A multiple cell battery charger configured in a parallel topology provides
constant current charging. The multiple cell battery charger requires
fewer active components than known serial battery chargers, while at the
same time preventing a thermal runaway condition. The multiple cell
battery charger in accordance with the present invention is a constant
voltage constant current battery charger that includes a regulator for
providing a regulated source of direct current (DC) voltage to the
battery cells to be charged. The battery charger also includes a pair of
battery terminals coupled in series with a switching device, such as a
field effect transistor (FET) and optionally a battery cell charging
current sensing element, forming a charging circuit. In a charging mode,
the serially connected FET conducts, thus enabling the battery cell to be
charged. The FETs are controlled by a microprocessor that monitors the
battery cell voltage and cell charging current and optionally the cell
temperature. The microprocessor periodically adjusts the charging current
of each cell to maintain a relatively constant current. When the
microprocessor senses a voltage or temperature indicative that the
battery cell is fully charged, the FET is turned off, thus disconnecting
the battery cell from the circuit. Accordingly, the battery charger in
accordance with the present invention utilizes fewer active components
and is thus less expensive to manufacture than known battery chargers
configured with a serial topography while at the same time providing
constant current charging to avoid a thermal runaway condition.