Float voltage is the voltage that is required to maintain a battery fully charged. Maintaining battery charge level at 100% is also called Trickle Charging. The idea is to keep the battery charging till it is needed.
Recommended Float Voltage Value
The appropriate float voltage depends on battery type as well as room temperature at which it is being charged. Typically, for a lead-acid battery the float voltage should be set at around 2.25V per cell at room temperature. So for a typical battery of 6 cells, float voltage should be around 13.5V. The current should be set less than 1amp.
Effect of Temperature
As you know, charging heats up a battery. But here we are talking about keeping the battery connected to a charger indefinitely. A float charger should have temperature sensors attached it. A charger having temperature sensors varies charging rate depending on battery temperature. So float voltage of the chargers will also be varied accordingly.
A battery charger may use any of two methods to maintain the float voltage. In the first method, a constant DC charge is applied to maintain constant voltage. In the second method, a particular phase of the AC current is used to apply float charge. The method used depends on type of battery being charged, e.g., automobile, personal gadgets, hospital equipment, etc.
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