JP7795992B2 - Lithium-ion secondary battery control system, charge/discharge control method, and device equipped with the same - Google Patents

Description

The present invention relates to a lithium-ion secondary battery control system and a device equipped with the same.

Because of their high energy density, lithium-ion secondary batteries have attracted attention as batteries for use in a variety of applications, from electrical equipment to vehicles such as trains and automobiles, and for storing electricity generated by solar or wind power generation and supplying it to the power grid. For example, when lithium-ion secondary batteries (hereinafter referred to as "batteries") are installed in electrical equipment, they can significantly improve the mobility of the electrical equipment. Furthermore, when they are installed in automobiles, such automobiles include zero-emission electric vehicles (without engines), hybrid electric vehicles (with both engines and secondary batteries), and plug-in hybrid electric vehicles (with batteries that are charged directly from the grid). They are also expected to be used as stationary energy storage systems to supply electricity in emergencies when the power grid is cut off.

For these diverse applications, batteries are required to have excellent durability. For example, they are required to have a low rate of decrease in rechargeable battery capacity, even when the ambient temperature rises or when repeated charge/discharge cycles are performed, and to have a high battery capacity retention rate (SOHQ) over the long term.

However, lithium-ion secondary batteries experience a decrease in battery capacity due to cycle degradation caused by leaving them in a high-temperature environment or repeated charge-discharge cycles. This capacity decrease is more pronounced when they are left at high voltage or when charge-discharge cycles are performed over a wide voltage range or at a large current. Furthermore, this capacity decrease varies depending on the usage environment, usage method, and charge-discharge method of the device or system in which the lithium-ion secondary battery is installed.

Patent Document 1 is a prior art document in this technical field. Patent Document 1 discloses that a deterioration characteristic value related to the degree of capacity deterioration of a secondary battery after a predetermined time has passed is compared with a threshold value to determine whether or not the charging voltage needs to be switched, and that the charging control circuit sets a second charging voltage value lower than the first charging voltage value in the charger when it determines that the deterioration characteristic value has reached the threshold value.

JP 2011-109840 A

The method described in Patent Document 1 changes the charging conditions after determining that the full charge capacity, a degradation characteristic value related to cycle degradation that occurs as a result of repeated charge/discharge cycles over a predetermined period of time, has reached a threshold value. Therefore, the problem is that the likelihood of extending the battery's lifespan is low.

In view of the above-mentioned problems, the present invention aims to provide a lithium-ion secondary battery control system, a charge/discharge control method, and a device equipped with the same that can limit charge/discharge conditions before the capacity retention rate of a lithium-ion secondary battery falls below a predetermined threshold, thereby reducing cycle degradation.

One example of the present invention is a lithium-ion secondary battery control system that controls the charging and discharging of a lithium-ion secondary battery. The system includes a detection unit that detects the state of charge of the lithium-ion secondary battery, and an arithmetic processing unit. When the arithmetic processing unit determines that the battery is empty based on the state of charge from the detection unit, it uses data from the detection unit to calculate a first DC resistance during charging from an empty state. When the arithmetic processing unit determines that the battery is fully charged based on the state of charge from the detection unit, it uses data from the detection unit to calculate a second DC resistance during discharging from a fully charged state, calculates a DC resistance ratio between the first DC resistance and the second DC resistance, and uses the DC resistance ratio to determine deterioration.

The present invention provides a lithium-ion secondary battery control system, a charge/discharge control method, and a device equipped with the same that can reduce cycle degradation of lithium-ion secondary batteries.

FIG. 1 is a graph showing the change in capacity retention rate over charge-discharge cycles under various cycle test conditions that are the basis for the examples. FIG. 2 is a diagram showing the conditions of each cycle test in FIG. 1. FIG. 1 is a graph showing the change in DC resistance at full charge with respect to charge/discharge cycles under various cycle test conditions that are the premise of the examples. FIG. 10 is a diagram showing the battery capacity dependency of DC resistance before and after a cycle test in an example. FIG. 1 is a graph showing the change in the ratio of the DC resistance during charging with no remaining battery charge to the DC resistance during discharging in a fully charged state with respect to the charge-discharge cycles under each cycle test condition in Examples. 1 is a configuration diagram of a lithium ion secondary battery control system according to an embodiment. FIG. 10 is a diagram showing experimental results of a cycle test in an example.

