FI123892B - Battery management - Google Patents

Abstract

Method and apparatus for equalizing the cell-specific charges of a battery pack comprised of cells (C1 - CN) connected in series, which battery pack comprises bidirectional power converters for transferring charges to/from a cell, which power converters comprise primary circuits and secondary circuits that are galvanically isolated from each other, of which secondary circuits each has its own corresponding cell, to which it is connected, and the primary circuits of which power converters are connected to a common DC circuit (power bus, PB), via which charges can be freely transferred from one power converter to another.

Description

BATTERY CHARGE MANAGEMENT

TECHNICAL FIELD The present invention relates to a method and apparatus for implementing charge management in a battery comprising a plurality of serially connected cells. In particular, the invention relates to the implementation of charge management by a power bus linking all charge management units for each cell or group of cells, which allows transfer of charge from any cell to any of 10 cells regardless of the size of the battery.

Background of the Invention and Prior Art

Battery cells are used eg. as an energy storage for vehicles using electric power processing (such as HEV, hybrid electric vehicle), and are therefore an indispensable component of such systems. Because the voltage of a single battery cell, for example when using modern lithium-based cells, is typically only about 3 volts, they must be connected in series to provide the source voltage required for power supply, such as 42V, 120V or 450V.

It is advantageous to select cells of the highest quality for the battery to ensure consistent performance. In practice, however, there are differences in the capacities of the cells (ability to absorb the charge), which can result, for example, in a deep discharge of a single weaker cell when the battery is being charged or similarly overcharged when the battery is being charged. Both cases imply activity outside the cell-specific operating range, which can result in permanent damage to the cell. Failure of a single cell, in turn, ° reduces the capacity of the battery as a whole and may even lead to the need for immediate service, oo 30

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x In order to prevent such an error situation,

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to emphasize cell charges, for which there are many well-known means.

For example, patent publication EP0814556 discloses a method of coupling shunt resistors to cells in order to weight charges at a level of 35. The method can prevent the cell from overcharging, but does not provide help to prevent deep discharge of the cell.

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US5498950, for its part, has become known in the art using a separate power supply which can be connected via relays to charge the desired cell to achieve the charge balance. The method can prevent deep discharge, but does not help against overcharging, and is also expensive and complicated due to relaying.

The disadvantages of the known charge management methods mentioned above are either the one-way charge transfer capability and the high hardware (such as relays) and thus expensive and complex system. To fully support battery cells in both charge and discharge conditions, it must be possible to transfer the charge from the stronger cells to the weaker ones, or vice versa. Thus, this requires a bidirectional transfer of charge, which in itself is achieved, for example, by connecting two DC / DC converters in parallel but in different directions. The solution leads to high costs for hel-15 mail because converters are needed for each cell.

US2005 / 0024015 discloses a method using a kind of charge pump for transferring a charge pulse from a cell to an adjacent cell. According to the method, complete charge management is in principle possible, but in practice only if the weak and strong cells are located close to each other.

US4600984 and US7046525 have become known as a bidirectional flyback converter which provides an advantageous option for transferring charges in both directions. Indeed, in US2008 / 0272735, such a method has been applied to a group of cells where charges can be transferred within the group. With multiple cell groups in the battery, this method does not provide the ability to equalize charges when a weak and strong cell is in different groups.

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Summary of the Invention 9 30 The object of the present invention is to provide a charge management method and apparatus which avoids the above drawbacks and

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listing the transfer of charges from any cell to any cell in the battery

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c \ j regardless of the number of connected cells. This purpose is achieved-

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g 35 according to the invention, characterized in that which is stated in the characterizing parts of the independent claims. Other preferred embodiments of the invention are claimed in the dependent claims.

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According to the invention, bi-directional charge transfer converters using transformers whose primary winding is galvanically separated from the 5 secondary windings (secondary windings) are used either per cell or per cell group. The invention does not place any restrictions on the converter operating principle, it can be any type of bidirectional charge / power transfer, preferably for example bidirectional flyback. According to the invention, the circuits associated with the primary windings of all converters are connected to the same power bus through which the charge / power transfer from one cell / group of cells to any other cell / group of cells is possible.

