float voltage

{{Short description|External voltage needed to keep a battery fully charged}}

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.{{Citation

| title = A Guide to Understanding Battery Specifications

| url = http://mit.edu/evt/summary_battery_specifications.pdf

| author = Team, M.I.T. Electric Vehicle

| accessdate = 2012-01-12

}} 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.{{cite book|author1=Bill Moeller|author2=Jan Moeller|title=RV Electrical Systems: A Basic Guide to Troubleshooting, Repairing and Improvement|url=https://books.google.com/books?id=y97AV1N8A20C&pg=PA34|accessdate=12 January 2012|date=1 October 1994|publisher=McGraw-Hill Professional|isbn=978-0-07-042778-5|page=34}} The appropriate float voltage varies significantly with the chemistry and construction of the battery, and ambient temperature.{{cite book|author=Whitham D. Reeve|title=DC power system design for telecommunications|url=https://books.google.com/books?id=G1HTJcUEoQsC&pg=PA239|accessdate=12 January 2012|year=2007|publisher=John Wiley and Sons|isbn=978-0-471-68161-8|page=239}}

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.{{Citation needed|date=January 2011}}

However, it should be understood that the concept of a float voltage does not apply equally to all battery chemistries. For instance, lithium ion 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 are 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.{{cite web|url=https://www.electronicsweekly.com/market-sectors/power/float-charging-lithium-ion-cells-2006-02/|title=Float charging lithium ion cells|date=February 2006|publisher=Electronics Weekly.com|accessdate=4 September 2018}}