The following describes an embodiment of the present invention with reference to the drawings. Note that this embodiment is not limited to the following content, and can be modified as desired without departing from the spirit of the invention.

First, a conventional method for detecting the degradation state of a lithium-ion secondary battery, which is the premise of this embodiment, will be described. SOH (States of Health) is a value that indicates the degradation state of a battery. When a battery deteriorates, changes in characteristics generally occur, such as an increase in internal resistance or a decrease in the capacity at full charge (capacity retention rate) compared to the initial state. A common method is to calculate the SOH from these characteristics that have changed due to deterioration or the ratio of these changed characteristics to the initial characteristics. The calculated SOH can be reflected in other calculations to detect the battery state taking degradation information into account, and can also be used as an indicator when determining the battery's lifespan. Examples of degradation states include changes in capacity retention rate and DC resistance.

However, SOH is a transition from the past to the present during equipment use, and while it is possible to track the user's usage environment history of the equipment, it is not possible to determine whether accelerated deterioration will occur under future usage environments.

As such, a conventional method for detecting the state of battery degradation involves evaluating the degradation characteristics under standard usage conditions in advance, and determining that degradation has occurred if it is determined that the capacity retention rate, which is related to cycle degradation caused by repeated charge-discharge cycles over a predetermined period of time, has fallen below a preset threshold. After detecting the state of degradation, subsequent degradation can be reduced by changing the charging conditions, such as by setting a lower end-of-charge voltage.

Figure 1 shows the change in capacity retention rate over charge/discharge cycles under various cycle test conditions, which are the basis for this example. Figure 2 also shows the various cycle test conditions in Figure 1. As shown in Figure 2, the various cycle test conditions were implemented by varying the ambient temperature, charge rate, discharge rate, upper limit voltage, and number of test cycles. Here, the charge/discharge rate refers to the magnitude of the current when a certain battery is energized. When a certain battery is discharged from a fully charged state at a certain current, the current value at which the battery is completely discharged in one hour (the state of charge (SOC) reaches 0%) is defined as 1C.

In Figure 1, under cycle test conditions Cy#5 and Cy#9, it can be seen that as cycle degradation caused by repeated charge/discharge cycles progresses, battery degradation, i.e., the decline in capacity retention, accelerates. This degradation is significantly affected by the battery's charge/discharge environment, which in turn is significantly influenced by the user's usage patterns for the device in which the battery is installed.

Meanwhile, Figure 3 shows the change in DC resistance at full charge relative to the charge/discharge cycle under each cycle test condition that is the premise of this example. The cycle test conditions are as shown in Figure 2. Figure 3 shows how DC resistance increases with an increase in the number of battery cycles, i.e., with increased degradation, but does not show signs of accelerated degradation as seen in Figure 1.

Figure 4 shows the dependency of DC resistance on battery capacity before and after the cycle test in this example. Figure 4 describes the cycle test condition Cy#9, with the open circles representing the results before the cycle test and the closed circles representing the results after 600 cycles, when the capacity retention rate had dropped to 60%. As shown in Figure 4, the cycle test resulted in an overall increase in DC resistance and a decrease in battery capacity. Here, we focus on the shape of the graph in Figure 4. In Figure 4, it can be seen that the shape on the right, where the charge rate is 0% (SOC 0%), remains almost the same before and after the cycle test, shifting in parallel, while the shape on the left, where the charge rate is 100% (SOC 100%), changes significantly.

In this embodiment, the ratio of the DC resistance at a charging rate of 0% to the DC resistance at a charging rate of 100% is calculated, and this DC resistance ratio is used as an index for determining deterioration diagnosis. For example, this DC resistance ratio is used to determine deterioration. This DC resistance ratio is also used as a trigger for changing the charging and discharging conditions. Details are explained below.

