Boat Charging Problem

by Stevie

OK then John Payne - I've paid the £35 and bought the "Bible", but I still can't find an explanation to my charging system!!

I have recently aquired a US-built Mainship 390 trawler yacht with a CAT 3126 350HP main engine with a 55 amp 12v alternator, with a pair of large D8 deep cycle 12v batteries, with shore powered Sentry smart charger system. There is a panel meter reading the voltage of both batteries and the engine volt meter also. All read the same, so I have eliminated dodgy gauges.

The alternator has also just been replaced with a new CAT exchange alternator - and yes I should probably have requested an 80 amp unit instead of a 55 amp!
On start-up, the voltage when the fuel valve solenoid is opened drops from 12v or so to read only 11 or 11.5 volts. No problem starting, and the voltage sits there at maybe 11 volts. After maybe 5 minutes, it starts to climb towards 12 volts and then flicks back down to 11 volts and then back up to 12 volts. After maybe 10 minutes flicking up and down, it finally climbs up to 13 volts and finally after maybe 15 minutes, when the engine is warm, it gets up to just under 14 volts and then stays there quite happily looking after bow thruster needs, radar, GPS, plotters etc with no problem.

No electrical book tells me if this is OK or I have a problem. Any ideas as to where I should look? I would have expected it to go straight to 14 volts or so and maybe drift back a little.