Is my battery no good

[chinook88]

I have an 8hp Johnson sailmaster out board on my boat. My battery is about 18 months old and I have noticed that if I go past about half throttle my voltage goes up to 16 - 18 volts. Below half throttle the voltage is between 12 and 14. Some people have told me that it is the battery others say the regulator. Anybody who can help please.

[noelex77]

First check the meter is reading correctly.

The output from most small outboards is unregulated. If the battery is full you need to switch off the output (there is some risk of damaging the diodes if you do this while running) otherwise the voltage can rise to dangerous levels.
16-18V is dangerous. It will damage the battery and there is some risk of an explosion is a spark ignites the gas given off.
Wear eye protection if you go near the battery when it has been at these voltages, but check the water level if its a flooded battery.
The maximum voltage you should allow is 14.8V flooded battery or 14.2V GEL or AGM.
So the regulator is not broken because there is probably not one fitted. The battery has very likely been damaged by the high voltage. If its reaching 18v with the small output from the outboard it also suggests its not very healthy.

There have been some threads on how to test the battery if you do a search.

[PaulfromNWOnt]

I would speculate that the very small charging circuit on an 8 horse motor uses voltage instead of amperage to charge your batteries. There is every possibility that they are fine. I wouldn't consider this as a time to panic, but I would certainly inquire with knowledgeable sources on the matter.

[fryewe]

If you buy one of the new microprocessor controlled battery chargers at an auto parts store, and put your battery on charge with it, it will tell you whether your battery is bad. Every time I have had a bad battery, more times than I like to admit, this type of charger has refused to charge the battery. The voltage indication on the front panel indicated 18.8 volts, I believe, and flashed, and the lights on the front showed no charging current. Cost: about 30 dollars. Will charge flooded, gel, or AGM from 115VAC.

[hellosailor]

Batteries can't make excess voltage above their rating, so the folks that say it is your battery are the folks you just smile at and ignore. they're dangerously misinformed.

As others said, your engine may not have a regulator in it. If it has a regulator, that's faulty. If if doesn't, you might want to add one. A battery won't be outright killed by 14-18V from time to time but electronics will be. And the battery will need water too often.

Even if all you have is the battery and some nav lights, at 18 volts the lights will burn out quickly. Unless the engine puts out so little power, that the voltage drops right down to 14-ish when the lights are turned on. That's possible too.