Figure 5 shows the change in the ratio of DC resistance during charging with no remaining battery power (0% remaining battery power) (SOC 0%) to DC resistance during discharging with a full charge (SOC 100%) for each charge/discharge cycle under each cycle test condition in this example. The cycle test conditions are as shown in Figure 2.

In Figure 5, for cycle test conditions Cy#5 and Cy#9 in Figure 1, where the capacity retention rate accelerated and deteriorated midway through the cycle, the DC resistance ratio dropped significantly at 100 cycles. In contrast, for cycle test conditions Cy#1 and Cy#2 in Figure 1, where the capacity retention rate showed moderate deterioration, the DC resistance ratio dropped gradually. On the other hand, for cycle test condition Cy#3 in Figure 1, where the capacity retention rate changed little, the DC resistance ratio remained almost constant.

Let's compare Figures 1 and 5. In the capacity retention curve, which is the SOH measured using the conventional method shown in Figure 1, an estimated battery life can be calculated under environmental conditions where accelerated degradation, such as past trends in capacity retention, does not occur, as indicated by an inflection point. By implementing control to suppress battery degradation when a set threshold is reached, a certain degree of battery life extension can be expected. However, with this method, under conditions where rapid acceleration occurs, such as cycle test conditions Cy#5 and Cy#9, significant degradation will have already occurred by the time the threshold is reached, and no life extension effect can be expected. On the other hand, by using the DC resistance ratios at 0% and 100% charge rates shown in Figure 5, it is possible to detect cycle conditions where rapid degradation progresses at an early stage. For example, while signs of degradation are apparent after 300 cycles in Figure 1, signs of degradation can be detected around 100 cycles in Figure 5.

Next, we will explain the lithium ion secondary battery control system that controls the charging and discharging of the lithium ion secondary battery in this embodiment.

Figure 6 is a configuration diagram of a lithium-ion secondary battery control system according to this embodiment. In Figure 6, the lithium-ion secondary battery control system 101 includes a calculation processing unit 107 that performs various calculations, a charge/discharge control circuit 105 that serves as an analog interface, a current measurement unit 110, a voltage measurement unit 111, and a switch 112. The calculation processing unit 107 is a signal processing device, typically comprised of a CPU (processor) and memory resources such as a storage device and memory. Specifically, the calculation processing unit 107 performs software processing to realize various functions by loading programs stored in the storage device into memory and executing the loaded programs via the CPU. As shown in Figure 6, the calculation processing unit 107 includes a degradation diagnosis unit 102, a battery state storage unit 103, and a DC resistance calculation unit 104. An external load 108 is connected to the lithium-ion secondary battery 106 via a switch 112, and a charging power source 109 for charging the battery 106 is also connected via the switch 112.

The lithium-ion secondary battery control system 101 can be installed as an external load 108 in electrical equipment, vehicles such as trains and automobiles, and even power storage systems that store electricity generated by solar power generation or wind power generation. For example, in electrical equipment, it is effective in rechargeable vacuum cleaners, which are subject to harsh environments for batteries. This is because rechargeable vacuum cleaners require high-rate discharge due to high power, and a wide voltage range for charge/discharge cycles, from full charge to empty.

Next, we will explain the charge/discharge control process for a lithium-ion secondary battery using the DC resistance ratio at a charge rate of 0% and 100% in this embodiment.

