I don't know where you are cruising, which ambient temperature?
What internal regulators many times missing is temperature compensation, because no temperature
sensor at the
battery mounted.
If you
cruise in the tropics very important. So an external
regulator should have always a temperature
sensor mounted at the
batteries.
If you are a sailor you have the risk of overcharging the
batteries.
For example: You are on the
engine and your
regulator is already from bulk to acceptance mode to float mode.
You stop the
engine, because you think there is enough
wind for sailing.
But 10 minutes later you start the engine again.
Regulator stays in acceptance mode on a fixed time based setting and was already in float.
At the time of restarting the engine regulator goes to bulk and AGAIN to the long acceptance mode, before going to float. At this point you have the risk off overcharging and damage your batteries, especially
AGM, wet cell loss only some more
water.
I just post my experience about the regulator I am using in another blog:
Arduino based Alternator Regulator Project - Cruisers & Sailing Forums
This regulator monitors also the amps. If the amps in acceptance mode are very low, means batteries are full, regulator switch to float mode. AND don't stay to long in Acceptance mode
So if you buy a external
alternator controller, look for one that also ""read"" the amps that goes into the
battery, your battery is more protected for overcharging.
Another protection on a external regulator is protecting from the
alternator for
overheating, with a temperature sensor on the alternator for regulation.