The power directed to a rechargeable battery is computed as the product of charging voltage and charging current (i.e., P=IV). Thus, a particular level of charging power can be achieved either by directing high charging current levels at relatively low charging voltage levels, or by directing lower current levels at relatively high voltage levels.
Generally, to “fast-charge” a rechargeable battery, high levels of electrical current are applied to the rechargeable battery. The use of high charging currents to charge rechargeable batteries requires relatively large and relatively expensive components, e.g., semiconductors, inductors, capacitors, etc., that are capable to handle high charging currents, in comparison to chargers that output relatively lower levels of current. On the battery side, protection circuits, e.g. fuses, Polymer thermal Cutoff (PTC's) fuses and/or Electronic Protection Boards, required for handling large charging currents become expensive and are often difficult to implement. Furthermore, the use of high charging currents to recharge a battery requires that the interface between the charger and the battery have large-sized physical dimensions to prevent melting and other deformation of the conductors. This, in turn, makes the interface bulky and less suitable to portable devices.
“Fast-charging” of a battery can alternatively be achieved by applying to the battery a high level of charging voltage at lower levels of charging current. However, under those circumstances, the higher voltage would have to be reduced (e.g., by using a step-down DC/DC converter, such as a buck converter), resulting in efficiency losses (typically 5 to 20%), quiescent drain, high costs, and added physical volume to accommodate the step-down converter.