In Figure 6, first, the arithmetic processing unit 107 outputs a signal to the charge/discharge control circuit 105 to connect switch 112 to the charging power source 109, thereby causing current to flow through the battery 106 and starting charging. Furthermore, the charge state of the battery 106 is measured by a charge state detection unit consisting of a voltage measurement unit 111 and a current measurement unit 110, and the charge/discharge control circuit 105 converts the measured data into a digital value and sends it to the arithmetic processing unit 107. As a result, the arithmetic processing unit 107 performs constant-current, constant-voltage charging while monitoring the charge state of the battery 106. Then, when the charge state data of the battery 106 from the charge/discharge control circuit 105 satisfies predetermined conditions, the arithmetic processing unit 107 determines that the battery 106 is fully charged, and outputs a signal to the charge/discharge control circuit 105 to open switch 112. The calculation processing unit 107 then outputs a signal to the charge/discharge control circuit 105 to connect the switch 112 to an external load, and the DC resistance calculation unit 104 calculates the DC resistance from the data from the voltage measurement unit 111 and current measurement unit 110, determines the DC resistance when the battery is discharged from a fully charged state, and records this in the battery state storage unit 103.

Furthermore, when the calculation processing unit 107 detects that the measurement value received from the charge/discharge control circuit 105 by the voltage measurement unit 111 is equal to the set lower limit voltage due to a decrease in the remaining battery charge caused by the external load 108, it determines that the battery is empty and sends a signal to the charge/discharge control circuit 105 to open the switch 112. When a start signal for the charging power supply 109 is input, the calculation processing unit 107 outputs a signal to the charge/discharge control circuit 105 to connect the switch 112 to the charging power supply, and the DC resistance calculation unit 104 calculates the DC resistance from the data from the voltage measurement unit 111 and the current measurement unit 110, determines the DC resistance during charging from an empty battery state, and records it in the battery state memory unit 103.

In the degradation diagnosis unit 102, the calculation processing unit 107 reads the DC resistance when the battery is empty and the DC resistance when fully charged from the battery state memory unit 103, calculates the DC resistance ratio between them, and when the DC resistance ratio falls below a threshold, transitions to processing to limit the charge and discharge conditions. The charge and discharge condition limit processing may, for example, lower the upper charge voltage limit or the charge and discharge current.

The DC resistance of a battery is measured by loading a constant current (I) onto the battery and calculating it using Ohm's law from the voltage difference before and after the load. Specifically, if the difference between the voltage before the load and the voltage after, say, 10 seconds of load is ΔV, then DC resistance R can be expressed as R = ΔV/I, and is calculated using this formula. Therefore, strictly speaking, DC resistance when the battery is empty is not the DC resistance when the battery is empty, but the DC resistance when charging from a state with no remaining battery power (0% SOC). Similarly, DC resistance when fully charged is not, strictly speaking, the DC resistance when fully charged, but the DC resistance when discharging from a state with full charge (100% SOC).

The DC resistance from the fully charged state and the DC resistance from the empty battery state do not have to be strictly from the fully charged state or the empty battery state. For example, as shown in Figure 4, the DC resistance may be obtained from a state near the fully charged state or the empty battery state a predetermined time after the fully charged state or the empty battery state, as long as a significant difference in the left and right shapes before and after the cycle test is maintained.

Figure 7 shows the experimental results of the cycle test in this example. In Figure 7, Experiment 1 corresponds to cycle test condition Cy#5 in Figure 1, and the battery was mounted in the lithium-ion secondary battery control system shown in Figure 6, charged and discharged, and the battery characteristics were evaluated using the method described below.

The battery was charged at 25°C with a current equivalent to 0.5 CA up to 4.20 V, then subjected to constant voltage charging at 4.20 V until the current reached 0.04 CA. After a 30-minute rest, it was discharged at a constant current equivalent to 1 CA down to 3.0 V. The discharge capacity at this time was taken as the initial capacity. The DC resistance ratio between the initial empty battery and the fully charged battery was 3.2.

Next, the battery was charged to 4.20 V at a current equivalent to 2.0 CA at 25°C, and then constant voltage charged at 4.20 V until the current reached 0.04 CA. After a 30-minute break, the battery was discharged to 3.0 V at a constant current equivalent to 4 CA, and then rested for 60 minutes.

