Battery bank voltage problem

[nwclassic]

We have a 2002 Classic with 4 T105 6V batteries in series for 12V use. They are 8yrs old and have always been charged with its 3 stage charger. Constantly plugged in, periodically added water and have until recently held +12.7V unplugged... now 12.45V after a few hours unplugged. Charger gives 14.4/13.6/13.2 when plugged in.
Suspecting a bad cell somewhere but hoped to get some good thoughts before separating, removing and testing. Popular wisdom says if one is found all get replaced.
Any help or advice before launching into the Rx would be much appreciated.

[idroba]

T 105's and their equal seldom last more than 8 to 10 years, even with excellent care and charging, such as you have been giving them. I was able to get 10 years out of mine, but that was very unusual. Honestly, I would just go ahead and replace them all. It is time, judging by their age.

By the way, you have a series / parallel arrangement, with two 6 volt batteries in series, connected in parallel with a second set of two 6 volt batteries in series. If all were connected in series you would have 24 volts

[nwclassic]

his has been my first experience with 6V batteries (yes, you're right about the wiring). I'll suck it up and get at the replacement. BTW, I'm at +11yrs with the tow vehicle batteries - still strong