DC power supply for vehicles

Abstract

To provide a DC power supply for vehicles that can reduce battery loss and suppress battery degradation. [Solution] A DC power supply for a vehicle that supplies a load current to a variable-speed motor for a vehicle includes a high-energy battery pack 1 consisting of a plurality of high-energy cells connected in series, a high-current battery pack 2 consisting of a plurality of low-resistance cells connected in series, battery management systems 3 and 4 that manage the two battery packs separately, and a wiring circuit that connects the two battery packs essentially in parallel. The high-energy cells have a different active material composition and a higher rated voltage than the low-resistance cells, and the high-energy battery pack has at least twice the internal resistance and at least twice the full charge capacity of the high-current battery pack under the same conditions, and the high-current battery pack has at least twice the cycle life of the high-energy battery pack.

Description

[Technical Field]

[0001] The present invention relates to a DC power supply for a vehicle that includes two batteries connected substantially in parallel. [Background technology]

[0002] By forming a DC power supply for an electric vehicle using two batteries connected in parallel, the reliability of the DC power supply can be improved. This DC power supply is called a parallel battery DC power supply. In this parallel battery DC power supply, when one of the two batteries becomes defective, the switch connected in series with the defective battery is opened. As a result, the electric vehicle can be normally driven by this DC power supply consisting only of the remaining normal battery.

[0003] Another problem with electric vehicle batteries is that their performance deteriorates in cold environments. To solve this problem, AC heating technology has been proposed, which supplies alternating current to the battery. The battery is heated by the supplied AC current. However, this AC heating technology requires a means to temporarily store the power discharged from the battery.

[0004] The applicant's patent application, Patent Document 1, proposes an AC heating technology that employs a DC power supply consisting of two batteries connected in parallel. The two batteries form an AC current circulation circuit through the secondary coil of a step-down transformer.

[0005] Another problem with electric vehicle batteries is described below. The discharge current of an electric vehicle battery varies depending on the driving environment of the electric vehicle. For example, the discharge current of an electric vehicle battery increases when climbing a slope or accelerating. As a result, the battery temperature increases due to internal losses in the battery that are proportional to the square of the current. However, the increase in battery temperature leads to deterioration of battery performance. For this reason, conventional electric vehicle batteries are typically forced to cool using a cooling fluid. However, such forced cooling mechanisms for batteries further increase power consumption, vehicle weight, and vehicle costs.

[0006] The internal loss of a battery can be reduced by reducing its internal resistance. The internal resistance of a battery can be reduced by reducing the thickness of the negative and positive electrode active material layers of the cell, known as electrode sheets. However, reducing the thickness of the electrode sheets results in a decrease in cell capacity. Therefore, the thickness of the electrode sheets is determined based on the required cell capacity and the allowable internal resistance. In other words, the amount of energy that can be stored in a cell is roughly proportional to the thickness of the active material layers. However, the internal resistance of the cell is also roughly proportional to the increase in the active material layer thickness, and the internal loss of the cell is roughly proportional to the active material layer thickness.

[0007] The relationship between the weight of a lithium-ion cell and the thickness of the active material layer will be further explained. The cell weight increases with increasing active material layer thickness. However, the increase in cell weight is not proportional to the increase in active material layer thickness. In addition to the weight of the active material layer, the cell weight is determined by the weight of the electrode metal, the cell case, and the metal tabs connecting the electrode metal to the electrode terminals.

[0008] When the cell current is low, the internal loss of the cell is not a problem. However, when the cell current increases, the internal loss of the cell increases significantly. For example, when the internal resistance of the cell is 10 milliohms, the relationship between the cell current and the cell loss can be explained. When the cell current is 1 A, the internal loss is only 0.01 W. However, when the cell current is 30 A, it becomes 9 W.