The DC resistance ratio was determined initially and every 20 cycles. The threshold DC resistance ratio at which charge/discharge limiting control was initiated was set at 2.5. Specifically, after 100 cycles, the DC resistance ratio fell below the threshold, and charge/discharge limiting control was initiated by limiting the upper charge voltage to 4.10 V. Tests were conducted from 101 to 1000 cycles. Regarding capacity retention, the battery was charged at 25°C at a current equivalent to 0.5 CA up to 4.20 V every 100 cycles, followed by constant voltage charging at 4.20 V until the current reached 0.04 CA. After a 30-minute break, the battery was discharged at a constant current equivalent to 1 CA down to 3.0 V, and the discharge capacity was measured to determine the capacity retention rate relative to the initial capacity. The resulting capacity retention rate was 80%.

Next, in Figure 7, Experiment 2 corresponds to cycle test condition Cy#9 in Figure 1, and the cycle test conditions were charging at 40°C with a current equivalent to 1.0 CA up to 4.20 V, followed by constant voltage charging at 4.20 V until the current reached 0.04 CA. After a 30-minute break, constant current discharge was performed at a constant current equivalent to 6 CA down to 3.0 V, followed by a 60-minute break.

The DC resistance ratio was determined initially and every 20 cycles, as in Experiment 1, and the threshold DC resistance ratio at which charge/discharge limit control was initiated was set at 2.5. Specifically, after 40 cycles, the DC resistance ratio fell below the threshold, and charge/discharge limit control was initiated, limiting the upper charge voltage to 4.10V, and testing was continued from the 41st cycle up to the 1000th cycle. The capacity retention rate was evaluated in the same manner as in Experiment 1, and the result was a capacity retention rate of 76%.

Next, in Figure 7, Experiment 3 corresponds to cycle test condition Cy#5 in Figure 1, and the cycle test conditions were the same as in Experiment 1: charging at 25°C at a current equivalent to 2.0 CA up to 4.20 V, followed by constant voltage charging at 4.20 V until the current reached 0.04 CA. After a 30-minute break, constant current discharge was performed at a constant current equivalent to 4 CA down to 3.0 V, followed by a 60-minute break. This cycle was repeated 1,000 times. No determination of the DC resistance ratio or charge/discharge limit control was performed. As a result, the capacity retention rate was 59%.

Next, in Figure 7, Experiment 4 corresponds to cycle test condition Cy#9 in Figure 1, and the cycle test conditions were the same as in Experiment 2: charging at 40°C with a current equivalent to 1.0 CA up to 4.20 V, followed by constant voltage charging at 4.20 V until the current reached 0.04 CA. After a 30-minute break, constant current discharge was performed at a constant current equivalent to 6 CA down to 3.0 V, followed by a 60-minute break. This cycle was attempted for 1,000 cycles, but the capacity retention rate reached 60% at 600 cycles, so the test was stopped. The DC resistance ratio was not determined, and charge/discharge limit control was not performed.

As shown in Figure 7, Experiments 1 and 2, which performed DC resistance ratio determination and charge/discharge limit control based on this determination, completed the 1000-cycle test without showing a sudden change in capacity retention, and maintained a high capacity retention even after 1000 cycles. In contrast, Experiments 3 and 4, which did not perform determination based on DC resistance ratio or charge/discharge limit control, showed a rapid decrease in capacity retention from around 300 to 400 cycles, and the rate of deterioration increased, as seen in cycle test conditions Cy#5 and Cy#9 shown in Figure 1. In this way, determination based on DC resistance ratio is more sensitive than conventional determination based on capacity retention, and allows for early detection of deterioration of lithium-ion secondary batteries.

As described above, this embodiment includes a detection unit that detects whether a lithium-ion secondary battery is empty or fully charged, measures the DC resistance at 0% SOC when charging from an empty state, and the DC resistance at 100% SOC when discharging from a fully charged state, and performs a deterioration diagnosis using the ratio of these values. When this ratio falls below a certain value, the charge/discharge conditions are restricted. In this way, by controlling the battery's charge/discharge conditions, it is possible to provide a lithium-ion secondary battery control system, a charge/discharge control method, and a device equipped with the same that can reduce cycle deterioration of the lithium-ion secondary battery and extend its lifespan.

