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.

However, it is important to understand that the concept of a float voltage does not apply at all to some battery chemistries. For instance, lithium ion cells cannot be safely trickle charged at any voltage, for the chemical system within the cell will be damaged to some extent by doing so. Some lithium ion variants are less tolerant than others, but generally overheating is likely and fire and explosion are not uncommon. It is important to make certain that the battery cell involved can be safely trickle charged, and that a charger circuit which automatically goes into trickle charge mode is safe and appropriate in each case.

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 (2 V) 3-cell (6 V) 6-cell (12 V)
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/°C (−2.17 mV/°F) 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 undercharging.

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