DC power supply with built-in battery

Abstract

According to the present invention, a secondary current of a step-down transformer is circulated to a current circulation circuit formed by two batteries and two secondary coils of the step-down transformer. The batteries supply a load current to an electric load through the secondary coils. A DC magnetic flux formed by the load current in the magnetic core of the step-down transformer is reduced by a DC magnetic flux reduction current supplied to a primary coil of the step-down transformer. The secondary coils apply alternately repeating positive and negative pulse voltages to each cell of the batteries. The internal resistance of each cell is detected on the basis of a voltage difference between the positive and negative pulse voltages and a battery current. One of the two batteries is connected to an external load through one of the two secondary coils, and the other one of the two batteries is connected to the external load through the other of the two secondary coils. The AC voltages induced in the two secondary coils have the same direction and equal number of turns.

Description

DC power supply with built-in battery Related Art References

[0001] Japanese Patent Nos. 7301208 and 7357175, owned by the present applicant, are prior patents related to this application, the entire contents of which are incorporated by reference.

[0002] The present invention relates to a battery-integrated DC power supply, and more particularly to a battery-integrated DC power supply that supplies DC power to a propulsion motor.

[0003] Electric vehicles have a DC power supply that includes a secondary battery such as a lithium-ion battery. An AC internal heating circuit is known that supplies AC current to this battery. When AC current is supplied, the battery heats up due to its ohmic losses.

[0004] Figure 1 shows an example of a conventional AC internal heating circuit. This circuit includes a transformer T, a switch S, and a diode D. The transformer T has a primary coil W1 and a secondary coil W2. The primary coil W1, the switch S, and the resistance R of the battery E form a closed discharge loop circuit. Furthermore, the diode D, the secondary coil W2, and the resistance R of the battery E form a closed charge loop circuit. When the switch S is turned on, a discharge current Id flows through the closed discharge loop circuit. When the switch S is turned off, a charge current Ic flows through the closed charge loop circuit.

[0005] Patent Documents 1 and 2 disclose a current circulation circuit formed by a battery, a storage element, and a secondary coil of a step-down transformer. When a primary AC current is supplied to the primary coil of the step-down transformer, a secondary AC current circulates in this circulation circuit. As a result, the battery can be heated by a compact step-down transformer.

[0006] Japanese Patent No. 7301208 Japanese Patent No. 7357175

[0007] An object of the present invention is to provide a highly reliable battery-powered DC power supply. Preferably, this battery-powered DC power supply is applied to electric vehicles. Electric vehicles of the present invention include vehicles with traction motors as well as ships, submarines, airplanes, drones, and the like with propulsion motors.

[0008] The battery-equipped DC power supply of the present invention has a current circulation circuit formed by a battery, a storage element, and the secondary coil of a step-down transformer. The storage element is composed of a battery or a smoothing capacitor. When a primary AC current is supplied from an oscillator to the primary coil of the step-down transformer, a secondary AC current circulates in this current circulation circuit. Furthermore, the battery supplies a load current to an electrical load through the secondary coil of the step-down transformer.

[0009] In a first aspect of the present invention, a DC flux reducing current is supplied to the primary coil of the step-down transformer to reduce the DC magnetic flux generated in the magnetic core of the step-down transformer by the load current, thereby allowing the use of a compact magnetic core.

[0010] In a second aspect of the present invention, a secondary coil of a step-down transformer applies alternately repeated positive and negative pulse voltages to each cell of a battery, and an electrical parameter related to the internal resistance of each cell is detected based on the difference between the positive and negative pulse voltages and the battery current, thereby enabling accurate detection of the electrical parameter related to the internal resistance of each cell.

[0011] In a third aspect of the present invention, a step-down transformer has two secondary coils and two battery packs. The two secondary coils and two battery packs form a current circulation circuit. The first battery pack is connected to an external load through the first secondary coil, and the second battery pack is connected to the external load through the second secondary coil. Two secondary voltages induced by the two secondary coils with equal numbers of turns have the same direction in the current circulation circuit. When one of the two battery packs is discharged, the other is charged. In other words, battery power discharged from one battery pack by the circulating current is temporarily stored in the other battery pack. Therefore, an oscillator connected to the primary coil of the step-down transformer only needs to supply power to the primary coil equivalent to the AC power loss due to the internal resistance of the two battery packs.

[0012] In a preferred embodiment, the current circulation circuit includes a first contactor and a second contactor. The first contactor is connected in series with the first battery pack and the first secondary coil. The second contactor is connected in series with the second battery pack and the second secondary coil. This allows a load current to be supplied to the external load even if one of the two battery packs fails.

[0013] In a preferred embodiment, the current circulation circuit includes a contactor. During a transient period when the contactor is open, the step-down transformer generates a secondary voltage to reduce the load current flowing through the secondary coil, thereby reducing contactor sparks.

[0014] In a preferred embodiment, the controller has a battery heating mode for heating a cold battery pack by circulating current, which can be implemented in conjunction with a battery discharging or charging operation.

[0015] In a preferred embodiment, the contactor is connected in parallel with a bypass circuit comprising a series-connected capacitor and diode. The diode allows the contactor surge current to charge the capacitor when the contactor is opened, and therefore bypasses the contactor surge current that occurs when the contactor is turned off.

