Constant-voltage (often called constant-potential) chargers maintain nearly the same voltage input to the battery throughout the charging process, regardless of the battery's state of charge. Constant-voltage chargers provide a high initial current to the battery because of the greater potential difference between the battery and charger. A constant-voltage charger may return as much as 70% of the previous discharge in the first 30 minutes. This proves useful in many battery applications involving multiple discharge scenarios. As the battery charges its voltage increases quickly. This reduces the potential that has been driving the current, with a corresponding rapid decrease in charge current as depicted in Figure 16. As a result, even though the battery reaches partial charge quickly, obtaining a full charge requires prolonged charging.

Given this behavior, constant-voltage chargers are frequently found in applications that normally allow extended charging periods to attain full charge. Constant-voltage chargers should not be used where there is frequent cycling of the battery. Repeated discharges without returning the cell to its full charge will eventually decrease the battery capacity and may damage individual cells.

Constant-voltage chargers are most often used in two very different modes: as a fast charger to restore a high percentage of charge in a short time or as a

Figure 16. Charge rate versus time for a typical constant-voltage charger.

charger to minimize the effects of overcharge on batteries having infrequent discharges as described below.

Float Charging

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" (discussed in the definitions and the section on Sealed Lead-Acid Batteries). 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.

Constant-Current Charging

Constant-current charging simply means that the charger supplies a relatively uniform current, regardless of the battery state of charge or temperature. Constant-current charging helps eliminate imbalances of cells and batteries connected in series. Single-rate, constant-current chargers are most appropriate for cyclic operation where a battery is often required to obtain a full charge overnight. At these high rates of charge there will be some venting of gases. Positive grid oxidation will occur at elevated temperatures or extended overcharge times. Normally the user of a cyclic application is instructed to remove the battery from a single-rate, constant-current charger within a period of time that permits full charge yet prevents excessive grid oxidation.

Another type of constant-current charger is the split-rate charger. A split-rate charger applies a high initial current to the cell and then switches to a low rate based on time of charge, voltage, or both. The choice of switching method and switch point may be affected by the relative priority of minimizing venting (early switching) versus maintaining good cell balance (later switching). In some splitrate chargers, the charger will alternate between the high and low rate as the battery approaches full charge. Split-rate chargers are useful when the discharge cannot be classified as float or cyclic, but lies somewhere between the two applications.