Battery Voltage Question

[Analgesic]

Four years ago I made the switch to four 6V golf cart batteries from Sam's Club for my house bank as recommended here.  They have served me well but for the last year whenever the boat was either on the mooring for a week or over the Winter ignored for any length of time, the resting voltage on the bank would drop to 11.85-11.90 range.  On the hard with my AC cord plugged in to charge or when motoring, it will zoom up to 14.5 V but drops right back down once unplugged.   
Yesterday I disconnected and removed all 4 batteries from the compartment and testing them individually, two had identical 6.05 V readings and two read 5.7 V. My questions: first why would this evolve when they all form one bank, can I try to charge up the two weaker ones away from the boat and see if they will hold steady, or can I replace the two weak ones and keep the two stronger ones using the same brand and model?  It's been an expensive Winter and new batteries were not in the planned budget.  Thanks for any help.
Brian McPhillips

[sailaway]

Brian I just went to my boat over the weekend, and checked the voltage on each of my 3 batts. They were bought at walmart 8 years ago at the same time. They set all winter in Ohio with the neg unhooked. They read 12.5v 12.6v an 12.7v  every spring I don't know why they are my house bank. I have a starter batt mouted aft. Charlie

[Stu Jackson]

Brian, most likely they sulphated.  I just replaced some four year old batteries (12V) but exhibited the same bad behavior.  They were Powerline.  I bought three new US Battery 12V 130 ah US31DCXC.  In discussion with the vendor, who sells a lot of batteries around here, he reiterated all of the "How to properly charge your batteries" stories that have been evident ever since Ample Power wrote their Primer years and years ago.  FLA batteries like to be charged, whether at a dock or by solar on a mooring.  Most of the stuff is in the "Electrical 101" topic.

[mainesail]

Brian,

When were they last equalized? Do you have a Charles Charger/ Battery Killer installed?

Sounds like you have a fairly heavy parasitic load on your batteries....

Batteries should never "zoom up to 14.5V" especially from 12V or so. It should take well over an hour to attain 14.5V unless your charger or alternator are HUGE..... This "zoom to 14.5V" is a prime indicator these batteries are sulfated from chronic undercharging or being left at states of charge below 100% far too often.

If your 6V series parallel batts were never pushed to 15.5V then they can remain "out of balance" when put in series. The best way to fix this is to wire all the batts in parallel and charge them to 7.75V before putting them into service, but this requires equipment you don't likely have.. It could also be because your bank wiring is done incorrectly and is causing an imbalanced situation.