Boat battery/electrical issue

Estragon

An older 20' inboard/outboard runabout with a niggly issue. 

The starting battery (flooded) is getting long of tooth and should be replaced soon.  Before doing so I'd appreciate any thoughts on the niggle. 

The boat is used to/from the cabin, ~10-15mins each way.  During the trips, the dash voltage meter spikes suddenly to max (~18v), and an alarm sounds (which I think is the halon system complaining about high voltage).  It can last anywhere from a second or two to several minutes, and can happen multiple times on a given trip, or not at all.  It never happens at low throttle or warming up at the dock, only underway, which makes it difficult to confirm with a different meter or troubleshoot.  I'm kind of suspecting a failing alternator, but I would have expected unregulated voltage to be anchored somewhat by the battery (as opposed to spiking suddenly from 12.x to >18 in a second.  Today I measured 12.2v before leaving the dock.  The dash meter showed voltage fairly stable for ~10mins, then spiked.

Maybe the battery is getting full enough to not anchor the voltage?

I've sanded,  greased, and retightened the battery connections to no apparent effect.

Marc Kurth

I would suspect the voltage regulator first.

Estragon

I'm pretty sure the alternator is internally regulated, which I guess means replacing it?

Marc Kurth

If it is internal, yes. But I wouldn't be in hurry unless it pegs that high voltage for long periods. If your battery has enough energy to start the engine, it is far from an "open circuit" to mess with the alternator. How old and what brand is the engine?

Estragon

It's a Mercruiser, likely original to the 1990 boat.  About 1/2 hour is the most I expect the boat to be underway, with 10-15mins being typical.

The battery does start the engine (grudgingly if cold), but is old and due for replacement anyway.  I'd rather not damage the replacement though.  Put a costly new lower drive on it a couple years ago, and hope to get a few more years out of it without putting a ton of money into it.

BB.

Have you ever had problems cranking the engine? Lights that were dim when the motor was not running? Possibly a bad wiring connection somewhere between the engine alternator and the battery? Or possibly a battery will a failing grid?

If you can put a current meter or DC Clamp DMM on the battery or alternator cable and see if you see a high surge of charging current (probably a bad internal regulator or other alternator issue), or not (then possibly wiring issue taking the battery out of the circuit and not buffering the minimum current output of the alternator).

-Bill

Estragon

I haven't noticed anything with dim lights, but rarely drive in the dark. It cranks "normally", considering battery age, temp, and thicker oil when cold.

Battery with a failing grid is possible.  It's flooded and mobile, so an intermittent shorted cell may be a suspect.  It's been going on for some time though, and doesn't seem to be related to rough conditions.  Today was rainy and smooth as glass.  Still, speed changes could slosh things around a bit and cause an intermittent short.  If that's the case, would an alternator voltage regulation typically limit the SC voltage?

mcgivor

Intermittent faults are the hardest to find, the likely cause is a poor connection as a component failure would be permanent. Where I would start is checking all negative grounding points including engine straps making sure all are clean and tight. Then move on to the positive side, rationale being a single ground negative fault can affect multiple positive functions by seeking negative paths through components such as motors and yes gauges. Lessons learned solving auto electrical faults.