Rapid charge lithium ion battery charger
A rapid charging circuit for a lithium ion battery. The battery charger in accordance with the present invention compensates for the voltage drops across the various resistance elements in the battery circuit by setting the charging voltage to a level to compensate for the initial resistance of the series resistances in the circuit and an additional resistance selected to take into account the anticipated increase in resistance of the various circuit elements over time. The battery charger in accordance with the present invention periodically monitors the open-circuit voltage of the battery cell and reduces the charging voltage to when the battery cell voltage reaches the optimal value. Thus, during a constant current charging mode, the battery cell is driven at a relatively optimal charging current to reduce the charging time. As such, the system is able to optimize the charging current supplied to a battery cell during a constant current mode of operation while compensating for circuit elements whose resistance may vary over time due to temperature or other factors, such as corrosion, while at the same time avoiding exceeding the maximum recommended voltage for the battery cell.
1. A battery charger having a constant current mode and a constant voltage mode, the battery charger comprising:
a pair of input terminals adapted to be coupled to a source of DC voltage;
a pair of output terminals adapted to be coupled to a battery cell;
a charging circuit coupled between said input terminals and said output terminals, said charging circuit comprising:
a converter comprising a voltage regulator for controlling the DC voltage applied to said input terminals, said regulator initially configured to provide a charging voltage in a constant current mode of operation relatively higher than said charging voltage in said constant voltage mode;
a switching device coupled between said input terminals and said output terminals, said switching device for selectively connecting and disconnecting said pair of output terminals from said charging circuit, said voltage comparator coupled to said regulator;
a voltage comparator for comparing the battery open circuit voltage with a reference voltage;
a microprocessor operatively coupled to said pair of output terminals for monitoring the open circuit voltage of said battery cell under charge and selectively providing a signal representative of the battery cell open circuit voltage to said voltage comparator forming a first feedback loop, said open circuit voltage representative signal also used to control said regulator in order to reduce said battery charging voltage during a constant voltage charging mode forming a secondary feedback loop when said open circuit voltage reaches a predetermined value less than said charging voltage in said constant current mode.
2. The battery charger as recited in claim 1 , wherein said microprocessor is configured to periodically monitor the open circuit voltage of said battery cell.
3. The battery charger as recited in claim 2 , wherein said microprocessor is configured to turn off charging current to when the voltage of said battery cell is being measured.
4. The battery charger as recited in claim 1 , wherein said microprocessor is configured to generate a pulse width modulated (PWM) signal having a controllable pulse width for charging said battery cell.