[0016] FIG. 1 is a circuit diagram showing a conventional battery heating circuit in which AC current flows between a transformer and a battery. It is a circuit diagram showing a battery-integrated DC power supply according to an embodiment. FIG. 3 is a flowchart showing battery pack connection switching. FIG. 4 is a schematic circuit diagram showing a positive component of a secondary current circulating in an AC current circulation circuit. FIG. 5 is a schematic circuit diagram showing a negative component of a secondary current circulating in an AC current circulation circuit. FIG. 6 is a schematic circuit diagram showing a secondary current for reducing an arc in a first contactor. FIG. 7 is a schematic circuit diagram showing a secondary current for reducing an arc in a second contactor. FIG. 8 is a schematic circuit diagram showing a circuit for detecting the internal resistance of each cell. FIG. 9 is a timing chart showing the operation of sequentially detecting the internal resistance of each cell. FIG. 10 is a flowchart showing various modes using a common step-down transformer. FIG. 11 is a circuit diagram showing a positive current flowing through an H-bridge acting as an oscillator. FIG. 12 is a circuit diagram showing a negative current flowing through an H-bridge acting as an oscillator. FIG. 13 is a timing chart showing the current and voltage of the H-bridge shown in FIGS. 11 and 12. Fig. 14 is a flowchart showing an internal resistance detection mode, Fig. 15 is a circuit diagram showing a modified example of the current circulation circuit, and Fig. 16 is a circuit diagram showing another modified example of the current circulation circuit.

[0017] A DC power supply of the present invention applied to a battery electric vehicle (BEV) will be described with reference to Figures 1-10. However, the present invention can also be applied to other DC power supplies, such as stationary DC power supplies.

[0018] The DC power supply shown in FIG. 2 includes a battery circuit 10, cables 141-143, and a contactor box 3. The battery circuit 10 includes a first battery pack 11, a second battery pack 12, a first battery management system (BMS) 110, and a second battery management system (BMS) 120. The battery pack 11 includes four series-connected cells 111-114. The battery pack 12 includes four series-connected cells 121-124. The contactor box 3 supplies DC power from the battery packs 11 and 12 to a motor drive circuit 2, which will be described later. The motor drive circuit 2 includes a three-phase inverter that applies a three-phase AC voltage to a traction motor (not shown).

[0019] The positive terminals of the battery packs 11 and 12 are connected to the contactor box 3 via a common cable 143. The negative terminal of the battery pack 11 is connected to the contactor box 3 via a cable 142. The negative terminal of the battery pack 12 is connected to the contactor box 3 via a cable 141.

[0020] The battery pack 11 is monitored by a battery management system (BMS) 110, and the battery pack 12 is monitored by a battery management system (BMS) 120. The BMS 110 detects the current of the battery pack 11 and the cell voltages of the cells 111-114. Similarly, the BMS 120 detects the current of the battery pack 12 and the cell voltages of the cells 121-124.

[0021] The contactor box 3 houses a high-side circuit 31, a low-side circuit 32, a step-down transformer 5, an oscillator 6, and a controller 7. The contactor box 3 has a high-side terminal 9H and a low-side terminal 9L for supplying DC power to the motor drive circuit 2.

[0022] The high-side terminal 9H is connected to the cable 143 through the high-side circuit 31. The high-side circuit 31 has a common contactor 43, a pre-charge contactor 44, a low-resistance element 45, a capacitor 81C, and a diode 81D. The common contactor 43 connects the cable 143 to the high-side terminal 9H. The pre-charge contactor 44 and the low-resistance element 45, which are connected in series, form a pre-charge circuit connected in parallel with the common contactor 43.

[0023] As is well known, the precharge contactor 44 is turned on before the common contactor 43 is turned on. As a result, a smoothing capacitor (not shown) connected to the high-side terminal 9H and the low-side terminal 9L is slowly charged through the low-resistance element 45. This smoothing capacitor reduces the switching noise voltage generated by the motor drive circuit 2.

[0024] Furthermore, a series-connected capacitor 81C and diode 81D form a bypass circuit connected in parallel with the common contactor 43. This bypass circuit prevents contact sparks that may occur when the common contactor 43 is turned off. The operation of this bypass circuit will be described later.

[0025] The low-side terminal 9L is connected to cables 141 and 142 through a low-side circuit 32. The low-side circuit 32 has contactors 41 and 42 and secondary coils 52 and 53. The low-side terminal 9L is connected to a cable 141 through a contactor 41. Furthermore, the low-side terminal 9L is connected to a cable 142 through a contactor 42.

[0026] The step-down transformer 5 has a primary coil 51 and two secondary coils 52 and 53, each wound around a magnetic core 50. The secondary coil 52 is connected in series with the contactor 42, and the secondary coil 53 is connected in series with the contactor 41. For example, the secondary coils 52 and 53 each have one turn, and the primary coil 51 has 100 turns. The secondary coils 52 and 53 have the same number of turns. The number of turns of the primary coil 51 can be selected as appropriate. The oscillator 6 applies a primary AC voltage to the primary coil 51. The oscillator 6 is an H-bridge circuit consisting of four MOSFETs.

[0027] The controller 7 controls the operation of the contactors 41-44 and the oscillator 6 based on the internal resistance values ​​of each cell calculated by the BMS 110 and the BMS 120.

[0028] The switching control of the two battery packs 11 and 12 will be explained with reference to FIG. 3. The common contactor 43 is turned on before the start of this battery pack switching control. First, it is determined whether the battery pack 11 is normal (S100). In this embodiment, it is determined whether all cells in the battery pack 11 have internal resistance values ​​within a predetermined tolerance range. If the internal resistance value of at least one cell in the battery pack 11 is outside this tolerance range, the contactor 42 is opened (S102). As a result, the motor drive circuit 2 is operated only by the battery pack 12.

