Boat battery/electrical issue

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.
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.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Comments
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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
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?
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.