The power bus voltage can be, for example, about 12Vdc or 24Vdc. According to a preferred embodiment of the invention, the power bus is connected directly or via a resistor / diode, for example, over four serially connected cells (12Vdc bus-15) or over eight serially connected cells (24Vdc bus). The bus can also be completely separate, without any galvanic connection to the cell. It may be advantageous from the point of view of isolation between the entry and the secondary to tie the bus potential midway through the battery, whereby the required level of insulation is only half that of connecting the bus to the other end of the battery, e.g.

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According to an essential feature of the invention, the battery management system ensures that the sum of the charges transferred to and taken from the power bus remains on average zero, i.e. the sliding time integral of the charge pulses is always close to zero. As a result, the bus voltage remains substantially constant, and, for example, if the bus is galvanically connected directly over a group of cells, it will not be the case. the cell group's charge status will drift in any direction due to joining the power bus.

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The method and apparatus of the invention allow complete control of the battery 9 30 regardless of the number of serially connected cells, which is a prerequisite for utilizing the full battery performance. The method is easy to implement and the hardware required is simple and straightforward.

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inexpensive.

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g 35 Brief Description of Drawings o

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The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: 4

Figure 1 shows a battery consisting of cells connected in series,

Figure 2 shows an example of a characteristic characteristic of a battery cell,

Fig. 3 is an example of a prior art 5 solution for balancing battery cells,

Fig. 4 is an example of a solution according to the invention for equalizing the charge of a battery cell,

Figure 5 shows another example of a solution according to the invention for equalizing battery cell charges, Figure 6 shows a bidirectional flyback converter, and

Figure 7 illustrates an example of power bus current pulses and their time integral.

Detailed Description of the Invention 15

Fig. 1 shows a battery of cells C 1 -C n connected in series with poles B + (positive terminal) and B- (negative terminal) connected to the external charge / load circuit. The number of cells N can be up to several hundreds, depending on the desired voltage Ub of the battery. The optimal load capacity of the cell requires that the state of charge (SoC) of the cells be close to each other, which is also generally reflected by the fact that the cell voltages Uci-Ucn are approximately equal. The external charge / load current iB passes through all the cells, thereby causing the same change in the charge of all serially connected cells (the charge is a time integral of the current-25).

Figure 2 shows the characteristic behavior of the cell voltage t? as a function of the remaining charge in the cell. The voltage has a certain lower limit Umin, ^ which lower voltage means the so-called. deep discharge mode, where operation can cause permanent damage. The cell voltage also has a certain upper limit, Umax, which can also cause permanent damage when charging a high amperage voltage. Normally, therefore, the aim is to operate between these voltage limits.

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The figure shows the characteristic curves of two cells differing in their charge capacity, with the full capacity of the cell 1 being m Q 35 Qd and the cell 2 correspondingly slightly higher QC2. It can be readily seen from the figure that if both cells are charged - from the seasonal limit (charge state 0) with the same current, the charge must stop when the cell 1 reaches full charge. In order to fully charge the cell 2, to utilize the full capacity of the entire battery, some balancing system must be used which transfers the charge from the cell 1 to the cell 2. Conversely, when loading both fully charged cells with the same current, 2 charges to cell 1 to maintain charge balance.

Figure 3 illustrates a known principle for preventing over-voltage of cells when charging a battery. The system includes a serial connection of a relay (Si - Sn) and a resistor Rbal connected to each cell. If an io cell threatens to overcharge, a resistor is connected to it so that a part of the sensor in question is connected. the cell discharges to the resistor.

However, this method cannot help prevent deep discharge of a weaker cell, but must be treated, for example, by stopping the battery in time as determined by the weakest cell.

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Fig. 4 is an example of a system according to the present invention for maintaining the charge balance of a battery cell. In the example, each cell is associated with a balancing unit CM that enables bidirectional charge transfer. The charge transfer is performed by a power converter having galvanically separated primary and secondary circuits, which primary circuit is located at the external power bus potential ΡΡΒ and which secondary circuit is located at the cell potential Pc.