[0009] When the internal loss of a cell increases, the amount of power that can be supplied to a load decreases and the cell temperature increases. Furthermore, this increase in cell temperature requires forced cooling of the cell to prevent cell degradation. This problem becomes particularly severe when driving on hills in the summer. Ultimately, it is understood that an increase in the thickness of the active material layer of a cell increases both the cell's energy storage capacity and its internal loss. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent 7301208 Summary of the Invention

[0011] An object of the present invention is to provide a DC power supply for a vehicle that can reduce battery loss and suppress battery deterioration.

[0012] The vehicular DC power supply of the present invention supplies a load current to an electric load having a high load current fluctuation rate. A typical example of an electric load having such a high load current fluctuation rate is an electric vehicle running in a city. This type of electric vehicle, such as an EV taxi, frequently repeats acceleration and deceleration. In other words, the battery of this type of electric vehicle repeatedly discharges and charges a large current in a relatively short period of time. As a result, battery loss increases, and battery deterioration is accelerated. Ultimately, it can be understood that a vehicular DC power supply driving an electric load having a high load current fluctuation rate will accelerate battery deterioration compared to a case in which a steady load current is supplied to the electric load.

[0013] In a first aspect of the present invention, a DC power supply for a vehicle that supplies a load current to a variable speed motor such as a traction motor includes a high-energy battery pack and a high-current battery pack that are substantially connected in parallel. The high-current battery pack comprises a plurality of low-resistance cells connected in series. The high-energy battery pack comprises a plurality of high-energy cells connected in series. The high-energy battery pack has at least twice the internal resistance and twice the full charge capacity of the high-current battery pack under the same conditions. However, the high-current battery pack has at least twice the cycle life of the high-energy battery pack.

[0014] For example, during a high current supply period in which the vehicle DC power supply supplies a high load current to the variable speed motor, the high-current battery pack supplies most of the load current. Therefore, during this high current supply period, the equivalent internal resistance of the vehicle DC power supply decreases, and resistance loss and heat generation in the vehicle DC power supply decrease. After this high current supply period ends, the voltage of the high-current battery pack, which has a relatively low storage capacity, drops below the voltage of the high-energy battery pack, which has a relatively high storage capacity. As a result, after the high current supply period ends, the high-energy battery pack slowly charges the high-current battery pack, and the SOC of the high-current battery pack is restored.

[0015] In a preferred embodiment, the high-energy battery pack has an internal resistance four times or more and a full charge capacity four times or more than that of a high-current battery pack under the same conditions. Furthermore, the high-current battery pack has a cycle life four times or more than that of a high-energy battery pack. This effectively reduces the internal resistance loss of the vehicle DC power supply.

[0016] In a preferred embodiment, the variable speed motor includes a traction motor for generating driving torque. For example, this traction motor drives electric vehicles, hybrid vehicles, electric motorcycles, electric buses, trains, etc. The load current required by such a traction motor varies greatly depending on the driving conditions. When climbing a slope or rapidly accelerating the vehicle, the traction motor requires a high load current. As a result, an increase in internal resistance loss causes a rise in the temperature of the battery pack, accelerating battery pack deterioration. This problem is effectively solved by the DC power supply for a vehicle of the present invention.

[0017] In a preferred embodiment, the high energy cells comprise LFP (lithium iron phosphate) cells and the high current cells comprise LTO (lithium titanate) cells.

[0018] In a preferred embodiment, the high-energy battery pack is connected to an external electrical load through a first switch, and the high-current battery pack is connected to the external electrical load through a second switch, so that when one of the two battery packs becomes defective, the electrical load can be driven only by the good battery pack.

[0019] When these two battery packs are simultaneously charged or discharged, the SOC of the high-current battery pack changes earlier than the SOC of the high-energy battery pack. If there is a concern that the high-current battery pack may be overcharged or overdischarged due to this sudden change in potential, the second switch connected in series with the high-current battery pack can be opened, thereby protecting the high-current battery pack from overcharge or overdischarge.

