don't mind taking stuff apart. Sometimes I even put it back together.

[billgewater]

Many boats have dedicated voltmeters on the instrument panel. Some with audible alarms. This is presumably because boats (cf cars) often have a heap of voltage sensitive instruments turned on when the engine is running.
16 V DC can really damage a lot of stuff and of course, you may cook the batteries as well. Really ruin your day out. The voltmeter connects when the starter key is turned to the instrument on position and stays on until the key is returned to its original position.
On my own tub, I have a dedicated bright digital voltmeter on a wall panel. It is always on. The little side benefit is that it provides a constant small light source which at night is very handy when you can't find the light switches.
It also seems that sometimes the owner or builder will install a secondary regulator between any charging source and the batteries. Consult a marine electrician.
The installation of the voltmeter can be done at the panel, so it is not a difficult task.

[donradcliffe]

A few questions.

Did you measure the 16 volts between the posts of the batteries with a decent digital multimeter, or is this some cheap analog panel meter?

Are the alternators internally or externally regulated? If external, is the voltage sense wire connected to the battery you want?

What do you want the voltage to be when the alternators are on and the batteries are charged? A dumb regulated 14.2 volts or a smart regulated 14.4 volts dropping back to 13.4 volts at float? Better yet, what type of batteries do you have?

I had to deal with this problem on a GB 32. Internally regulated alternator, as I recall of Italian origin. New alternator did not solve problem, and alternator shop claimed alternator tested good. New regulator in new alternator did not solve problem. I converted the alternator to external regulation and used a $25 Napa external regulator to get a 14.2 regulated voltage. Problem solved, and has remained solved for 5 years.

[Nauti Cat]

It's internal regulator and agm batteries. volts measured at battery is 15.75. anywhere between 13 and 14.4 would be fine , but not 15+.

Did another experiment. Drained house battery some by turning off charger and running microwave thru inverter, then started port engine. volts were great at 14 for several minutes, then went back up to 16 volts (i assume as batteries came up to full charge)

it seems when house battery not fully charged, alternator puts out correct voltage, then when house battery gets up to full, alternator starts overvoltage.

Must be just a bad regulator in alternator?

[donradcliffe]

That would by my conclusion. I'd take the alternator out and bring it to a local alternator repair shop. Ask them what make and model it is. If they can't or won't tell you, walk out. Emphasize to them that the alternator puts out power, but the voltage regulator is not working. If the estimate for repair is over $100, look at your online options.

[smac999]

Originally Posted by Nauti Cat

It's internal regulator and agm batteries. volts measured at battery is 15.75. anywhere between 13 and 14.4 would be fine , but not 15+.

Did another experiment. Drained house battery some by turning off charger and running microwave thru inverter, then started port engine. volts were great at 14 for several minutes, then went back up to 16 volts (i assume as batteries came up to full charge)

it seems when house battery not fully charged, alternator puts out correct voltage, then when house battery gets up to full, alternator starts overvoltage.

Must be just a bad regulator in alternator?

The alternator will never put out correct volts untill the batteries are fuller. When batteries are low the voltage slowly climbs Untill the set voltage. Seems yours keeps just keeps going up.