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 method for fast charging a lithium ion battery cell having a maximum recommended battery cell voltage, the method comprising:
(a) providing a charging current during a constant current mode of operation during which a first charging voltage and a constant current are applied to said battery cell;
(b) periodically measuring the battery cell open circuit voltage to determine if it has reached a predetermined value;
(c) maintaining 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;
(d) switching to a constant voltage mode of operation when said open circuit battery cell voltage reaches said predetermined value, wherein in said constant voltage mode of operation a constant second voltage is applied to said battery cell during said constant voltage mode of operation;
(e) monitoring the charging current supplied to said battery cell during said constant voltage mode of operation; and
(e) terminating charging when said charging current during said constant voltage mode of operation drops below a predetermined value.
2. The method as recited in claim 1 , wherein step (a) comprises: (a) providing a first charging voltage that has been selected to compensate for the maximum anticipated resistance in a battery charging circuit coupled to the battery cell including an internal battery cell resistance, battery terminals, and wire connected to the circuit.
3. The method as recited in claim 1 , wherein step (b) comprises turning off the charging current to the battery cell while the battery cell voltage is being measured.
4. The method as recited in claim 3 , wherein step (b) includes the step of periodically turning off the charging current to the battery cell.
5. The method as recited in claim 3 , wherein step (b) includes the step of turning off the charging current to the battery cell for a predetermined time period t and taking one or more battery cell voltage measurements.