[0020] In a preferred embodiment, the two battery packs form an AC circulation circuit together with the two secondary coils of the step-down transformer. With a very small number of turns, the resistance loss of the two secondary coils can be almost ignored. This allows most of the power energy of the battery packs to be used for heating the battery packs. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a block circuit diagram showing an example of a DC power supply for a vehicle. [Figure 2] FIG. 2 is an equivalent circuit diagram of the DC power supply for a vehicle in a non-discharging state. [Figure 3] FIG. 3 is an equivalent circuit diagram of a DC power supply for a vehicle in a discharging state. DETAILED DESCRIPTION OF THE INVENTION

[0022] A DC power supply for a battery electric vehicle (BEV) is described with reference to Fig. 1. The DC power supply for a vehicle includes a high-energy battery pack 1, a high-current battery pack 2, a battery management system 3, a battery management system 4, a step-down transformer 5, an oscillator 6, a contactor 7, a contactor 8, and a controller 9.

[0023] The high-energy battery pack 1 consists of a number of LFP cells (11-1M) connected in series. LFP cells, also known as lithium iron phosphate batteries, are known to have low manufacturing costs. The high-current battery pack 2 consists of a number of LTO cells (21-2N) connected in series. LTO cells, also known as lithium titanate batteries, have superior cycle life compared to other lithium-ion cells such as LFP cells.

[0024] SCiB (registered trademark) is a representative example of an LTO cell that uses lithium titanate. This SCiB cell has a cycle life of over 20,000 cycles and a wide charge / discharge range from 0% SOC to 100% SOC. However, LTO cells have a lower energy storage capacity per unit weight than other types of lithium-ion cells.

[0025] Each LFP cell (11-1M) is connected to a battery management system 3, which detects the state of each LFP cell (11-1M). Each LTO cell (21-2N) is connected to a battery management system 4, which detects the state of each LTO cell (21-2N). Furthermore, the battery management system 3 performs a self-balancing operation to reduce SOC variations among the LFP cells (11-1M), and the battery management system 4 performs a self-balancing operation to reduce SOC variations among the LTO cells (21-2N).

[0026] The high potential end of the high-energy battery pack 1 is connected to the high potential output terminal VH of the vehicle DC power supply through a contactor (first switch) 7 and a secondary coil 52. The high potential end of the high-current battery pack 2 is connected to the high potential output terminal VH of the vehicle DC power supply through a contactor (second switch) 8 and a secondary coil 53.

[0027] The secondary coils 52 and 53 are wound around the magnetic core of the step-down transformer 5. When the temperature of the high-current battery pack 1 is low, the oscillator 6 applies an AC voltage of approximately 5-10 kHz to the primary coil 51 wound around this magnetic core. This induces two secondary voltages in the secondary coils 52 and 53, which circulate a battery pack heating current through the two battery packs 1 and 2 via the contactors 7 and 8. In one example, the step-down transformer 5 has a turns ratio of 70-100, and the secondary coils 52 and 53 each have one turn. The circulating current supplied by the secondary coil 52 to the battery packs 1 and 2 is in the same direction as the circulating current supplied by the secondary coil 53 to the battery packs 1 and 2. As a result, most of the AC power supplied by the oscillator 6 to the primary coil 51 can be consumed within the battery packs 1 and 2.

[0028] An external electric load (not shown) has a high potential end connected to the high potential output terminal VH of the vehicle DC power supply, and a low potential end connected to the low potential output terminal VL of the vehicle DC power supply. The external electric load includes a three-phase inverter that drives a traction motor.

[0029] The controller 9 detects a failure in the battery packs 1 and 2 based on information about the battery packs 1 and 2 received from the battery management systems 3 and 4. Furthermore, the controller 9 opens the contactor 7 when the battery pack 1 is defective, and opens the contactor 8 when the battery pack 2 is defective. This allows the vehicle DC power supply to supply three-phase AC power to the traction motor even if either the battery packs 1 or 2 is defective.

[0030] Furthermore, switch 7 is opened when battery pack 1 deviates from its preferred SOC range, and switch 8 is opened when battery pack 2 deviates from its preferred SOC range, thereby preventing overcharging and over-discharging of battery packs 1 and 2.

