Typical 12-volt lead-acid batteries have a voltage of about 14 volts when fully charged and 11 volts fully discharged. Most amateur radio equipment doesn't operate properly below 11.
Can a lead acid battery be charged at a full charge?
Test show that a heathy lead acid battery can be charged at up to 1.5C as long as the current is moderated towards a full charge when the battery reaches about 2.3V/cell (14.0V with 6 cells). Charge acceptance is highest when SoC is low and diminishes as the battery fills.
This mode works well for installations that do not draw a load when on standby. Lead acid batteries must always be stored in a charged state. A topping charge should be applied every 6 months to prevent the voltage from dropping below 2.05V/cell and causing the battery to sulfate. With AGM, these requirements can be relaxed.
Not doing so increases the risk of damage to the battery and if taken down deep enough into the batteries charge, will ruin the battery. Having said that, the absolute lowest level a lead acid battery can be discharged to, UNDER LOAD, is 10.8 volts but this is not recommended (more on this below).
How many volts should a lead acid battery be cut off?
The truth is that any lead acid battery, be it a Gel Cell, AGM or flooded batteries such as DCBs, should be cut-off at 11.6 volts. Not doing so increases the risk of damage to the battery and if taken down deep enough into the batteries charge, will ruin the battery.
The charge time is 12–16 hours and up to 36–48 hours for large stationary batteries. With higher charge currents and multi-stage charge methods, the charge time can be reduced to 8–10 hours; however, without full topping charge. Lead acid is sluggish and cannot be charged as quickly as other battery systems. (See BU-202: New Lead Acid Systems)
The recommended float voltage of most flooded lead acid batteries is 2.25V to 2.27V/cell. Large stationary batteries at 25°C (77°F) typically float at 2.25V/cell. Manufacturers recommend lowering the float charge when the ambient temperature rises above 29°C (85°F).