You obviously have an external regulator, the make and model would be helpful.
A disconnected “sense” lead might show those symptoms. A 16 volt level, (I assume this is the voltage the alternator is putting on the battery terminals) sounds like it might be an equalizing level. Check to see if the regulator is somehow set on “equalize”. Perhaps it is the maximum voltage the alternator can put on the battery and the alternator is running wide open. You could try disconnecting the regulator and applying 12VDC directly to the alternator to check it’s max output. It could also be that the regulator is defective. I suggest you install an ammeter on the output of the alternator to really see what it is putting out. There are available clamp-on DC ammeters as test instruments, a shunt/millivolt meter combo, or just a simple DC ammeter in line with the alternator output. For my boat, I built an alternator control panel that allows me to choose between a 4-stage regulator and a big rheostat to feed the field on the alternator. It also has an ammeter and a battery selection switch (start, house, both, off) with field disable in the off position. My main engine oil pressure switch is a spst two-terminal type. I have it rigged to supply 12VDC to the engine instruments and alternator field supply only when the engine is running. All this gives me great versatility but I find that what I actually use almost always when underway is the alternator running on manual regulation with the full-on rheostat setting charging the house battery (which supplies my underway loads – radar, plotter, running lights, autopilot and so on) at about 40 amps. My alternator is a old style Chrysler externally regulated unit. Running this way it puts out about 40 amps. When I apply 12VDC to the field directly from the start battery positive terminal the output jumps up to about 55 amps so I suspect that the wiring through the oil pressure switch limits the current to the field thus limiting the alternator output to 40 amps. Full-out field current is normally around five amps so it doesn’t take much restriction in the field supply wiring to slow things down. Not-underway house battery charging is from PV/wind generator/genset, not the main engine. I have one of Andina’s very excellent Combiners (a voltage sensitive relay that connects two batteries [typically House and Start] together when one of them is being charged) installed between the start and house batteries. It is the old style and the controlling battery is the House since it is the one most charged. With the new Combiner models either battery that is being charged is the controlling battery. This removes the work in keeping two batteries topped up. I keep close track of the House battery condition and just check the Start battery s.g. and water level occasionally. Norm S/V Bandersnatch Lying Julington Creek FL 30 23.8N 081 25.7W
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