Float Charge

A float charge is a lower, steady voltage a charger applies after a lead-acid battery is full, to keep it full without overcharging. It is the third stage of lead-acid charging.

Battery University gives a float voltage of 2.25 to 2.27 volts per cell for most flooded lead-acid batteries, and notes that not every charger has a float stage. A charger that stays at the higher topping voltage can overcharge the battery.

Lithium-ion does not use a float charge. The guide to lithium vs lead-acid charging explains the difference, and the car battery charging time calculator estimates the time before float begins.