[0029] Next, it is determined whether the battery pack 12 is normal (S104). In this embodiment, it is determined whether all the cells in the battery pack 12 have internal resistance values ​​within a predetermined tolerance range. If the internal resistance value of at least one cell in the battery pack 12 is outside this tolerance range, the contactor 41 is opened (S106). This allows the motor drive circuit 2 to be operated only by the battery pack 11. Next, if the internal resistance values ​​of all the cells included in the battery packs 11 and 12 are within this tolerance range, the two contactors 41 and 42 are closed (S108). This reduces losses in the battery packs 11 and 12. Furthermore, it is possible to prevent the SOC difference between the battery packs 11 and 12 from increasing.

[0030] 2 has a pair of battery pack 11 and contactor 42 connected in series, and a pair of battery pack 12 and contactor 41 connected in series. Furthermore, these two pairs are connected in parallel. This allows the two battery packs 11 and 12 to be operated in parallel or individually.

[0031] Next, the operation of supplying AC current to the battery packs 11 and 12 by the step-down transformer 5 will be described with reference to FIGS. 4 and 5. This AC current supply operation is typically performed in cold environments, but can also be performed to improve the electrical characteristics of the battery packs. The oscillator 6 applies an AC voltage VT1 to the primary coil 51. The AC voltage VT1 consists of alternating positive and negative pulse voltages. When the AC voltage VT1 is applied to the primary coil 51, a secondary voltage VT2 is induced separately in the secondary coils 52 and 53. The directions of these two secondary voltages VT2 are the same.

[0032] Battery pack 11 has an open-circuit voltage VB and an internal resistance r. Battery pack 12 has an open-circuit voltage VB and an internal resistance r20. The open-circuit voltages of the two battery packs 11 and 12 are actually different. For simplicity, it is assumed that battery packs 11 and 12 have the same open-circuit voltage VB. The two secondary voltages VT2 circulate a secondary current IC through the current circulation circuit consisting of the two battery packs 11 and 12. The two battery packs 11 and 12 supply a load current IL to the motor drive circuit 2 through a connection point 54 between the two secondary coils 52 and 53. The current component supplied to the motor drive circuit 2 by the secondary coil 52 has an equal amplitude and an opposite direction to the current component supplied to the motor drive circuit 2 by the secondary coil 53. As a result, the secondary coils 52 and 53 do not supply a secondary current to the motor drive circuit 2.

[0033] 4 shows the current flow during the period when the positive pulse voltage is applied. The secondary voltage VT2 of the secondary coil 52 reduces the current component supplied to the motor drive circuit 2 by the battery pack 11. Conversely, the secondary voltage VT2 of the secondary coil 53 increases the current component supplied to the motor drive circuit 2 by the battery pack 12. As a result, the load current IL is not affected by the two secondary coils 52 and 53.

[0034] 5 shows the current flow during the period when a negative pulse voltage is applied. The secondary voltage VT2 of the secondary coil 52 increases the current component supplied from the battery pack 11 to the motor drive circuit 2. Conversely, the secondary voltage VT2 of the secondary coil 53 decreases the current component supplied from the battery pack 12 to the motor drive circuit 2. As a result, the load current IL is not affected by the two secondary coils 52.

[0035] Consequently, when the secondary coil 52 connected in series with the battery pack 11 has the same number of turns as the secondary coil 53 connected in series with the battery pack 12, the AC current IC can circulate only through the battery packs 11 and 12. Consequently, the secondary coils 52 and 53 do not affect the load current IL.

[0036] However, the inductance of the secondary coil 52 or 53 will enhance the spark of the contactor 41 or 42. A contactor protection operation to solve this problem will be explained with reference to FIGS.

[0037] 6 shows the current flow during a transient period when the contactor 42 is open. The battery pack 11 supplies a current I1 to the motor drive circuit 2, and the battery pack 12 supplies a current I2 to the motor drive circuit 2. The oscillator 6 applies a positive pulse voltage VT1 to the primary coil 51 during this transient period. This causes the secondary coils 51 and 52 to circulate a positive circulating current ICP through the battery packs 11 and 12 and the contactors 41 and 42. As a result, the battery pack 11 is charged, and the battery pack 12 is discharged. Consequently, the current flowing through the contactor 42 during the transient period when the contactor 42 is open is (I1 - ICP). As a result, sparks at the contactor 42 are reduced.

[0038] 7 shows the current flow during the transient period when the contactor 41 is open. The battery pack 11 supplies a current I1 to the motor drive circuit 2, and the battery pack 12 supplies a current I2 to the motor drive circuit 2. During this transient period, the oscillator 6 applies a negative pulse voltage VT1 to the primary coil 51. This causes the secondary coils 51 and 52 to circulate a negative circulating current ICN through the battery packs 11 and 12 and the contactors 41 and 42. As a result, the battery pack 11 is discharged and the battery pack 12 is charged. Consequently, the current flowing through the contactor 41 during the transient period when the contactor 41 is open is (I2 - ICN). As a result, sparks at the contactor 42 are reduced.

