Often a boat will have a lead-acid starting battery but you would prefer to use a low maintenance, deep cycle marine battery for the "house" loads. The problem is, older alternators have a built-in regulator set for over 15 volts which will damage an AGM or GEL style battery.
The traditional solutions to this problem are to stick with a lead-acid battery for the house loads and lose all the benefits you wanted, or scrap the lead-acid starting battery, modify the alternator and/or add a smart external regulator.
HERE IS A SIMPLE SOLUTION
Click picture for details.
There is a feature built into the C150 combiners and some models of C100 Combiners that makes it act as a voltage regulator for AGM/GEL style batteries. One of the cables or terminals on the top is labelled "HI VOLTS". When this control is connected to a voltage of about 14.2 volts, it disables the combiner until the voltage drops below this threshold.
You were going to need a combiner anyhow, to isolate the batteries when they are not on charge, so for no extra cost you get a regulator as well.
You can download the owner's manuals from the specification pages (Click on picture above) and connect the "HI VOLTS" terminal to the positive terminal of the AGM/GEL battery bank. This added jumper connection carries no current so a 16 or 18 gauge wire is more than adequate.
In operation, while the house bank is below its maximum voltage, and the starting battery is receiving a charge, the combiner will come on and the green light will show while both banks are receiving a charge. When the house bank reaches the HI VOLTS cut-off voltage, the red light will come on and within a second or two, the green light will go off. The house bank is now taken off charge. If there are loads on the house bank that cause it to discharge below full, the red light will go off, and about 30 seconds later, the green will come back on as it again puts the house bank on charge. The rate of cycling and how long it stays on green will depend on how much load you have on the house bank, and how much current the alternator is putting out.
WHAT ABOUT THE SHORE CHARGER?
If the shore charger is not adjustable and is designed for lead-acid batteries, then connect all its ouputs to the starting battery. The combiner and the HI VOLTS regulator will make sure the house bank is safely and fully charged.
If the shore power charger is adjustable for the type of battery you have your choice. Either set it for lead-acid battery voltages and connect as described above, or set it for AGM/GEL voltage and connect it to the house bank. In this case, the combiner will feed backwards to the starting bank however the starting bank will not be getting a full "lead-acid" peak charging voltage, but the difference will be minor. Depending on the actual voltage put out by the charger, and the internal setting of the HI VOLTS feature, there is the chance that the combiner in this case would see the maximum voltage has been reached on the house bank, and disconnect the starting battery from it. This will probably not be a problem since periodically loads on the house bank will lower its voltage enough that the combiner will come on and transfer any residual charge required to the starting battery.
WARNING: This method will only work if you have the lead-acid battery installed. The regulation feature of the the Combiners CANNOT be used between the alternator on its own, and the batteries on charge. The combiner must ALWAYS have a battery connected to each of its main current terminals. If you put it between the alternator and a battery as a regulator, the first time it turns off for HI VOLTS, the alternator may be DESTROYED due to the high voltage spike cause when the alternator suddenly has no load.