Device and method for continuously equalizing the charge state of lithium ion battery cells
A method of equalizing charge states of individual cells in a battery includes measuring a previous cell voltage for each cell, measuring a previous shunt current for each cell, calculating, based on the previous cell voltage and the previous shunt current, an adjusted cell voltage for each cell, determining a lowest adjusted cell voltage from among the calculated adjusted cell voltages, and calculating a new shunt current for each cell.
1. A method of equalizing charge states of individual cells in a battery, the method comprising:
measuring a previous cell voltage for each cell;
measuring a previous shunt current for each cell;
calculating, based on the previous cell voltage and the previous shunt current, an adjusted cell voltage for each cell;
determining a lowest adjusted cell voltage from among the calculated adjusted cell voltages; and
calculating a new shunt current for each cell, wherein the calculating the new shunt current for each cell comprises:
setting the new shunt current for the cell having the lowest adjusted cell voltage to zero; and
calculating a new shunt current for each of the remaining cells according to the following equation:
ISNn =(ΔCTp)(VNpi −VLpi)/(B)(VH-VL), where
ISNn is the new shunt current,
ΔCTp is the difference between the nominal battery capacity at charge termination and the nominal battery capacity at discharge termination at temperature Tp,
VNpi is the adjusted cell voltage,
VLpi is the lowest adjusted cell voltage from among the adjusted cell voltages of each of the N cells,
B is the target time to substantially fully equalize the charge states,
VH is the cell charge voltage limit, and
VL is the cell discharge voltage limit.
2. The method of claim 1 , wherein the calculating the previous shunt current comprises:
multiplying the previous shunt current by a temperature-corrected nominal cell impedance to produce a result; and
adding the result to the previous cell voltage.
3. The method of claim 1 , further comprising, when the adjusted cell voltage of each of the cells is substantially equal to the cell charge voltage limit:
setting the new shunt current for each of the cells to zero;
applying a charge voltage to a stack, which includes all of the cells, to generate a battery stack charge current through the stack;
limiting battery stack charge current to approximately ΔCTp/100; and
setting the battery stack charge current to zero when the battery stack charge current is less than or equal to approximately ΔCTp/500.
4. A device for equalizing charge states of individual cells in a battery, the device comprising:
control circuitry including
a voltage sense amplifier which measures a previous cell voltage for each cell,
a current sense amplifier which measures a previous shunt current for each cell,
a part which calculates, based on the previous cell voltage and the previous shunt current, an adjusted cell voltage for each cell,
a part which determines a lowest adjusted cell voltage from among the calculated adjusted cell voltages, and
a part which calculates a new shunt current for each cell, wherein the part which calculates the new shunt current for each cell:
sets the new shunt current for the cell having the lowest adjusted cell voltage to zero; and
calculates new shunt currents for each of the remaining cells according to the following equation:
ISNn =(ΔCTp)(VNpi −VLpi)/(B)(VH-VL), where
ISNn is the new shunt current,
ΔCTp is the difference between the nominal battery capacity at charge termination and the nominal battery capacity at discharge termination at temperature Tp,
VNpi is the adjusted cell voltage,
VLpi is the lowest adjusted cell voltage from among the adjusted cell voltages of each of the N cells,
B is the target time to substantially fully equalize the charge states,
VH is the cell charge voltage limit, and
VL is the cell discharge voltage limit.
5. The device of claim 4 , wherein the part which calculates a new shunt current for each cell:
multiplies the previous shunt current by a temperature-corrected nominal cell impedance; and
adds the result of the multiplying to the previous cell voltage.
6. The device of claim 4 , further comprising:
a part which sets the new shunt current for each of the cells to zero when the adjusted cell voltage of each of the cells is substantially equal to the cell charge voltage limit;
a part which applies a charge voltage to a stack, which includes all of the cells, to generate a battery stack charge current through the stack;
a part which limits the battery stack charge current to approximately ΔCTp/100; and
a part which sets the battery stack charge current to zero when the battery stack charge current is less than or equal to approximately ΔCTp/500.
7. The device of claim 4 , further comprising a plurality of control circuits, each of which corresponds to a fixed number of individual cells.
8. The device of claim 7 , wherein
the battery includes eight cells, and
the plurality of control circuits comprises:
a first control circuit corresponding to the first and second cells;
a second control circuit corresponding to the third through fifth cells; and
a third control circuit corresponding to the sixth through eighth cells.
9. The device of claim 4 , further comprising:
a shunt current control amplifier which controls the current being shunted a given individual cell based on the calculated new shunt current corresponding to the individual cell;
a shunt pass element connected to the shunt current control amplifier; and
a shunt resistance connected to the shunt pass element and through which the shunt current flows.
10. A system for equalizing battery cell charge states, the system comprising:
a battery including first through eighth cells;
a first control circuit corresponding to the first and second cells;
a second control circuit corresponding to the third through fifth cells; and
a third control circuit corresponding to the sixth through eighth cells,
wherein each of the first through third control circuits comprises:
a voltage sense amplifier which measures a previous cell voltage for each of the corresponding cells;
a current sense amplifier which measures a previous shunt current for each of the corresponding cells;
a part which calculates, based on the previous cell voltage and the previous shunt current, an adjusted cell voltage for each of the corresponding cells;
a part which determines a lowest adjusted cell voltage from among the calculated adjusted cell voltages; and
a part which calculates a new shunt current for each of the corresponding cells, and
wherein the part which calculates the new shunt current for each of the corresponding cells:
sets the new shunt current for the cell having the lowest adjusted cell voltage to zero; and
calculates new shunt currents for each of the remaining cells according to the following equation:
ISNn =(ΔCTp)(VNpi −VLpi)/(B)(VH-VL), where
ISNn is the new shunt current,
ΔCTp is the difference between the nominal battery capacity at charge termination and the nominal battery capacity at discharge termination at temperature Tp,
VNpi is the adjusted cell voltage,
VLpi is the lowest adjusted cell voltage from among the adjusted cell voltages of each of the N cells,
B is the target time to substantially fully equalize the charge states,
VH is the cell charge voltage limit, and
VL is the cell discharge voltage limit.