Tel Co's float their lead-acid batteries at 2.17 volts per cell. Nominal cell voltage is considered to be 2.08 volts per cell. At this float charge rate, they generally last for 10-15 years before needing to be replaced. Most of the time, new strings are charged (at 2.5 volts per cell) for a pre-determined amount of time (generally in the range of 1-1 1/2 weeks), then the plant voltage is dropped back to float voltage for the string. The most damaging thing for a lead acid cell is the discharge. If it goes too deep, they are prone to the cell reversing polarity. Once that happens, it is very difficult to undo, and almost always causes some permanent diminished capacity in the cell. Easiest way to determine if a cell is reversed, is to measure each cell individually (in a string of attached cells). A reversed cell will generally measure twice the expected voltage indicating it is really acting like a resistor.
Edit: These are large cells; 400-500 pounds each. I wouldn't recommend charging your car battery at that rate for that long. You would probably boil it dry in that amount of time.
Dialtone