The power bus PB thus links all the balancing units CM. The voltage of the power bus preferably corresponds to the voltage of a few cells connected in series, for example to the voltage of four cells (about 12 Vdc), as shown in the figure. The power bus may be floating, completely detached from the battery, or it may be connected to one of the cell groups, e.g. The bus voltage can also be controlled higher than the voltage of the connected 'cell array', whereby the bus charge pulses do not circulate through the o ™ cell array. In this case, for example, a diode o 30 Vpb as shown in the figure can be used, which prevents the discharge of the higher voltage of the power bus to the voltage of the lower cell array.

g According to the invention, the control unit 0_CU controlling the operation of the accumulator controls the operation of the balancing units CM via the control bus CB so that the cells transferred from the cells to the power bus and the cells transferred from the power bus to the cells

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g 35, the sum of the charges is an average of 0. In this way, when the power bus is connected to a cell group Cgi, not to the cell in question. the charge balance of the cell group was disturbed by connection to the bus. The charge balance of the power bus can be monitored by measuring the current of one of the interconnectors (1, 2), for example by means of the shunt resistor Rpb according to Fig. 4-6, shown as an alternate connection by dashed lines. If the power bus is disconnected from the battery or more high voltage than the group connected to it via diode Vpb shown as an alternate connection with dashed lines, the corresponding charge balance control is achieved by monitoring the voltage of the te-5 bus; at equilibrium, the voltage remains within the set limits.

Figure 5 shows another example of a system according to the invention. In it, the CGM balancing units are cell group specific; In the case of the ίο pattern, the battery is divided into four cell groups CGi - CGn, each of which has its own common unit. By the way, the operation is the same as in Fig. 4, but now the balancing unit has as many secondary circuits as there are cells connected to the unit, i.e. in this case 4 secondary circuits. The advantage of such a system is the lower cost than the cost per cell-15 system of Figure 4. Of course, the number of cells in the same group may be more than 4. The maximum number is mainly limited by practical implementation problems, such as, for example, the number of secondary windings that can fit on the same frame in a power converter transformer.

The control unit CU, the control bus CB, the current measuring shunt Rpb 20 and the voltage isolation diode Vpb may also have the same meaning in the example of FIG. 5 as in the example of FIG. 4, for reasons of clarity only. The control unit CU may be a separate unit, or it may advantageously be integrated into a larger balancing unit CGM.

Figure 6 illustrates, in simplified form, a preferred embodiment of a charge transfer converter related to the invention. This is a so-called. a bidirectional flyback converter comprising a primary circuit (primary winding NP, primary switch Vp) and one (with respect to the example of FIG. 4) or more (with respect to the example of FIG. 5 ^) secondary circuits (secondary windings Nsi to Nsn, secondary switches Vsi to Vsn). The switches are preferably MOSFET transistors whose internal structure o is known to include a diode. A capacitor Cpb may be coupled to the voltage equalizer of the primary circuit power bus interface, which is of advantageous importance especially

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when the power bus is disconnected from the battery or its voltage is higher than c \ j the voltage of the cell group connected to it via the diode, m § 35 When the converter according to the figure is to transfer the charge pulse

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c \ j from the power bus to the cell, the primary switch Vp is initially directed to conductive. The flyback transformer primary winding current will then increase linearly until the switch is controlled non-conducting. The energy charged to the magnetic circuit of the transformer Ti is then discharged into the secondary circuit with the lowest voltage. If no secondary switch is controlled to conduct, the energy is discharged through the secondary diodes' internal diodes to the cell with the lowest voltage. By using power fetuses with a low channel resistance such that their conductive state voltage is significantly lower than the output voltage of the fetal diode 5 (about 0.7 V), for example 0.2 V, control of the secondary switches allows to select which cell the transformer magnetic circuit is discharged, regardless of small voltage differences between the cell voltages.

Correspondingly, when the converter is to transfer the charge pulse from the cell to the power bus, the secondary switch Vs conducting-10 corresponding to the desired cell is initially controlled. The secondary winding current will then increase linearly until the switch is controlled non-conducting. The energy of the magnetic circuit of the transformer Ti is then discharged to the primary circuit (= power bus) via the internal diode of the primary switch.