[0039] Sparks that occur during the period when the common contactor 43 is open are reduced by the bypass circuit 81 shown in Fig. 2. The bypass circuit 81, which is connected in parallel with the common contactor 43, consists of a capacitor 81C and a diode 81D connected in series. The diode 81D bypasses the spark current of the common contactor 43. During the transition period when the common contactor 43 changes from a closed state to an open state, the bypass current flows through the capacitor 81C and the diode 81D. As a result, the arc of the common contactor 43 is effectively extinguished.

[0040] When the common contactor 43 is closed, the capacitor 81C is short-circuited through the common contactor 43. However, the diode 81D blocks the current caused by this short circuit. As a result, the charge stored in the capacitor 81C is slowly discharged through the high reverse resistance of the diode 81D during the on-period when the common contactor 43 is closed.

[0041] In this embodiment, contactor 42 is opened when at least one cell in battery pack 11 is faulty, and contactor 41 is opened when at least one cell in battery pack 12 is faulty, so that the electric vehicle can continue to operate even if one cell is faulty.

[0042] Internal short circuits in cells due to dendrite growth are known to be a major cell failure. It is also known that battery fires caused by thermal runaway in cells can occur due to these internal short circuits. Furthermore, it is known that the internal resistance of a cell gradually decreases due to dendrite growth. Therefore, by monitoring the internal resistance of a cell, it is possible to predict the occurrence of an internal short circuit in the near future. However, detecting the internal resistance of a cell to detect an internal short circuit essentially needs to be performed constantly. This means that the internal resistance detection operation of each cell must be performed even during battery discharging or charging. However, detecting the internal resistance of each cell during discharging or charging is difficult due to fluctuations in the discharge current or charge current.

[0043] 8 and 9 disclose a novel cell resistance detection operation using a step-down transformer 5. FIG. 8 shows a part of a BMS 110 that detects the cell voltages of a battery pack 11. A BMS 120 that detects the cell voltages of a battery pack 12 is essentially the same as the BMS 110. In FIG. 8, only four cells 11A-11D of the battery pack 11 are shown. The BMS 110 transmits the cell voltages V11-V14 of the cells 11A-11D to the controller 7 in sequence.

[0044] Cell 11A has an open-circuit voltage (OCV) V1 and an internal resistance (IR) r1. Cell 11B has an open-circuit voltage V2 and an internal resistance r2. Cell 11C has an open-circuit voltage V3 and an internal resistance r3. Cell 11D has an open-circuit voltage V4 and an internal resistance r4. Cells 11A-11D supply a load current IL to the motor drive circuit 2. Furthermore, the secondary voltage induced in the secondary coils 52 and 53 circulates a circulating current IC through cells 11A-11D. As a result, the internal resistances r1-r4 of cells 11A-11D generate voltage drops due to the load current IL and the circulating current IC, respectively. Therefore, the cell voltage V11 of cell 11A is V1 + r1 (IL + IC), and the cell voltage V12 of cell 11B is V2 + r2 (IL + IC). Similarly, the cell voltage V13 of cell 11C is V3+r3(IL+IC), and the cell voltage V14 of cell 11D is V4+r4(IL+IC).

[0045] The oscillator 6 alternately applies a positive pulse voltage and a negative pulse voltage to the primary coil 51 of the step-down transformer 5. The secondary coils 52 and 53 circulate a substantially constant positive pulse current ICP to the battery packs 11 and 12 during a positive pulse period when the positive pulse voltage is applied to the primary coil 51. Furthermore, the secondary coils 52 and 53 circulate a substantially constant negative pulse current ICN to the battery packs 11 and 12 during a negative pulse period when a negative pulse voltage is applied to the primary coil 51. The integral of the positive pulse current ICP is equal to the integral of the negative pulse current ICN.

[0046] The cell voltages V11-V14 are selected in turn by the BMS 110 during the positive pulse period, converted into digital signals by the A / D converter 60, and then sent to the controller 7. Similarly, the cell voltages V11-V14 are selected in turn by the first BMS 110 during the negative pulse period, converted into digital signals by the A / D converter 60, and then sent to the controller 7. In this way, eight cell voltages are sent to the controller 7.

[0047] The operation of detecting the internal resistance r1 of cell 11A will now be described. First, during the positive pulse period when a positive pulse current ICP flows through cell 11A, the cell voltage V11 is detected. This cell voltage V11 is referred to as the positive cell voltage V11P. Next, during the negative pulse period when a negative pulse current ICN flows through cell 11A, the cell voltage V11 is detected. This cell voltage V11 is referred to as the negative cell voltage V11N. It is assumed that the positive pulse current ICP and the negative pulse current ICN have equal amplitudes. It is also assumed that cell 11A has a constant open-circuit voltage V1 during the positive pulse period and the negative pulse period.

[0048] Next, the voltage difference ΔV (= V11P - V11N) between the two cell voltages V11P and V11N is detected. This voltage difference ΔV does not include the open-circuit voltage V1 of cell 11A. Furthermore, when the load current IL is constant, the voltage difference ΔV does not include the voltage drop (r1 · IL) across the internal resistance r1 due to the load current IL. Therefore, the voltage difference ΔV is 2IC · r1, and the internal resistance r1 is ΔV / 2IC. Ultimately, by utilizing the AC circulating current supplied to the battery pack 11 by the step-down transformer 5, the internal resistance r1 can be detected without being affected by the load current IL or the open-circuit voltage V1 while the battery pack 11 is discharging to the motor drive circuit 2. The internal resistances of cells 11B-11D are also detected in a similar manner.

