Float voltage

Float voltage is the voltage at which a battery is maintained after being fully charged to maintain that capacity by compensating for self-discharge of the battery. The voltage could be held constant for the entire duration of the cell's operation (such as in an automotive battery) or could be held for a particular phase of charging by the charger. The appropriate float voltage varies significantly with the chemistry and construction of the battery, and ambient temperature.

With the appropriate voltage for the battery type and with proper temperature compensation, a float charger may be kept connected indefinitely without damaging the battery.

Accepted average float voltages for lead-acid batteries at 25 °C can be found in following table:

Compensation per cell of approximately −3.9 mV per °C (respectively -2.17 mV per Fahrenheit) of temperature rise is necessary.

A 12 V (6 cell) battery at 30°C (86°F) (+5 °C change):
$(-3.9 mV/°C) \times (6 cells) \times (5 °C change) = -117 mV$
$13.4 V (flooded battery float) + (-117 mV) = 13.28 V$

A 12 V (6 cell) battery at 20°C (68°F) (−5 °C change):
$(-3.9 mV/°C) \times (6 cells) \times (-5 °C change) = +117 mV$
$(13.4 V flooded battery float) + (117 mV) = 13.52 V$

Not compensating for temperature will shorten battery life by over or under charging.

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Wikipedia