Dual battery question

[Daz1971 Lurker Find all posts View Profile Close Daz1971]

Running a dual battery system & suspected alternator failure.
Question is- does the alternator put charge into secondary battery?
Both batteries without running were 12.6 and when running the secondary battery was high 13s. Does this seem right?
Have ex mining vehicle with isolator switch & wondering if this could be the cause.
Have traced problem to dodgy earth so hopefully no alternator outlay for now anyway.
TIA

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Alternator charges both batteries, 13.8V-14.4V is ok. 120 OEM alternator is temperature compensated and should have no problem charging both batteries.