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).
A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to end the charge The correct setting of the charge voltage limit is critical and ranges from 2.30V to 2.45V per cell.
Setting the voltage threshold is a compromise and battery experts refer to this as “dancing on the head of a pin.” On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, over-saturation by not switching to float charge causes grid corrosion on the positive plate.
This loss is larger if the battery requires periodic deep discharges. (See also BU-806a: How Heat and Loading affect Battery Life.) Figure 3: Capacity loss on standby.
A regulated current raises the terminal voltage until the upper charge voltage limit is reached, at which point the current drops due to saturation.
Recharge every 6 months while in storage; AGM every 6–12 months.
These described voltage settings apply to flooded cells and batteries with a pressure relief valve of about 34k Pa (5psi).
If continually deprived, the battery will eventually lose the ability to accept a full charge and the performance will decrease due to sulfation.
The float charge in the third stage maintains the battery at full charge.