question

Sorry new to this site and to me not very clear to use! my message was Victron energy Phoenix 12/50 charger the main connection is to two batteries in a bank, and a lead going to engine battery, the main battery bank is fine, at the engine battery only getting 6.?v taken the front of the charger in case it was a bad connection of lose wire, but at the charger it is exactly the same, I have checked the four 20amp fuses but they seem all ok is there somewhere else I need to check ? for info it has ben running almost non stop for a few years, but only just notice the very low voltage on the engine battery (which I will admit it is a very old battery)