A passive equalization method and device for lithium iron phosphate batteries

Abstract

The present invention relates to a passive equalization method and device for lithium iron phosphate batteries, including an equalization marking stage and an equalization stage; in the equalization marking stage, the maximum value of the single cell voltage Vmax ≥ V 3 If Vmin≥V 1 , no marking is performed; if Vmin<V 1 When all voltages in the battery system exceed V 2 The monomers are labeled; among them, V 3 is a setting value close to the upper limit of the single cell voltage; V 1 is the upper voltage limit during the charging platform period, V 1 <V 2 <V 3 ; In the balancing stage, the marked monomer is passively balanced. The beneficial effects of the present invention include: the balancing mark is judged at the charging end, and V 1 and V 2 , by properly choosing V 1 With V 2 , so that a single balancing coverage range is large and will not cause a single balancing over-limit problem; the mark is at the end of charging, but the balancing process can be performed during charging, wake-up, and discharge.

Description

Technical Field

[0001] The present invention relates to the technical field of battery equalization, and in particular to a method and device for passive equalization of a lithium iron phosphate battery. Background Art

[0002] As lithium iron phosphate battery systems have high safety and low-cost advantages, they are being used more and more widely as national subsidies are gradually withdrawn. However, after vehicles have been operated in the market, the consistency deviation of the batteries has become increasingly larger, the available capacity of the battery system has gradually decreased, and the vehicle's cruising range has seriously declined. This part of the lost capacity is reversible, and the system balancing function needs to be managed to increase the system capacity, so the balancing ability of the management system is particularly important.

[0003] The passive balancing method mainly includes a marking judgment stage and a balancing stage. In the marking stage, the single cells that need to be balanced are marked, and in the balancing stage, discharge balancing is performed. In the existing method, the voltage method and the △SOC method are mainly used for judgment in the marking stage. The voltage method generally marks higher single cell voltages during the charging process, and the balancing range is small and the efficiency is low. The △SOC method generally needs to be triggered when the shelving conditions are met at the end of the discharge, which requires a long shelving time and a low triggering probability, making the management system's balancing ability weak. In addition, most current management systems do not have a balancing diagnostic strategy. Once the balancing circuit fails (the balancing circuit is short-circuited), the battery continues to discharge through the balancing circuit, causing the battery to be over-discharged, resulting in serious property losses. Summary of the invention

[0004] The purpose of the present application is to provide a method and device for passive equalization of lithium iron phosphate batteries to solve the problem of low equalization efficiency in the prior art.

[0005] To achieve the above-mentioned purpose, the present invention proposes a passive equalization method for lithium iron phosphate batteries, including an equalization marking stage and an equalization stage; in the equalization marking stage, when the maximum value of the single cell voltage Vmax ≥ V3 and the charging current is within a predetermined range, if Vmin ≥ V1, no marking is performed; if Vmin < V1, all single cells in the battery system whose voltage exceeds V2 are marked; wherein V3 is a set value close to the upper limit value of the single cell voltage; V1 is the upper limit value of the voltage during the charging platform period, V1 < V2 < V3; in the equalization stage, the marked single cells are passively balanced.

[0006] Furthermore, passive balancing is performed by periodically turning on / off the balancing circuit.

[0007] Furthermore, when the maximum value of the single cell voltage Vmax≥Vm, the open balance time is T1, the closed balance time is T2, and the duty cycle = T1 / (T1+T2); when the maximum value of the single cell voltage Vmax<Vm, the open balance time is T3, the closed balance time is T4, and the duty cycle = T3 / (T3+T4); T1 / (T1+T2)<T3 / (T3+T4).

[0008] Furthermore, when the total equalization start time reaches a predetermined time, the equalization ends.

[0009] Furthermore, during the balancing process, the temperature of the balancing circuit is detected in real time. When the maximum temperature is greater than the set temperature upper limit, the balancing is interrupted, and the balancing mark and the time when the balancing is turned on are retained; when the maximum temperature is lower than the set temperature lower limit, the balancing is resumed.

[0010] Furthermore, it also includes a wake-up phase; in the wake-up phase, all monomers are balanced, and if after the balance, the voltage drop of a monomer is lower than the preset value, the fault of the equalization circuit corresponding to the monomer is reported.

[0011] The present invention also provides a passive equalization device for lithium iron phosphate batteries, comprising a processor and a memory, wherein the memory stores a computer program, and the processor implements the above method when executing the computer program.

[0012] The beneficial effects of the present invention include: the beneficial effects of the present invention include: the equalization mark is judged at the charging end, and V1 and V2 are set. By reasonably selecting V1 and V2, a large number of single equalization coverage ranges are made, and the problem of single equalization exceeding the limit is not caused; the mark is at the charging end, but the equalization process can be performed during charging, waking up, and discharging.

[0013] By enabling balancing according to the duty cycle, the single effective balancing capacity is greatly increased, thus improving the balancing capability of the management system.

[0014] Increase the temperature collection of the balancing circuit, interrupt the balancing when a certain temperature is reached, and resume the balancing after the temperature drops to avoid overheating or circuit burning.

