BATTERY MANAGEMENT

This invention relates to battery management, and in particular to a method and apparatus for equalising the charge on a plurality of series connected cells forming a battery.

Series strings of storage cells are extensively used in many applications; for example PDA's, laptops, standby power supplies, wind, wave or solar generating systems, and electric vehicles. Inbalances in the charges of these cells tend to occur and grow over time, as the string is charged and discharged. This reduces the efficiency of the charging and discharging process and also limits the life of the battery. It is therefore very important to ensure the uniformity of charge for all cells in a battery string, which is a major part of what is now known as 'managing' the battery. .

Prior art attempts to do this have included providing means for measuring the voltage of each cell, and then switching a resistor across those with a higher charge in order to discharge them to the level of the cell with the lowest charge. However this system causes a wasteful loss of energy and also produces heat, which is undesirable in a battery container.

Alternative arrangements use DC to DC converters connected such that energy can be transferred between cells . However these arrangements require isolating windings, and expensive and bulky components.

Accordingly there is a need for a battery management system that is small and efficient.

According to the present invention there is provided an apparatus for

regulating the output voltages of at least two cells connected in series, the apparatus comprising a charge storage element connectable across each cell, and a control circuit having an input connectable across the charge storage element, wherein the control circuit is arranged to: for each cell, connect the charge storage element across the cell, disconnect the charge storage element from the cell, and connect the charge storage element to the control circuit input to measure the voltage of the cell; compare the measured voltages of each cell, determine a high voltage cell having a voltage higher than at least one other cell, and determine a low voltage cell having a voltage lower than at least one other cell; and connect the charge storage element across the high voltage cell thereby to discharge the high voltage cell, disconnect the charge storage element from the high voltage cell, and connect the charge storage element across the low voltage cell thereby to charge the low voltage cell. The process may be repeated a number of times with a small amount of energy transferred during each complete operation. Thus the invention may discharge a cell with a higher voltage, store the charge in the charge storage element, and use this to charge a cell with a lower voltage, in order to equalise the charges on the cells. This is done using a single charge storage element or capacitor and is thus less bulky and expensive than prior art arrangements. The apparatus has the advantage of being relatively small. Also, the switches operate at substantially zero current, which mitigates switching losses, and allows high frequency switching.

The apparatus may also be adapted for use with a further parallel string of cells, by the use of a further set of switches.

Preferably the apparatus also comprises an inductor connectable between one terminal of each cell and the charge storage element. The control circuit may be arranged to connect the charge storage element to the high voltage cell via the inductor for an 'on' period substantially proportional to the resonant frequency of the LC circuit (for example equal to approximately one half of the resonant frequency), and then to reverse the connections across the high voltage cell for a further 'on' period. This "ringing" action may be repeated to increase the charge in the charge storage element as required. This enables the charge storage element to be charged to a higher level and reduces the energy loss occurring as a result of the operation. Furthermore, the control circuit may similarly be arranged to connect the charge storage element across the low voltage cell via the inductor for such an "on" period, and then to reverse the connection across the low voltage cell for a further "on" period. This assists in discharging the capacitor fully.

The present invention also provides an apparatus for regulating the output voltage of at least two cells connected in series, the apparatus comprising: a charge storage element; a set of first cell switches for coupling a first terminal of each cell respectively to a first plate of the storage element; a set of second cell switches for coupling a second terminal of each cell respectively to a second plate of the storage element; and a pair of level shifting switches for coupling the first and second plates of the storage element respectively to a control circuit; the control circuit being arranged to operate each of the switches. The apparatus preferably comprises an inductor connectable by means of an inductor switch between the first set of cell switches and the first plate of the storage element.

The invention also comprises a method for regulating the output voltage of

at least two cells connected in series, comprising (a) connecting a charge storage element across a first cell in the string; (b) isolating the charge storage element from the cell and then connecting the charge storage element to a control circuit; (c) measuring the voltage across on the charge storage element with the control circuit; (d) repeating steps (a)-(c) for the remaining cells of the string in turn; (e) comparing the voltages so as to identify a high voltage cell, which has a higher charge than at least one other cell; (f) connecting the high voltage cell across the charge storage element to charge the charge storage element; (g) determining a low voltage cell having a lower charge than at least one other cell; and (h) connecting the charge storage element across the low voltage cell to charge that cell.

The method preferably also comprises, between steps (e) and (f): coupling an inductor to one plate of the charge storage element, connecting the cell in step (f) for an "on" period substantially proportional to the resonant frequency of the LC circuit, and then reversing the connection across the cell for a further "on" period further to charge the charge storage element.

The method may also comprise, between steps (g) and (h): coupling an inductor to one plate of the charge storage element, connecting the cell in step (h) for an "on" period as defined above, and then reversing the connection across the cell for a further "on" period.

The invention will now be described with reference to the accompanying drawings in which: -

Figure 1 is a circuit diagram of an apparatus according to an embodiment of the present invention; and

Figure 2 is a circuit diagram of a switching circuit suitable for use in the apparatus of Figure 1.

A battery comprises a string of cells 2, including adjacent cells labeled n+1, n, and n-1. For clarity, only the switches required to describe the functions in relation to the cell n are shown in the diagram. These include a first cell switch 4 for coupling the positive side of the cell n to a first or upper plate 12 of a charge storage element in the form of a capacitor 14. A further first cell switch 6 is provided for coupling the positive side of the adjacent cell n-1 to the upper plate 12 of a capacitor 14. Further first cell switches will be required for connecting the positive sides of the remaining cells in the string to the upper plate 12 of the capacitor.

