CC/2C/1000Time in hoursConstant voltagecharge rateLead-Acid Storage BatteriesDOE-HDBK-1084-95MAINTENANCERev. 0Page 31BatteriesFigure 16. Charge rate versus time for a typical constant-voltage charger.charger to minimize the effects of overcharge on batteries having infrequentdischarges as described below. FloatChargingFloat charging is most commonly used for backup and emergency powerapplications where the discharge of the battery is infrequent. During float chargingthe charger, battery, and load are connected in parallel. The charger operates offthe normal power supply which provides current to the load during operation. Inthe event of normal power supply failure, the battery provides backup power untilthe normal power supply is restored. Since most equipment requires alternatingcurrent, a rectifier circuit is usually added between the battery and the load. Floatchargers 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, keepsthe charging current low and thus minimizes the damaging effects of high-currentovercharging. For valve-regulated batteries, an important consideration when float charging isthe possible occurrence of a phenomena called "thermal runaway" (discussed in thedefinitions and the section on Sealed Lead-Acid Batteries). The best way ofpreventing thermal runaway is through the use of a temperature-compensatedbattery charger. A temperature-compensated charger adjusts the float voltagebased upon battery temperature. Temperature-compensated chargers will increasethe reliability and prolong the life of the battery/charger system. They areespecially useful for batteries located in areas where temperatures may besignificantly above ambient conditions.
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