[0031] The resistance loss of this vehicle DC power supply will be explained with reference to Figures 2 and 3. Figures 2 and 3 are equivalent circuits of the vehicle DC power supply shown in Figure 1. High-energy battery pack 1 has an internal resistance r1, and high-current battery pack 2 has an internal resistance r2. In Figure 2, the discharge current flowing from the high-potential output terminal VH of this vehicle DC power supply to the low-potential output terminal VL through an electrical load (not shown) is zero. As a result, the voltage drops across the internal resistances r1 and r2 are zero, and the output voltage Vo of the vehicle DC power supply is equal to the voltage of battery packs 1 and 2.

[0032] The internal resistance r2 can be reduced by reducing the thickness of the active material layer of the high-current battery pack 2. In this embodiment, the internal resistance r1 of the battery pack 1 is set to four times the internal resistance r2 of the battery pack 2. However, by reducing the thickness of the active material layer of the high-current battery pack 2, the stored power amount of the high-current battery pack 2 is reduced. According to this embodiment, the stored power amount (full charge capacity) of the high-current battery pack 2 is set to 5% of the stored power amount (full charge capacity) of the high-energy battery pack 1. For example, the full charge capacity of the high-energy battery pack 1 is 50 kWh, and the full charge capacity of the high-current battery pack 2 is 2.5 kWh.

[0033] In FIG. 2, the internal resistance r1 of the battery pack 1 having a thick active material layer is 40 milliohms, and the internal resistance r2 of the battery pack 2 having a thin active material layer is 10 milliohms.

[0034] In Figure 3, battery pack 1 discharges current I1 to an external electrical load through internal resistance r1, and battery pack 2 discharges current I2 to an external electrical load through internal resistance r2. When the voltages of battery packs 1 and 2 are equal, discharge current I2 is four times the discharge current I1. As a result, the combined internal resistance of this vehicle DC power supply is 8 milliohms, and a combined load current (I1 + I2) is discharged to the external electrical load.

[0035] In the high-current discharge mode, where the combined load current (I1 + I2) increases, the SOC of the high-current battery pack 2, which discharges a high current despite its low full-charge capacity, drops rapidly, causing its terminal voltage to drop. As a result, a balancing current I3 flows from battery pack 1 to battery pack 2 due to the potential difference between the voltage V1 of the high-energy battery pack 1 and the voltage V2 of the high-current battery pack 2. This causes battery pack 1 to discharge and battery pack 2 to charge. Ultimately, the balancing current I3 enhances the discharge of battery pack 1 and suppresses the discharge of battery pack 2. When the discharge from the vehicle DC power supply to the external electrical load is completed, the discharge currents I1 and I2 become zero. Thereafter, the balancing current I3 continues to flow through the internal resistances r1 and r2 until the potential difference between battery packs 1 and 2 becomes zero.

[0036] An example of the relationship between the discharge current and discharge loss of a vehicle DC power supply is described below. In this example, the voltage of the vehicle DC power supply is assumed to be 335 V. First, when the combined discharge current (I1 + I2) is 5 A, the discharge power of the vehicle DC power supply is approximately 1.7 kW. The resistive loss of battery pack 1 is 0.04 W, and the resistive loss of battery pack 2 is 0.16 W. Next, when the combined discharge current (I1 + I2) is 50 A, the discharge power of the vehicle DC power supply is 17 kW. The resistive loss of battery pack 1 is 4 W, and the resistive loss of battery pack 2 is 16 W. Next, when the combined discharge current (I1 + I2) is 200 A, the discharge power of the vehicle DC power supply is 68 kW. The resistive loss of battery pack 1 is 64 W, and the resistive loss of battery pack 2 is 256 W.