A second cell switch 8 is arranged to couple the negative side of the cell n to the second or lower plate 16 of the capacitor 14. A further second cell switch 10 is arranged to couple the negative end of the adjacent cell n+1 to the lower plate 16 of the capacitor 14. Further second cell switches will be required for the remaining cell of the string. Thus it will be understood that two switches are provided for each inter-cell node; one first cell switch and one second cell switch corresponding to the adjacent cells.

The switches may conveniently be switching circuits, for example as shown in Figure 2. These circuits comprise two N channel mosfet devices connected such that they have common gate and source connections, as will be described later.

It can be seen that each of the cell switches is connected to a control circuit 18 (as shown by broken lines), for operating the switches. The circuit also

comprises an inductor 20 which is connectable between the first cell switches 4, 6 and the upper plate 12 of the capacitor 14 via an inductor switch 22. Finally, a pair of level shifting switches 24, 26 are operable by the control circuit 18 to couple the upper and lower plates 12, 16 of the capacitor 14 respectively to the control circuit 18.

The operation of the apparatus for battery management will now be described. The circuit is first used as a multiplexer to measure the voltage across each cell. This is done by connecting the capacitor 14 across a first cell n in the string using the respective first and second cell switches 4, 8, with the inductor switch 22 closed to short circuit the inductor 20. After the cell switches 4, 8 have been closed for a sufficiently long time period, the capacitor 14 will charge through the on resistance of the switches to a voltage substantially the same as the voltage of the cell n. The first and second switches 4, 8 are then opened, with the charge remaining in the capacitor 14. The level shifting switches 24, 26 are then closed such that the voltage on the capacitor 14 is presented to an input of the control circuit 18. This may for example be a microprocessor analogue to digital converter input. The voltage of the cell n is thus measured by the control circuit 18. This process is repeated for each of the remaining cells in the string in turn. It will be appreciated that the apparatus acts as a multiple input/single output system, or multiplexer, which traps and then level shifts the voltage of any cell in the string, such that a low voltage control circuit can read the voltage level on the cell. High string voltages are blocked from the control circuit by the alternate switching action of the level shifting switches 24, 26 and the first and

second cell switches e.g. 4, 8.

After the voltages across each cell have been ascertained by the control circuit, it is possible to compare them and identify a cell which has a lower voltage than other cells in the string. The inductor 20 is then connected between the first switches 4, 6 and the top plate 12 of the capacitor 14 by opening the inductor switch 22. The capacitor 14 will be charged with a residual voltage following its use in the multiplexing mode.

If for example it has been determined that a particular cell (e.g. n) has a higher voltage than at least one other cell (e.g. adjacent cell n-1), then the high voltage cell n is connected across the capacitor 14 using the respective first and second cell switches 4, 8 via the inductor 20. Conventional current will thus start to flow in the inductor 20 and consequently the capacitor charge will increase. The cell switches 4,8 are closed for an "on" period substantially proportional to the resonant frequency of the LC circuit. For example, the "on" period may be approximately one half of the resonant frequency. At the end of the "on" period, the inductor current will have fallen back to zero and the capacitor will be charged to approximately twice the difference between the residual voltage and the voltage across the high voltage cell n.

The connections are then reversed by connecting the positive terminal of the high voltage cell n to the lower plate 16 of the capacitor using the respective second cell switch 10, and connecting the negative terminal of the high voltage cell n to the top plate 12 of the capacitor using the respective first cell switch 6. The switches are closed for a further "on" period as defined above to charge the capacitor further. This so-called "ringing" action is used to remove energy from

the cell n and store it in the capacitor 14.

The charge stored on the capacitor 14 may then be used to charge the adjacent low voltage cell n-1. This is done by closing the first and second cell switches 6, 28 across the cell n-1 for an "on" period substantially proportional to the resonant frequency of the LC circuit, (with the other cell switches open). Because the voltage of the capacitor 14 is greater than the voltage of the low voltage cell n-1, current will flow from the capacitor via the inductor into the low voltage cell n-1. Due to the nature of the LC circuit some energy will be remain in the capacitor but with reversed voltage polarity. This energy can be transferred to the cell n-1 by reversing the connection of the cell n-1, and by closing the second switch 8 and a further first switch (not shown) to connect the negative side of the cell n-1 to the top plate of the capacitor 14 (via the inductor 20). This is done for a further "on" period substantially proportional to the resonant frequency of the LC circuit.The above sequence of switch operation is repeated until little energy is left stored in the capacitor. hi this way the low voltage cell n-1 is charged to a higher voltage, and the high voltage cell n is discharged, such that the voltages may be equalised. This process may be used for any cell or string of adjacent cells within the string 2.

The switches referred to above may comprise any suitable switching circuit. Referring now to Figure 2, one example of a switching circuit is shown which is suitable for use as any of the first or second cell switches, inductor switch and level shifting switches of the circuit of Figure 1. The switching circuit may for example comprise two N channel mosfet devices 30, 32 which are connected such that they have a common gate 34 and a common source 36

connection. A resistor 38 is connected between the two common terminals 34, 36, and a Zener diode is also connected in parallel with the resistor 38 between the common terminals 34, 36.

It can be seen that such a switching circuit will block current flow in both directions in the off state, whilst conducting in both directions in the on state. In the on state, it is predominately a resistance. Thus the circuit displays the main characteristics of a switch required for use in the multiplexing circuit.

The control circuit may be arranged to carry out this measuring and equalising process frequently, so as to maintain a relatively constant uniform charge on the cells.