Hamish, welcome. In order for us to help you, we need some more info. Please confirm that it was a direct Hitachi
replacement alternator? [I know you did say it was a direct replacement] External or internal regulator
? If external, what settings?
You may be unaware of the Hitachi alternator issues:
Some alternators though, such as those made by Hitachi and found on Yanmar diesels, are dumber than a pound of beetle poop. Actually, to the alternator, they are pretty smart but to your batteries and the speed of charging
they are flat out stupid. Why?
Hitachi alts with dumb regulators, and some others, limit voltage but also reduce voltage
based on alternator temperature. This is a self protective feature installed in the internal dumb regulator
to prevent the alternator from cooking
itself. Remember voltage is the pressure that allows more current
to flow. So, if we reduce the absorption voltage, then we also reduce the current
the alternator is supplying.. The battery
simply will not accept the same current at 13.4V that it did at 14.4V and as a result the alternator will run cooler. What do you suppose this does to your batteries over time.......?
The problem is that when cold you will get 14.3V to 14.4V out of the Hitachi but as the alternator heats up the dumb regulator begins to reduce the CV/voltage limit based on the alternators internal temperature. It is not uncommon to find a Hitachi alternator at 13.4V when hot. This is REALLY, REALLY DUMB....
If you have a dumb regulator, and notice the voltage dropping, it is likely a temp compensated dumb regulator. Get rid of it or plan to buy new batteries more often.
If you have a temp compensated alternator or a Hitachi alternator on a Yanmar you really are in dire need of external regulation if deep cycling a larger battery
This is from:
Musings Regarding External Regulation - SailboatOwners.com
and these, too:
Hitachi Alternator and Smart Regulator Instal Question
Most practical way to upgrade Yanmar alternator?