The above describes an embodiment of the present invention. The present invention can provide a lithium-ion secondary battery control system and a device equipped with the same that can reduce cycle degradation of lithium-ion secondary batteries, thereby reducing the amount of resources required. This can reduce carbon emissions and prevent global warming, contributing to the realization of the SDGs (Sustainable Development Goals), particularly item 7, energy.

Furthermore, the present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those having all of the described configurations.

101: Lithium-ion secondary battery control system, 102: Degradation diagnosis unit, 103: Battery state memory unit, 104: DC resistance calculation unit, 105: Charge/discharge control circuit, 106: Lithium-ion secondary battery (battery), 107: Processing unit, 108: External load, 109: Charging power supply, 110: Current measurement unit, 111: Voltage measurement unit, 112: Switch

Claims (11)

A lithium ion secondary battery control system that controls charging and discharging of a lithium ion secondary battery,
a detection unit that detects the charging state of the lithium ion secondary battery;
A calculation processing unit is provided,
The arithmetic processing unit
when it is determined that the battery is empty based on the charging state from the detection unit, calculating a first DC resistance during charging from an empty battery state using data from the detection unit;
When the charge state detected by the detection unit is determined to be fully charged, a second DC resistance during discharge from the fully charged state is calculated using data from the detection unit;
A lithium ion secondary battery control system, comprising: a DC resistance ratio between the first DC resistance and the second DC resistance; and a deterioration determination using the DC resistance ratio. 2. The lithium ion secondary battery control system according to claim 1,
a battery state storage unit;
a switch for turning on and off the connection between the lithium ion secondary battery and the external load and the charging power source;
The arithmetic processing unit
When it is determined that the battery is depleted, the switch is connected to the charging power source, and the first DC resistance is calculated and recorded in the battery state storage unit;
When it is determined that the battery is fully charged, the switch is connected to the external load, and the second DC resistance is calculated and recorded in the battery state storage unit;
A lithium ion secondary battery control system characterized by reading the first DC resistance and the second DC resistance from the battery state memory unit, calculating the DC resistance ratio, and determining deterioration using the DC resistance ratio. 2. The lithium ion secondary battery control system according to claim 1,
The lithium ion secondary battery control system is characterized in that the arithmetic processing unit limits charge and discharge conditions when the DC resistance ratio is equal to or less than a threshold value. 2. The lithium ion secondary battery control system according to claim 1,
The lithium ion secondary battery control system is characterized in that the detection unit is a voltage measurement unit and a current measurement unit. 4. The lithium ion secondary battery control system according to claim 3,
The lithium ion secondary battery control system is characterized in that the restriction on the charge and discharge conditions is to lower an upper limit charge voltage. 4. The lithium ion secondary battery control system according to claim 3,
The lithium ion secondary battery control system is characterized in that the restriction on the charge and discharge conditions is to reduce the charge and discharge current. A method for controlling charging and discharging of a lithium ion secondary battery, comprising:
calculating a first DC resistance when charging the lithium ion secondary battery from an empty state and a second DC resistance when discharging the lithium ion secondary battery from a fully charged state;
calculating a DC resistance ratio between the first DC resistance and the second DC resistance;
A charge/discharge control method for a lithium ion secondary battery, characterized in that deterioration is determined using the DC resistance ratio. 8. The method for controlling charge and discharge of a lithium ion secondary battery according to claim 7,
A method for controlling charging and discharging of a lithium ion secondary battery, comprising restricting charging and discharging conditions when the DC resistance ratio is equal to or less than a threshold value. 9. The method for controlling charge and discharge of a lithium ion secondary battery according to claim 8,
The method for controlling charging and discharging of a lithium ion secondary battery is characterized in that the restriction of the charging and discharging conditions is performed by lowering the upper limit charging voltage or lowering the charging and discharging current. An electrical device equipped with the lithium-ion secondary battery control system described in claim 1. A rechargeable vacuum cleaner equipped with the lithium-ion secondary battery control system described in claim 1.