Fig. 7 shows the characteristic behavior of the power bus current ipe and its time integral 15 JipB when using the flyback converter of Fig. 6 and the charge transfer zero method according to the invention. The current iPB curve shape is triangular in nature, and when the battery management system ensures that the positive and negative current pulse areas within any reasonably long time window, such as 1 ms, are equal to py-20, the current time integral JiPB also remains reasonably close to zero is zero). This is a prerequisite to ensure that the charge balance of the cell array connected to the bus does not drift in any direction and that the voltage of the bus disconnected from the battery or bus connected to the cell but with higher voltage remains within the desired limits.

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It will be apparent to one skilled in the art that the various embodiments of the invention are not limited solely to the examples above, but may vary within the scope of the following claims.

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Claims (11)

A method for equalizing cell-specific charges in a battery consisting of series-connected cells (Ci-Cn), wherein the battery utilizes bidirectional power converters to transfer charges to / from the cell, characterized in that the power converters have primary and secondary circuits it is interconnected and having the primary circuits of the power converters connected in parallel to a common direct current circuit (power bus, PB) through which the charges can be freely transferred from one power converter to another. Method according to Claim 1, characterized in that the charge transfer is controlled such that the moving average of the sum of the charges transferred from the battery cells (Ci to Cn) to the power bus and the charges transferred from the power bus to the battery cells is substantially zero. 20 3. A method according to claim 1 or 2, characterized in that the power bus is connected to a positive and negative terminal of a cell group (CGI-Cgn) having at least two cells and a result of the measurement result is used to control the charge balance of the power bus. The method according to claim 1 or 2, characterized in that the power bus 9 is galvanically isolated from the battery and o the power bus voltage is measured and power is applied from the measurement result to control the bus charge balance advantageously so that the power bus voltage CL remains substantially constant. <x »C \ J LO § 35 Method according to claim 1 or 2, characterized in that the power bus is connected via a diode (VpB) to the positive and negative terminals of a cell array, and the power bus voltage is measured and used to control the power bus charge balance preferably so that the power bus voltage remains substantially constant. and higher than that. voltage of the cell array. 6. Apparatus for equalizing cell-specific charge in a battery consisting of a series-connected cell (Ci-Cn) having two-way power converters for transferring charge to / from the cell, characterized in that the power converters have galvanically separated primary and secondary circuit to which it is connected, whose primary converters of the power converters are connected in parallel to a common direct current circuit (power bus, PB) through which the battery management control unit (CU) is adapted to control the transfer of charges from one power converter to another. Apparatus according to claim 6, characterized in that the battery management control unit (CU) 20 is arranged to control the charge transfer such that the moving average of the sum of the charges from the battery cells to the power bus and from the power bus to the battery cells is substantially zero. 8. Apparatus according to claim 6 or 7, characterized in that the power bus is connected to a positive and negative terminal of a cell group (CGI-Cgn) having at least two cells, and the control unit controlling the battery is adapted to measure from the cell group to the power bus. transmitting current and using the measurement result to control the charge balance of the power bus Tj-9 30. 00 0 9. Apparatus according to claim 6 or 7, characterized in that the power bus C \ 10 01 is galvanically isolated from the battery and the control unit controlling LO § 35 is adapted to measure power bus voltage and control power bus charge balance based on the measurement result essentially constant. 10. Apparatus according to claim 6 or 7, characterized in that the power bus is connected via a diode (VPB) to the positive and negative terminals of a cell array, and a control unit controlling 5 batteries is adapted to measure the power bus voltage and control the power bus charge the voltage remains substantially constant and higher than the current. voltage of the cell array. ίο Apparatus according to one of the preceding claims 6 to 10, characterized in that the power converter is a two-way flyback converter comprising a primary circuit having a primary winding (NP) and at least one primary switch (VP) and one or more secondary circuits having one or more secondary windings (NSi - NSn) and at least one secondary switch (VSi - VSn) · 15 CO δ c \ already CO o X cc CL C \ l CD CM LO O) OO CM