Originally Posted by Lady Fitzgerald
Snip.......However, I have had batteries show full voltage at rest but drop dramatically when a light to medium load was placed on them (usually, an open or high resistance internal connection is to blame). Even a partial charge (usually what is called a surface charge) can show full voltage but, when loaded, will discharge rapidly. The only way to accurately check a battery with a meter is to put a load on it. Even a hydrometer test will can be invalid if the battery has an open or high resistance connection that allowed a trickle of current in to charge the battery.
I agree with Jeannie 100%.
To really know the condition of the battery, do a hydrometer test of each individual cell, then measure the battery voltage under a small load with an accurate voltmeter. At full charge with no load, it will measure 12.7 volts, and discharged under load will measure about 10.5 to 11.0 volts, for a 6-cell lead acid battery.