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Float charging is most commonly used for
backup and emergency power applications where the discharge
of the battery is infrequent. During float charging the
charger, battery, and load are connected in parallel. The
charger operates off the normal power supply which provides
current to the load during operation. In the event of normal
power supply failure, the battery provides backup power
until the normal power supply is restored. Since most
equipment requires alternating current, a rectifier circuit
is usually added between the battery and the load. Float
chargers are typically constant-voltage chargers that
operate at a low voltage. Operating the charger at a low
voltage, usually less than about 2.4 V per cell, keeps the
charging current low and thus minimizes the damaging effects
of high-current overcharging.
For valve-regulated batteries, an important
consideration when float charging is the possible occurrence
of a phenomena called "thermal runaway". The
best way of preventing thermal runaway is through the use of
a temperature-compensated battery charger. A
temperature-compensated charger adjusts the float voltage
based upon battery temperature. Temperature-compensated
chargers will increase the reliability and prolong the life
of the battery/charger system. They are especially useful
for batteries located in areas where temperatures may be
significantly above ambient conditions.
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