[0037] Next, a case will be described in which the vehicle DC power supply consists only of a high-energy battery pack 1. The vehicle DC power supply supplies only a discharge current I1 to an external electrical load. First, when the discharge current I1 is 5A, the discharge power of the vehicle DC power supply is approximately 1.7kW. The resistive loss of the battery pack 1 is 1W. Next, when the discharge current I1 is 50A, the discharge power of the vehicle DC power supply is approximately 17kW. The resistive loss of the battery pack 1 is 100W. Next, when the discharge current I1 is 200A, the discharge power of the vehicle DC power supply is approximately 68kW. The resistive loss of the battery pack 1 is 1600W. In conclusion, when a high-current battery pack 2 with low internal resistance is added to the vehicle DC power supply, the loss of the vehicle DC power supply through which a large current flows is significantly reduced.

[0038] In one preferred example, high-energy battery pack 1 consists of 100 series-connected LFP cells, and high-current battery pack 2 consists of 134 series-connected SCiB cells. When the SOC of the LFP cells is 80%, the SOC of the SCiB cells is nearly 100%.

[0039] The DC power supply for a vehicle of this embodiment, which is essentially composed of two battery packs connected in parallel, is called a dual-battery-pack type DC power supply for a vehicle. This power supply has a higher manufacturing cost than a conventional single-battery-pack type DC power supply for a vehicle. However, by connecting a high-energy battery pack and a high-current battery pack in parallel, it is possible to effectively prevent an increase in power loss and an increase in battery temperature, even in a driving environment where temporary increases in load current frequently occur. As a result, vehicle safety is improved and the maximum driving distance can be extended.

[0040] Preferably, this dual-battery pack type vehicle DC power supply can have a switch for isolating each battery pack. When these switches are simultaneously opened, the DC power supply can be electrically isolated from the load, and only the battery pack for which charging or discharging is not desired can be disconnected from the DC power supply. For example, with a conventional single-battery pack type vehicle DC power supply, if a battery pack fails while the vehicle is running, the vehicle may suddenly stop. The dual-battery pack type vehicle DC power supply of this embodiment can solve this problem.

[0041] This dual battery pack type vehicle DC power supply can also supply AC current to the two battery packs through a step-down transformer in cold environments, thereby improving the charging and discharging operations in cold environments, which is a major problem with conventional lithium-ion batteries.

[0042] In this embodiment, the high-energy battery pack can be formed using various types of conventionally known cells other than LFP cells. The cells used in this high-energy battery pack can have a relatively thick active material layer compared to the cells of conventional single-battery pack-type vehicle DC power supplies. In this embodiment, the high-current battery pack can be formed using cells other than LTO cells, such as all-solid-state cells. However, the cells used in the high-current battery pack must have lower internal resistance, better cycle life, and better rapid discharge performance than the cells used in the high-energy battery pack.

[0043] In this embodiment, the high-current battery pack has a smaller storage capacity than the high-energy battery pack. For example, the high-current battery pack preferably has a storage capacity of 5 to 15% of that of the high-energy battery pack. This allows the cells of the high-current battery pack to have a thinner active material layer. As a result, the internal resistance and manufacturing costs of the high-current battery pack can be reduced.

[0044] The number of turns in the secondary coil of the step-down transformer should be reduced as much as possible, preferably to one turn, so that the copper loss in the secondary coil during charging or discharging of the battery pack can be reduced to a negligible level.

Claims

[Claim 1] a high-energy battery pack consisting of a plurality of high-energy cells connected in series; a high current battery pack consisting of a plurality of low resistance cells connected in series; a battery management system that manages the two battery packs separately; a wiring circuit that connects the two battery packs in parallel; A DC power supply for a vehicle that supplies a load current to a variable speed motor for a vehicle, comprising: the high energy cell has a different active material composition and a higher rated voltage than the low resistance cell; The high-energy battery pack has an internal resistance that is at least twice as large and a full charge capacity that is at least twice as large as the high-current battery pack under the same conditions; the high-current battery pack has more than twice the cycle life of the high-energy battery pack; the wiring circuit includes a step-down transformer having a primary coil, a first secondary coil, and a second secondary coil; the high-energy battery pack is connected to an external electrical load through the first secondary coil; the high current battery pack is connected to the external electrical load through the second secondary coil; The two secondary coils circulate a battery heating current through the two battery packs.

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