So the first question is, was the system working at least for a short time with the Anco alt? Or was the problem right from the get go? If right from the get go, it could be a
wiring or compatibility issue. If it failed after awhile, it will be the charge controller.
The issue is that the next step regulator is a "smart"
charger. It provides the three steps of
charging, Bulk, absorption and float. If these fail, then what you are experiencing is usually the result. You are lucky the
batteries have held the voltage down at 15V. The big issue with these and as far as I know, all makes of theses alt smart chargers is there is no protection to stop the Alt from going over voltage. Either the batteries boil and can be wrecked, or worse, the system voltage goes so high that
electronics are damaged. Of course, the alt. can also be fried from excessive
current production causing it to over heat. During absorption
charging, the voltage will go over 15V, but the current should be low. So do ensure that it isn't just in absorption.
If anyone knows of a make that is protected from over voltage, please let me know.