A battery charger is a type of AC-to-DC converter capable of restoring the charge in storage batteries. Industrial batteries are mainly used in forklifts, scissors lifts, golf carts and small material-handling equipment. They are also found in telecommunications, uninterrupted power supplies, medical equipment and in standby applications such as engine-starting batteries, in mobile homes, yachts, ambulances and emergency rescue vehicles.
Two predominant types of technology are used for converting the AC input in battery chargers:
Switch-mode: Switch-mode battery chargers are electronic devices that use a switching circuit to convert AC input at a very high frequency. They use predominantly solid-state components and do not require large transformers. These chargers are more compact, more energy efficient, more tolerant of input AC voltage fluctuation and slightly more expensive than those that use linear technology.
Linear: Linear battery chargers use an older technology with lower-efficiency transformers that generate more heat than switch-mode chargers. The charger is larger and heavier.
Types of Battery Chargers
There are three main types:
These three types of chargers have similar average conversion efficiencies (ranging from 80 to 90 percent, depending on the manufacturer), but have different peak electrical demands. Depending upon the electrical demand charge arrangement between you and your utility company, this difference may be significant.
Magnetic-amplifier chargers have the lowest peak electrical demand of the three types. Magnetic-amplifier chargers consist of three one-phase saturable reactors, a three-phase step-down transformer (with or without an interphasing reactor) and a diode rectifier (with or without a DC choke or 'inductor'). The saturable reactors regulate the charging rate and are controlled by a DC current through a control resistor. This charger is rugged and suitable for industrial use.
Overall power loss is high: for forklift chargers, loss is about eight percent of the rated output. Loss can be reduced to about one percent of the output power by replacing the saturable reactors with silicon controlled rectifiers (SCRs). The efficiency of this charger drops gradually from the beginning to the end of the charging cycle.
Ferroresonance chargers have a high peak electrical demand. They consist of three one-phase ferroresonant transformers and a diode rectifier.
These transformers are usually connected in a delta-to-double-wye configuration, and the secondary transformers operate into and out of the transformer core saturation. The core is constructed such that the magnetic path between the primary winding and the secondary winding has an air gap, thus providing the necessary isolation for the primary input when the core is partially saturating.
The secondary has two windings: the first is a centre-tapped winding that supplies output power, and the other is connected to a capacitor in series with a resistor. The capacitor provides automatic output regulation, and does not require any control signal.
The total power loss of the three resistors in the charger can be as high as seven percent. Connecting a fuse across the resistor can eliminate these losses. This type of charger does not typically have a DC choke.
The charging current for a ferroresonance charger automatically varies with the voltage difference between the transformer output and the battery voltage, resulting in very high initial charging rates, and much lower rates as charging proceeds. This high initial charging rate may cause some additional losses in the circuit. This can be improved by connecting a small capacitor for the initial charging rate control. Another capacitor can be switched in near the end of the charging cycle to maintain the charging current. The instantaneous efficiency of this type of charger drops rapidly from the beginning to the end of the charging cycle.
SCR-bridge chargers have the highest peak electrical demand of the three types. SCR-bridge chargers usually consist of a three-phase step-down transformer and an SCR bridge (with or without a DC choke).
The SCRs are used for AC-to-DC power conversion and for control of the charging rate. No additional power components are therefore needed for output regulation. The control is usually implemented by using a current feedback signal to vary the SCR firing pulses. The SCR usually has a higher on-state voltage drop than that of a diode, causing a power loss of about one percent of output power.
The SCRs in the charger are usually connected as a 6-pulse bridge. The transformer feeding the bridge converter can reduce the transformer's rating by 20 to 50 percent compared to feeding a double-wye-connected converter. However the bridge connection will increase the converter loss by about 3 percent of the output power, compared to the double-wye connection.
The Canadian Standards Association (CSA) has no performance standard for industrial battery chargers at present (2005). The National Electrical Manufacturers Association (NEMA) standard is PE 5 Utility Type Battery Chargers, which provides a large number of test methods to determine the performance of the battery charger, including an efficiency test. The NEMA standard also specifies requirements for AC input, DC output, operating environment and mechanical design; however the standard does not specify energy-efficiency levels.
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