[0049] The circuit of the BMS 110 will be further explained with reference to Figure 8. The positive terminal of cell 11A is connected to signal line 71 through switch S1. The positive terminal of cell 11B is connected to signal line 72 through switch S2. The positive terminal of cell 11C is connected to signal line 71 through switch S3. The positive terminal of cell 11D is connected to signal line 72 through switch S4. The negative terminal of cell 11D is connected to signal line 71 through switch S5. Each of switches S1-S5 consists of two MOSFETs connected in series in an anti-reverse direction. The two signal lines 71 and 72 are connected to a capacitor 70.

[0050] When switches S1 and S2 are turned on, the voltage of capacitor 70 is equal to cell voltage V11 of cell 11A. When switches S2 and S3 are turned on, the voltage of capacitor 70 is equal to cell voltage V12 of cell 11B. When switches S3 and S4 are turned on, the voltage of capacitor 70 is equal to cell voltage V13 of cell 11C. When switches S4 and S5 are turned on, the voltage of capacitor 70 is equal to cell voltage V14 of cell 11D.

[0051] Each voltage of the capacitor 70 is sent to the A / D converter 60 by turning on two of the four switches S6-S9. The A / D converter 60 converts each received voltage of the capacitor 70 into a digital signal, and these digital signals are sent to the controller 7. Like the switches S1-S4, the switches S6-S9 are each made up of two MOSFETs connected in series in the reverse direction.

[0052] 9 is a timing chart showing the switching operations of switches S1-S9. In the first and second halves of period T1, switches S1 and S2 are turned on, and the voltage of capacitor 70 is equal to cell voltage V11. Similarly, in the first and second halves of period T2, switches S2 and S3 are turned on, and the voltage of capacitor 70 is equal to cell voltage V12. In the first and second halves of period T3, switches S3 and S4 are turned on, and the voltage of capacitor 70 is equal to cell voltage V13. Similarly, in the first and second halves of period T4, switches S4 and S5 are turned on, and the voltage of capacitor 70 is equal to cell voltage V14.

[0053] In the first half of period T1, switches S6 and S9 are turned on immediately after switches S1 and S2 are turned off. In the second half of period T1, switches S7 and S8 are turned on immediately after switches S1 and S2 are turned off. Similarly, in the first half of period T2, switches S6 and S9 are turned on immediately after switches S2 and S3 are turned off. In the second half of period T2, switches S7 and S8 are turned on immediately after switches S2 and S3 are turned off.

[0054] In the first half of period T3, switches S6 and S9 are turned on immediately after switches S3 and S4 are turned off. In the second half of period T3, switches S7 and S8 are turned on immediately after switches S3 and S4 are turned off. Similarly, in the first half of period T4, switches S6 and S9 are turned on immediately after switches S4 and S5 are turned off. In the second half of period T4, switches S7 and S8 are turned on immediately after switches S4 and S5 are turned off.

[0055] As a result, each cell voltage is transmitted to the capacitor 70 in sequence under the control of the switches S1-S4. Furthermore, the voltage of the capacitor 70 is transmitted to the A / D converter 60 under the control of the switches S6-S9, which are implemented after the switches S1-S4 are turned off. The switches S6-S9 have the function of reversing the direction of the voltage of the capacitor 70. The positive pulse current ICP flows in the first half of the period T1-T4 of the circulating current IC, and the negative pulse current ICN flows in the second half of the period T1-T4 of the circulating current IC.

[0056] Therefore, two types of cell voltages of one cell are detected in each of the periods T1-T4. For example, if the battery pack 11 consists of 100 cells connected in series, it takes 100 periods to detect the internal resistances of all the cells. In a modified embodiment, the capacitor 70 and the switches S6-S9k can be omitted. According to this embodiment, the A / D converter 60 directly converts the cell voltages V11-V14 into digital signals.

[0057] The internal resistance r of the cell is ΔV / (ICP+ICN), where ΔV is the voltage difference between the cell voltage value when the positive pulse current ICP flows through the battery pack and the cell voltage value when the negative pulse current ICN flows through the battery pack. Because the positive pulse current ICP and the negative pulse current ICN flow in opposite directions, the voltage difference ΔV can be increased.

[0058] Other electrical parameters of each cell can also be calculated based on the internal resistances r1-r4 of the cells 11A-11D. For example, the deterioration of the cells 11A-11D can be estimated based on the internal resistances r1-r4. Furthermore, the voltage drop across each internal resistance r1-r4 can be calculated from the internal resistance values ​​r1-r4 of the cells 11A-11D. The open-circuit voltages V1-V4 of each cell 11A-11D can be calculated from these voltage drops and the cell voltages V11-V14.

[0059] An example of control by the controller 7 will be described with reference to the flowchart shown in FIG. It is assumed that the contactors 41-43 are pre-closed. First, it is determined whether the temperature Tb of the battery packs 11 and 12 is lower than a predetermined threshold value Tth (S1000). If the determination result is Yes, an AC heating subroutine (S1020) is executed to supply AC current to the battery packs 11 and 12. If the determination result is No, this AC heating subroutine is not executed.

[0060] In this AC heating subroutine, the oscillator 6 supplies a primary AC current to the primary coil 51. This induces secondary voltages in the same direction in the secondary coils 51 and 52, and a secondary circulating current is supplied to the battery packs 11 and 12. As a result, the battery packs 11 and 12 are heated. When the temperatures of the battery packs 11 and 12 reach a predetermined value, this AC heating subroutine is terminated.

