Float voltage

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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.[1] 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.[2] The appropriate float voltage varies significantly with the chemistry and construction of the battery, and ambient temperature.[3]

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.

Lead-acid batteries[edit]

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

Lead-acid battery type single cell (2V) 3 cell (6V) 6 cell (12V)
Gel battery 2.18 6.53 13.05
Flooded lead–acid cell 2.23 6.7 13.4
Absorbent glass mat 2.27 6.8 13.6
Temperature compensation

Compensation per cell of approximately −3.9 mV per °C (respectively -2.17 mV per Fahrenheit) of temperature rise is necessary.[4]

Example 1

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

Example 2

A 12 V (6 cell) battery at 20°C (68°F) (−5 °C change):
(-3.9 mV/°C) × (6 cells) × (-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.

See also[edit]


  1. ^ Team, M.I.T. Electric Vehicle, A Guide to Understanding Battery Specifications (PDF), retrieved 2012-01-12 
  2. ^ Bill Moeller; Jan Moeller (1 October 1994). RV Electrical Systems: A Basic Guide to Troubleshooting, Repairing and Improvement. McGraw-Hill Professional. p. 34. ISBN 978-0-07-042778-5. Retrieved 12 January 2012. 
  3. ^ Whitham D. Reeve (2007). DC power system design for telecommunications. John Wiley and Sons. p. 239. ISBN 978-0-471-68161-8. Retrieved 12 January 2012. 
  4. ^ John A. O'Connor, Unitrode Application Note: Simple Switchmode Lead-Acid Battery Charger (PDF), retrieved 2012-11-10