The first question is how much difference? At 10 to 20 points difference, you might just need a good equalizing charge.  Run your batteries up to about 15.0 volts (that’s for 12-volt systems, you folks at higher voltages can figure it out) and hold them at between 15.0 and 15.5 volts for three to five hours.  This will even out any small differences between cells that have developed over time.  It’s a good idea to do an equalization every two to six months; it’s like a minor tune-up for your batteries.  Equalizing is more important in the winter when batteries tend to run at lower charge levels than in the summer.

At more than 20 points differences, you’re probably looking at a dying cell.  Replacing a single battery out of larger bank usually isn’t a great idea.  If the batteries have seen 50% or more of their typical life expectancy, it probably isn’t a good idea to simply replace the bad battery.  For instance, a set of golf cart batteries (3 to 5 years typical) with 2 years on them is not a good candidate for a single battery replacement.  A new battery installed in an older battery bank will be dragged down to the performance level of the worst cell in the bank.  So don’t mix old and new batteries unless you’re willing to sacrifice a considerate amount of life expectancy on the new cells.  The battery book for your PV home covers in detail what I’ve lightly brushed over here.