[0061] Next, an internal resistance detection subroutine is executed (S1040). In this internal resistance detection subroutine, positive and negative pulse currents are alternately supplied to each cell to detect the internal resistance of each cell of the battery packs 11 and 12. As a result, the BMS 110 detects the internal resistance value of the battery pack 11, and the BMS 120 detects the internal resistance value of each cell of the battery pack 12.

[0062] Next, it is determined whether all internal resistance values ​​detected from the battery pack 11 are within a predetermined tolerance range (S1060). If the determination result is No, the process proceeds to step S1080. In step S1080, a contactor protection subroutine is executed to protect the contactor 42. According to this contactor protection subroutine, the oscillator 6 generates a secondary circulating current. This secondary circulating current reduces the current flowing through the contactor 42. Preferably, this secondary circulating current is approximately equal to the load current IL. As a result, the current flowing through the contactor 42 becomes approximately zero. Next, the contactor 42 is opened (S1100). As a result, no arc occurs across the contactor 42.

[0063] If all the internal resistance values ​​of the battery pack 11 are within the allowable range, it is determined whether all the internal resistance values ​​of the battery pack 12 are within the allowable range (S1120). If the determination result is No, the process proceeds to step S1140.

[0064] In step S1140, a contactor protection subroutine is executed to protect the contactor 41. According to this contactor protection subroutine, the oscillator 6 generates a secondary circulating current. This secondary circulating current reduces the current flowing through the contactor 41. Preferably, this secondary circulating current is approximately equal to the load current IL. This reduces the current flowing through the contactor 41 to approximately zero. Next, the contactor 41 is opened (S1160). As a result, no arc occurs on the contactor 41.

[0065] If all the internal resistance values ​​of the battery packs 11 and 12 are within a predetermined tolerance range, the contactors 41 and 42 are closed (S1180). This allows the battery packs 11 and 12 to supply the load current IL to the motor drive circuit 2. In particular, it is important to determine whether the internal resistance r of each cell is higher than a predetermined minimum value. If the internal resistance r is lower than this minimum value, an internal short circuit in the cell is presumed.

[0066] The above-described embodiment illustrates an example of a DC power supply having a step-down transformer 5 for supplying AC current to the battery circuit 10. The secondary coils 52 and 53 of the step-down transformer 5 are disposed in a load current path that supplies DC current from the battery circuit 10 to the motor drive circuit 2. As a result, when the battery circuit 10 supplies a load current IL to the motor drive circuit 2, the load current IL generates magnetic flux in the magnetic core 50 of the step-down transformer 5. This magnetic flux is called load magnetic flux. The motor drive circuit 2, which generally comprises a three-phase inverter, includes a smoothing capacitor connected to the high-potential terminal 9H and the low-potential terminal 9L shown in FIG. 2. Therefore, the load current IL supplied from the battery circuit 10 to an electrical load such as the motor drive circuit 2 is primarily DC current. This DC current component may include a low-frequency current component.

[0067] The load current IL is composed of a DC current I1 flowing through the battery pack 11 and a DC current I2 flowing through the battery pack 12. When the DC current I1 flows through the secondary coil 52, a DC magnetic flux F1 is generated in the magnetic core 50. When the DC current I2 flows through the secondary coil 53, a DC magnetic flux F2 is generated in the magnetic core 50.

[0068] When the battery packs 11 and 12 have the same open circuit voltage and internal resistance, the current I1 is equal to the current I2, and the currents I1 and I2 are each half the load current IL. Therefore, when the battery packs 11 and 12 have the same open circuit voltage and internal resistance, the DC magnetic flux difference (F1-F2) formed in the magnetic core 50 becomes zero.

[0069] However, when the battery packs 11 and 12 have different open circuit voltages, the currents I1 and I2 are different. Similarly, when the battery packs 11 and 12 have different internal resistances, the currents I1 and I2 are different. Therefore, the DC magnetic flux difference (F1-F2) formed in the magnetic core 50 is not zero. As a result, when the primary coil 51 forms an AC magnetic flux in the magnetic core 50, this AC magnetic flux is biased by the DC magnetic flux difference (F1-F2).

[0070] Eventually, both this AC magnetic flux and the DC magnetic flux difference (F1-F2) flow through the magnetic core 50, making the magnetic core 50 more susceptible to saturation. This magnetic saturation increases distortion of the secondary AC voltage generated in the secondary coils 52 and 53. The magnetic saturation of the magnetic core 50 due to the DC magnetic flux difference (F1-F2) can be resolved by increasing the cross-sectional area of ​​the magnetic core 50. However, this increases the size, weight, and loss of the step-down transformer 5. Techniques for miniaturizing the step-down transformer 5 by reducing the DC magnetic flux difference (F1-F2) are described below.

[0071] 11 and 12 show an oscillator 6 formed by an H-bridge. The oscillator 6 is made up of four MOS transistors (MOSFETs) 61-64. The H-bridge consists of a first leg made up of series-connected MOS transistors 61 and 62, and a second leg made up of series-connected MOS transistors 63 and 64. The output terminal of the first leg is connected to one end of the primary coil 51, and the output terminal of the second leg is connected to the other end of the primary coil 51. The oscillator 6 supplies a primary AC current Ip to the primary coil 51.

