Rapid charge lithium ion battery charger
View Patent ↗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 charging circuit for charging a lithium ion battery cell having a maximum recommended battery cell voltage, the battery charger comprising:
(a) a charging circuit for charging a battery cell configured with a constant current during a constant current mode of operation during which a first charging voltage and a constant current is supplied to said battery cell and a constant voltage mode of operation during which a constant second charging voltage is applied to said battery cell during a charging cycle, said charging circuit configured to intermittently sense the battery cell open circuit voltage when no current is flowing in the charging circuit and maintain said constant current mode of operation until said battery cell open circuit voltage reaches a predetermined value less than said maximum recommended battery cell voltage, said charging circuit further configured to switch to said constant voltage mode when said open circuit battery cell voltage reaches said predetermined value and thereafter reduce the charging voltage to said constant second charging voltage at or below the maximum recommended battery cell voltage.
2. The battery charger as recited in claim 1 , wherein said charging circuit is configured to turn off charging current to said battery cell when the open circuit voltage of said battery cell is being measured.
3. The battery charger as recited in claim 1 , wherein said battery cell voltage is used in a feedback loop to control when the battery charger switches from a constant current mode to a constant voltage mode.
4. The battery charging circuit as recited in claim 1 , wherein said circuit is configured to check if a battery cell has been inserted into the circuit.
5. The battery charging circuit as recited in claim 1 , wherein said circuit is configured to check the temperature of a battery cell to be charged.
6. The battery charging circuit as recited in claim 5 , wherein said circuit includes a thermistor.
7. The battery charging circuit as recited in claim 1 , wherein said circuit is configured to check if a battery cell has been fully charged.
8. The battery charging circuit as recited in claim 7 , wherein said battery charging circuit is configured to determine the value of the charging current being applied to a battery cell connected to the circuit and provide a signal that represents that the battery cell is fully charged when the charging current reaches a predetermined value.
9. The battery charging circuit as recited in claim 8 , wherein said predetermined value is less than 200 ma.
10. The battery charging circuit as recited in claim 8 , wherein said predetermined value is less than 100 ma.
11. The battery charging circuit as recited in claim 1 , wherein said constant value of said charging voltage during said constant voltage mode of operation is 4.2 volts.
12. The battery charging circuit as recited in claim 5 , wherein said circuit checks whether the battery cell temperature is within a predetermined temperature range.
13. The battery charging circuit as recited in claim 12 , wherein said predetermined temperature range is −7° C. and 46° C.
14. The battery charging circuit as recited in claim 12 , wherein said circuit is configured to repeat the check of battery cell temperature when the battery cell temperature range is outside said predetermined temperature range.
15. The battery charging circuit as recited in claim 8 , wherein said circuit is configured to terminate charging if the battery cell is configured to be fully charged.
16. The battery charging circuit as recited in claim 8 , wherein said first charging voltage is relatively higher than said maximum recommended battery cell voltage.