Float voltage is the
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
at which a battery is maintained after being fully charged to maintain that capacity by compensating for
self-discharge
Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes or any external circuit. Self-discharge decreases the shelf life
Shelf life ...
of the battery. The voltage could be held constant for the entire duration of the cell's operation (such as in an
automotive battery
An automotive battery or car battery is a rechargeable battery that is used to start a motor vehicle. Its main purpose is to provide an electric current to the electric-powered starting motor, which in turn starts the chemically-powered internal ...
) 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.
However, it should be understood that the concept of a float voltage does not apply equally to all battery chemistries. For instance,
lithium ion
A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
cells have to be float charged with extra care because if they are float charged at just a little over optimum voltage, which is generally the full output voltage of the lithium cell, the chemical system within the cell will be damaged to some extent. Some lithium ion variants are less tolerant than others, but generally overheating, which shortens cell life is likely, and fire and explosion possible other outcomes. It is important to make certain that the battery cell involved can be safely float charged, and that in the absence of protection from a
battery management system, that the charger circuit goes into float charge status when full charge is achieved.
Lead-acid batteries
Accepted average float voltages for
lead-acid batteries at 25 °C can be found in following table:
;Temperature compensation
Compensation per cell of approximately −3.9 mV/°C (−2.17 mV/°F) of temperature rise is necessary.
;Example 1
A 12 V (6-cell) battery at 30 °C (86 °F) (+5 °C change):
;Example 2
A 12 V (6-cell) battery at 20 °C (68 °F) (−5 °C change):
Not compensating for temperature will shorten battery life by over- or undercharging.
See also
*
Trickle charging Trickle charging means charging a fully charged battery at a rate equal to its self-discharge rate, thus enabling the battery to remain at its fully charged level; this state occurs almost exclusively when the battery is not loaded, as trickle cha ...
References
Battery charging
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