[0072] 11 shows the flow of primary AC current Ip during the positive half-cycle (P+). Switch 61 is always on, and switch 62 is always off. The second leg is PWM switched. When switch 64 is on, primary AC current Ip flows from switch 61 through primary coil 51 to switch 64. When switch 64 is off, freewheel current If flows through switch 63 and switch 61.

[0073] 12 shows the flow of primary AC current Ip during the negative half-cycle (P-). Switch 63 is always on, and switch 64 is always off. The first leg is PWM switched. When switch 62 is on, primary AC current Ip flows from switch 63 through primary coil 51 to switch 62. When switch 62 is off, freewheel current If flows through switches 61 and 63.

[0074] 13 is a timing chart showing waveforms associated with the primary AC current Ip. The positive half-cycle (P+) shown in FIG. 11 and the negative half-cycle (P-) shown in FIG. 12 alternate. The duty ratio D+ of the PWM-switched switch 64 during the positive half-cycle (P+) is greater than the duty ratio D- of the PWM-switched switch 64 during the negative half-cycle (P-). As a result, the positive component of the primary AC current Ip during the positive half-cycle (P+) has a higher amplitude than the negative component of the primary AC current Ip during the negative half-cycle (P-).

[0075] The primary AC current Ip is divided into an AC current component IAC and a DC current component IDC. The positive and negative components of the AC current component IAC have equal amplitudes. The PWM duty ratio of switch 64 is D+ during the positive half-cycle (P+), and the PWM duty ratio of switch 62 is D- during the negative half-cycle (P-). When the duty ratios D+ and D- are substantially equal, the DC current component IDC is zero. When the duty ratio D+ is greater than the duty ratio D-, the DC current component IDC is positive. When the duty ratio D+ is less than the duty ratio D-, the DC current component IDC is negative. In FIG. 13, the DC current component IDC is positive. Consequently, the H-bridge 6 is able to supply to the primary coil 51 both an AC current component IAC with the required waveform and a DC current component IDC with the required amplitude.

[0076] Generally, the open circuit voltages and internal resistances of the battery packs 11 and 12 are different from each other. Therefore, the currents I1 and I2, which are parts of the load current, are generally different from each other. Similarly, the currents I1 and I2, which are parts of the charging current, are generally different from each other.

[0077] A DC magnetic flux Fdc is formed in the magnetic core 50 by the current difference (I1-I2). Consequently, the AC current component IAC and DC current component IDC flowing through the primary coil 51, the current I1 flowing through the secondary coil 52, and the current I2 flowing through the secondary coil 53 form a magnetic flux in the magnetic core 50.

[0078] For example, assume that the magnetic reluctance of magnetic core 50 is Rm, the number of turns in primary coil 51 is 100, and the number of turns in secondary coils 52 and 53 is each 1. The total magnetic flux in magnetic core 50 is (100IAC + 100IDC + (I1 - I2)) / Rm. Therefore, when the DC magnetic flux Fdc (= (I1 - I2) / Rm)) has an amplitude equal to and opposite to the DC magnetic flux (100IDC / Rm) created by the DC current component IDC, the magnetic flux is created by only the AC current component IAC.

[0079] Ultimately, by adjusting the DC current component IDC flowing through the primary coil 51 in accordance with the current difference (I1-I2), it is possible to significantly reduce the DC magnetic flux within the magnetic core 50. As a result, a compact step-down transformer 5 can be realized.

[0080] 14 is a flowchart showing control for reducing the DC magnetic flux in the magnetic core 50. This DC magnetic flux reduction control is performed by adjusting the PWM duties of the switches 62 and 64 by the controller 7. First, the current I1 flowing through the battery pack 11 and the current I2 flowing through the battery pack 12 are detected (S200). The current I1 is the discharge current or charge current of the battery pack 11. The current I2 is the discharge current or charge current of the battery pack 12.

[0081] Next, the current difference (I1 - I2) between the two currents I1 and I2 is calculated (S202). Next, the DC current component Idc supplied to the primary coil 51 of the step-down transformer 5 by the oscillator 6 is determined. When the number of turns in the secondary coils 52 and 53 is each 1 and the number of turns in the primary coil 51 is N, the DC current component Idc (= I2 - I1) / N) is supplied to the primary coil 51. The DC magnetic flux formed in the magnetic core 50 by the DC current component Idc flows in the opposite direction to the DC magnetic flux formed in the magnetic core 50 by the current difference (I1 - I2). As a result, the DC magnetic flux flowing in the magnetic core 50 is approximately zero.

[0082] Next, the primary AC current IAC supplied by the oscillator 6 to the primary coil 51 is determined. This primary AC current IAC can be calculated from the secondary AC current and the turns ratio of the step-down transformer 5 (S204).

[0083] Next, the PWM duty ratio D+ of the switch 64 and the PWM duty ratio D- of the switch 62 are determined (S206). The relationship between the duty ratios (D+, D-) of the switches 62 and 64 and the primary current Ip flowing through the primary coil 51 is stored in advance in a map.

[0084] Next, in step S208, the PWM duty ratio of the switch 64 is controlled to D+ during the positive half cycle (P+), and the PWM duty ratio of the switch 62 is controlled to D- during the negative half cycle (P-). As a result, the primary current Ip supplied to the primary coil 51 by the oscillator 6 is the sum of the primary AC current IAC and the primary DC current IDC. Therefore, only the circulating AC current Ic circulating through the battery packs 11 and 12 substantially forms a magnetic flux in the magnetic core 50. As a result, the magnetic core 50 becomes compact. Ultimately, the magnetic effect of the load current or charging current on the magnetic core 50 is reduced by the DC current supplied to the primary coil 51.

