Dennis's points are very good, and I second the Trojan
website:
http://www.trojanbattery.com/
We have their batteries in a PV-light system in a high
mtn cabin.  Their tech support is excellent, too.

Anyway, one point:  To use voltage to measure the charge
of the battery, not only must it be "open" voltage as
Dennis points out--no load--but the batteries must "rest"
for some period of time.  Trojan says 6 hours min and
preferably 24 hours.  That is to allow electrolyte to
completely mix.  Under charging or under load, the solution
next to the plates reflects what's happening and the
electrolyte isn't uniform in the cell.

See:
http://www.trojanbattery.com/Tech-Support/BatteryMaintenance/Testing.aspx

                                Richard A Jones
                                Boulder, Colorado