A battery charger typically includes a charger housing and a charging circuit supported by the housing. The battery charging circuit is connectable to a power source and to a rechargeable battery and which is operable to charge the battery. A battery charger includes a switch which transfers charge from a power source such as a regulated DC voltage supply to battery cells, and a controller which controls the amount of charge supplied to the battery. The controller might also include battery sensing circuitry which measures the battery voltage and charging current. These measurements may be used to determine when a battery is fully charged or when the charging current to the battery is too great. Battery chargers for electric shavers, mobile phones, fax machines, cordless phones, and other electronic devices generally include a relatively simple, low cost, low frequency transformer circuit which has a simple diode rectifier at the output of a secondary winding. Modern battery chargers often include a circuit for measuring the voltage across a battery as it is being charged. A charging circuit, in turn, applies a selected current to the battery based on the voltage across the battery. Many chargers also include a load resistor that is selectively connected across the battery. Once the battery charging cycle has been completed, this resistor is connected across the battery and the voltage across it is measured. This voltage measurement is used to determine the state of the cells integral with the battery. This measure of cell state is, in turn, used as measure of the utility of a battery. Cellular phone battery chargers are commonly available as single pocket and dual pocket devices. Dual pocket devices are known to be used for charging a spare battery, as well as the battery connected to the cellular phone. In general, a battery charger is so designed that it charges a battery by supplying it with an electric current while the voltage across the battery is lower than a predetermined level, and that it thereafter stops charging when the voltage across the battery reaches that predetermined level as the result of the charging. 

Generally, either a constant voltage or a constant current power source is used for charging and recharging batteries. Both constant current chargers and voltage switched chargers require a circuit to provide power from the input power source to drive the series pass device or the series switch. Constant-voltage charging is most popular in float mode applications. In constant-voltage charging, the voltage across the battery terminals is held constant, with the state of the battery determining the charge current level. The charging process normally terminates after a certain time limit is reached. Constant voltage battery charging is provided in a battery charger with a regulated voltage source for constant voltage charging in which a first voltage measurement across the battery is taken while the battery is being charged, and a second voltage measurement across the battery is taken while the charging current is interrupted. A determination of the potential difference between the first voltage and the second voltage identifies a voltage drop across the terminals and/or protection circuitry of the battery pack, which is used for adjusting the setting of the voltage source to provide the optimum voltage level at the rechargeable cells of the battery pack. Automatic compensation of the voltage source thus provides battery cells under charge with the optimum constant voltage level for recharging, which is particularly well suited for charging lithium ion batteries. Constant voltage chargers generally have a switch in series with the input voltage source, an inductor, and the battery that is being charged. Constant-current charging holds the charging current constant. Constant current chargers regulate the battery charging current by varying the impedance of a series pass device, essentially dissipating power in the pass device to maintain regulation. Due to the relatively high assembly power dissipation, constant current chargers are unsuitable for charging NiMH batteries and typically are used only for charging low capacity batteries. Constant voltage charging may be used in charging nickel-cadmium (NiCd), nickel metal hydride (NiMH), alkaline, as well as lithium ion batteries and the like.

Battery chargers supply charge in a controlled manner to rechargeable batteries. Battery chargers can be classified into two general groups, including linear regulator battery chargers and switching regulator battery chargers. Linear regulator battery chargers typically employ single stage charge control with hardware-based voltage regulation. Linear regulator battery chargers require an external current limit to protect the battery from a charger fault condition that could result in excessive current being supplied to the battery. A switching regulator circuit includes a specially-designed charge control integrated circuit (IC) device for use with the other circuit elements. The integrated circuit (IC) is connected within the switching regulator circuit in constant-current mode. With the regulator operating in constant-current mode, charging continues at a constant current until the voltage reaches the pre-set maximum charging voltage. The circuit then limits the output charging voltage to the maximum charging voltage, using a pulse-width modulation technique. They typically consist of a low frequency power transformer along with an uncontrolled bridge rectifier. They are typically less efficient and have slow dynamic response. Such a charger is suited for constant-voltage charging, where the battery's state of charge sets the charging current. This type of charger could cause damage to batteries because it lacks control of the charging current. Alternatively, controlled chargers offer the ability to control the charging current as well as to implement both constant-voltage and constant-current charging methods. The simplest form of controlled chargers is silicon controlled rectifier (SCR) chargers, consisting of a low frequency transformer, an SCR bridge rectifier, and a DC choke. Transistor controlled chargers consist of a low frequency transformer, an uncontrolled bridge rectifier, and a series pass transistor. These chargers can implement both constant-voltage and constant-current charging methods and have fast dynamic response. However, they have low efficiencies and are generally bulky due to the low frequency transformer. Switch mode power supply (SMPS) based chargers offer improved performance compared to the SCR and the transistor controlled chargers. The basic components of an SMPS charger include an input filter stage, an input rectification stage, a power factor correction stage (if required), a high frequency power conversion power stage, a high frequency isolation transformer, and an output rectification and filtering stage. A central analog/digital controller is normally employed to regulate the charger voltage/current and to implement the desired charging algorithm. Switch mode power supply based chargers offer high efficiency power conversion due to high frequency operation. These chargers have fast dynamic response.

Rechargeable batteries are generally charged by a battery charger having a power supply that can provide a supply of DC current. A rechargeable battery accepts the electrical current and converts it into chemical energy. As the battery becomes more fully charged, the internal chemical reaction slows and more of the electrical energy input is converted into heat energy. This continues until the chemical reaction effectively ceases and virtually all of the electrical energy input is converted into heat energy. The heat generated by a rechargeable battery is used as a parameter to sense that it has reached a fully charged state. High speed battery chargers, which charge a battery at a rapid rate, involve high energies, both for the charging pulses and the depolarization pulses. Battery chargers for lead-acid batteries typically apply a charge current to a battery cell or cells under charge. The charge current drives an electrochemical reaction that causes lead ions to precipitate out of an electrolytic solution onto a metal plate. Once a maximum amount of lead is removed from the solution, the battery is fully charged. A typical method of charging a battery for a portable consumer electronics involves providing a constant charging current into the battery until the battery reaches a certain voltage. The voltage is then regulated with a voltage control loop at a constant level causing the current to gradually reduce over time, which is usually fulfilled by controlling battery charger circuitry with a combination of a current feedback amplifier and a voltage feedback amplifier. A battery charger for a cellular phone charges a battery through a connection terminal at the rear of the battery. In many cases, battery chargers are provided in which the battery may be taken from the phone and charged. Alternatively, some battery chargers are adapted to charge the battery while it is in the phone. Such chargers allow the phone to be available for use while its battery is charging.