[0085] 2, step-down transformer 5 has secondary coil 52 connected in series with battery pack 11 and secondary coil 53 connected in series with battery pack 12. The secondary voltages induced in the two secondary coils form an AC current circulating through the two battery packs. Furthermore, H-bridge 6 can be controlled by various known PWM control methods.

[0086] 15 is a circuit diagram showing a first modified embodiment. In this figure, a secondary AC voltage induced in a secondary coil 54 of a step-down transformer 5 circulates an AC current in a current circulation circuit formed by a battery pack 11, a secondary coil 54, and a smoothing capacitor 8. A load current supplied from the battery pack 11 to a motor drive circuit 2 flows through the secondary coil 54.

[0087] 16 is a circuit diagram showing a second modified embodiment. In FIG. 16, battery pack 11 consists of two battery modules 160 and 170 connected in series. Battery pack 12 consists of two battery modules 180 and 190 connected in series. The secondary coil 54 of step-down transformer 5 connects intermediate potential point C1 of battery pack 11 to intermediate potential point C2 of battery pack 12. Battery packs 11 and 12 supply load current to motor drive circuit 2. When the two intermediate potential points C1 and C2 have the same potential, no load current flows through secondary coil 54.

[0088] However, the open circuit voltages and internal resistances of the four battery modules 160, 170, 180, and 190 are generally different from one another. As a result, the two intermediate potential points C1 and C2 have different potentials, and part of the load current flows through the secondary coil 54.

[0089] 15 and 16, the DC current for canceling the DC magnetic flux as described above can be supplied to the primary coil 51. Furthermore, in the modified embodiments shown in Figures 15 and 16, the internal resistance of each cell of the battery pack 11 can be detected by utilizing the secondary circulating current supplied from the step-down transformer 5 to the battery pack 11.

Claims

1. A battery-integrated DC power supply comprising: a battery having a number of cells connected in series; a step-down transformer having a secondary coil connected in series with said battery; a storage element forming a current circulation circuit together with said battery and said secondary coil; an oscillator that supplies a primary current to the primary coil of said step-down transformer; and a controller that controls said oscillator to control said primary current, wherein said controller controls said primary current in order to reduce DC magnetic flux formed in the magnetic core of said step-down transformer by a load current supplied from said battery to an external load through said secondary coil.

2. A DC power supply with built-in battery as recited in claim 1, wherein the step-down transformer has two secondary coils with the same number of turns, the battery and the storage element are made up of a first battery pack and a second battery pack with a number of cells connected in series, the first battery pack is connected to an external load through one of the two secondary coils, and the second battery pack is connected to the external load through the other of the two secondary coils, and the two secondary voltages induced in the two secondary coils have the same direction in the current circulation circuit.

3. The DC power supply with built-in battery according to claim 1, wherein said controller has a battery heating mode for heating said battery by circulating said secondary current through said current circulation circuit when said battery is at a low temperature.

4. The DC power supply with built-in battery according to claim 1, wherein the battery supplies a load current to an external load through the secondary coil and the contactor, and the controller reduces the load current by applying a pulse voltage to the primary coil during a transient period when the contactor is open.

5. A battery-integrated DC power supply comprising: a battery having a number of cells connected in series; a step-down transformer having a secondary coil connected in series with the battery; a storage element forming a current circulation circuit together with the battery and the secondary coil; an oscillator that supplies a primary current to the primary coil of the step-down transformer; and a controller that controls the oscillator to control the primary current, wherein the controller has the steps of: detecting a positive cell voltage of the cell when a positive pulse voltage is applied to the primary coil; detecting a negative cell voltage of the cell when a negative pulse voltage is applied to the primary coil; and calculating an electrical parameter related to the internal resistance of the cell based on the voltage difference between the positive cell voltage and the negative cell voltage.

6. A DC power supply with built-in battery according to claim 5, wherein the step-down transformer has two secondary coils with the same number of turns, the battery and the storage element are made up of a first battery pack and a second battery pack with a number of cells connected in series, the first battery pack is connected to an external load through one of the two secondary coils, and the second battery pack is connected to the external load through the other of the two secondary coils, and the two secondary voltages induced in the two secondary coils have the same direction in the current circulation circuit.

7. The DC power supply with built-in battery according to claim 5, wherein said controller has a battery heating mode for heating said battery by circulating said secondary current through said current circulation circuit when said battery is at a low temperature.

8. The DC power supply with built-in battery according to claim 5, wherein the battery supplies a load current to an external load through the secondary coil and the contactor, and the controller reduces the load current by applying a pulse voltage to the primary coil during a transient period when the contactor is open.

9. A battery-integrated DC power supply comprising: a battery having a number of cells connected in series; a step-down transformer having a secondary coil connected in series with the battery; a storage element forming a current circulation circuit together with the battery and the secondary coil; an oscillator that supplies a primary current to the primary coil of the step-down transformer; and a controller that controls the oscillator to control the primary current, wherein the step-down transformer has two secondary coils with the same number of turns; the battery and the storage element comprise a first battery pack and a second battery pack having a number of cells connected in series; the first battery pack is connected to an external load through one of the two secondary coils; and the second battery pack is connected to the external load through the other of the two secondary coils; and the two secondary voltages induced in the two secondary coils have the same direction in the current circulation circuit.

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