Circuit and method for evaluating cells in a battery

1. A circuit, comprising:

a balancing circuit, configured to inductively transfer charges between cells in a battery using current pulses, and

a control unit configured to control the balancing circuit to provide the current pulses, wherein an average value of the current pulses oscillates over time;

wherein the oscillation of the average value is sinusoidal; and where a frequency of the oscillation is lower than 1 kHz.

2. The circuit of claim 1 , wherein

at least one of:

peak values of the current pulses;

time differences between adjacent current pulses; and

widths of current pulses

varies in an oscillating manner over time.

3. The circuit of claim 1 , wherein

the balancing circuit comprises a transformer,

wherein the transformer comprises a primary coil and a plurality of secondary coils, wherein

the primary coils is coupled across the battery, and

a respective secondary coil of the plurality of the secondary coils is coupled across a respective cell of the battery.

4. The circuit of claim 3 , wherein

the balancing circuit further comprises:

a plurality of second switching means, wherein a respective of the second switching means is configured to control a current flow through a respective secondary coil, wherein

the plurality of second switching means are controlled by the control unit.

5. The circuit of claim 4 , wherein

the balancing circuit further comprises:

a first switching means configured to control a current flow through the primary coil, wherein

the first switching means is controlled by the control unit.

6. The circuit of claim 1 , further comprising

a shunt resistor coupled in series to the battery.

7. The circuit of claim 6 , further comprising

a measuring unit configured to determine a voltage across the shunt resistor and a respective voltage across a respective cell.

8. The circuit of claim 7 , further comprising

a plurality of filters configured to reject frequencies above a frequency of the oscillations of the average value of the current pulses, wherein

one of the filters is coupled to the shunt resistor and filters the voltage across the shunt resistor; and

respective filters are coupled to respective cells and filter the respective voltage across the respective cell.

9. The circuit of claim 8 , wherein

a cutoff frequency of the filters is adapted to the frequency of the oscillations of the average values of the current pulses.

10. The circuit of claim 8 , further comprising

a calculation unit configured to determine an impedance of a cell based on the filtered voltages across the shunt resistor and the filtered voltage across the respective cell.

11. A method for evaluating cells of a battery, comprising:

balancing a voltage of a cell using charge pulses, wherein

the charge pulses are modulated with an oscillating test signal;

measuring a current through the cell and measuring a voltage across the cell;

demodulating the measured current and the measured voltage; and

determining an impedance based on the demodulated current and the demodulated voltage.

12. The method of claim 11 , wherein

the test signal is periodic.

13. The method of claim 11 , wherein

the charge pulses are inductively generated current pulses.

14. The method of claim 11 , wherein

the test signal modulates an average value of the charge pulses.

15. The method of claim 14 , wherein

the test signal modulates at least one of the following:

an amplitude of the pulses;

a duration of the pulses;

a phase of the pulse; and

a frequency of the pulses.

16. The method of claim 11 , wherein

a charge pulse during a first time period removes charge from the cell and a charge pulse during a second time period transfers this charge to the battery; or

a charge pulse during a first time period removes charge from the battery and a charge pulse during a second time period transfers this charge to the cell.

17. The method of claim 16 , wherein

the removal of charge during the first time period and the transfer of charge during the second time period is repeated until the cells of the battery are balanced.

18. The method of claim 16 , wherein

the modulation of the charge transfer takes place during at least one of:

first time periods, and

second time periods.

19. The method of claim 16 , wherein

a period of the test signal is chosen to be larger by a factor of 10 than the time period defined by the first time period together with the second time period.

20. The method of claim 11 , wherein

demodulating the measured current and the measured voltage comprises:

demodulating a sampled signal representing the measured current through the battery; and

demodulating a sampled signal representing the measured voltage across the cell.

21. The method of claim 20 , wherein

demodulating comprises filtering the corresponding signal to suppress frequencies higher than a fundamental frequency of the test signal.

22. The method of claim 20 , further comprising

determining the complex impedance of the cell using the demodulated signals.

23. The method of claim 22 wherein

the impedance of the cell is used to determine at least one of:

a state of charge; and

a state of health

of the cell.

24. The method of claim 22 , further comprising

determining the impedance of the cell at other frequencies of the test signal.

25. The method of claim 11 , wherein

all the cells of the battery are evaluated.

26. A circuit, comprising:

an active balancing circuit, configured to balance voltages of cells of a battery by inductively transferring charges between a respective cell and the battery using current pulses;

a pulse generating unit, configured to generate the current pulses, wherein the current pulses are modulated with a given frequency;

a measuring unit, configured to measure a voltage across the cell and a current flow through the cell;

a demodulation unit, configured to determine a voltage and a current at the given frequency from the measured voltage across the cell and current flow through the cell; and

a calculation unit, configured to calculate an impedance of the cell from the voltage and the current determined at the given frequency.