[0015] During the wake-up phase, the management system determines whether the balancing circuit is damaged based on the changes in the monomer voltage to avoid serious property losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the charging curve of lithium iron phosphate battery;

[0017] Figure 2 is a schematic diagram of the battery system structure of an embodiment of the present invention;

[0018] Figure 3 It is a flow chart of the passive equalization method of the present invention. DETAILED DESCRIPTION

[0019] Passive balancing method embodiment

[0020] like Figure 2 The lithium iron phosphate battery system in the vehicle includes several battery cells (Cell1, Cell2...Celln) connected in series. Each single cell is equipped with a balancing circuit and a sampling circuit. The MCU collects the voltage of the single cell through the sampling circuit (including the collection of the temperature of the balancing circuit). The MCU controls the switch devices (such as MOS tubes) in the balancing circuit to discharge and balance the single cell. The MCU is connected to the master control through the CAN bus, and the MCU is the slave control.

[0021] Figure 1 This is the charging curve of the lithium iron phosphate battery. According to the characteristics of the lithium iron phosphate battery charging curve, the voltage curve is smooth in the early stage of charging, forming a plateau period, and it is difficult to accurately identify the consistency differences between batteries. However, after exceeding this platform, that is, at the end of charging, the charging voltage rises rapidly. If the battery consistency is good, all single cell voltages will be above V1 at the end of charging, and there is no need to start the management system for balancing; at the end of charging, when a single cell voltage is less than V1, it means that there is a certain degree of consistency deviation in the battery system, and the single cells with a voltage higher than V2 can be marked. V1 is the upper voltage limit of the charging voltage plateau period.

[0022] The selection of V1 and V2 needs to be calibrated experimentally so that the charging capacity between V1 and V2 is greater than the capacity of a single balancing process. In other words, the difference between V2 and V1 should not be too large, as a too large difference will result in a small number of single markings and lower balancing efficiency. It should not be too small either, as a too small difference will result in a lower voltage after balancing, causing high and low voltages to switch back and forth, resulting in invalid balancing.

[0023] like Figure 3 As shown, the passive balancing method is described in detail below.

[0024] Balanced marking phase

[0025] When at the end of charging, that is, the maximum single cell voltage Vmax ≥ V3 (V3 is close to the voltage upper limit) appears in the battery system, and the charging current is within the predetermined range, it is determined whether the minimum single cell voltage Vmin ≥ V1. If Vmin ≥ V1, the current charging equalization mark is 0; if Vmin < V1, all single cells in the battery system that exceed the voltage of V2 are marked.

[0026] Equilibrium stage

[0027] When the maximum value of the single cell voltage Vmax ≥ Vm, the master control sends a balancing start instruction to the slave control, and the slave control starts balancing for the marked single cell. The balancing circuit is periodically turned on / off, with the balancing time on being T1 and the balancing time off being T2. The duty cycle = T1 / (T1+T2). The heat generated during the on-time is dissipated and cooled during the off-time. Among them, Vm is the upper voltage limit, which is determined by experiments: according to the current balancing circuit, balancing is continuously performed under different battery voltage input conditions. When the input voltage continues to balance and the temperature rise reaches the maximum temperature rise requirement, this voltage is Vm.

[0028] When the maximum value of the single cell voltage Vmax<Vm, the master control sends a balancing start instruction to the slave control, and the slave control starts balancing for the marked single cell. The balancing circuit is periodically turned on / off, with the balancing start time being T3 and the balancing stop time being T4, and the duty cycle = T3 / (T3+T4). T1 / (T1+T2)<T3 / (T3+T4); in this case, the duty cycle can be appropriately increased while ensuring that the temperature is not too high.

[0029] When the total balancing start time accumulates to T0, the master controller sends a balancing end instruction, the slave controller ends balancing, and clears the balancing mark.

[0030] During the balancing process, the slave controller detects the temperature of the balancing circuit in real time. When the highest temperature is ≥ t2, the master controller controls the slave controller to interrupt balancing, and the balancing mark and the time when balancing has been turned on are retained; when the highest temperature is < t1 after the balancing is interrupted, balancing is restored, that is, the master controller controls the slave controller to turn on balancing, and the time when balancing is turned on continues to accumulate. t1 and t2 are the set temperature values.

[0031] Balance diagnosis

[0032] In the wake-up phase of the management system, the master controller sends a balanced start command to all slave controllers and determines the voltage changes before and after the start. If all monomers V i -V i+1 >△V1, the balancing circuit is normal; if there is a single cell V after closing i -V i+1<△V2, it means that there is a problem with the balancing system and it cannot be effectively balanced. Therefore, the short circuit or open circuit fault of the balancing circuit is reported for the monomer that meets this condition: inform the service personnel to go and deal with it to avoid long-term battery imbalance or over-discharge problems. △V1 and △V2 are preset voltage difference thresholds. According to the balancing circuit, the battery operating voltage range is determined. For example, the upper limit of the battery operating voltage is V1 and the lower limit is V2; then after the balancing is turned on, the battery voltage drops in the range of [V2 / R, V1 / R], and R is the balancing equivalent resistance. Therefore, V2 / R≤△V1≤V1 / R, △V2<V2 / R. When the voltage change before and after closing the balancing is <△V2, the balancing circuit is open (the voltage does not change before and after the open circuit is closed, and maintains the normal value) or short circuit (the short circuit is always on before and after closing, and the battery voltage is always low before and after closing).

[0033] Passive balancing device embodiment

[0034] Regarding the passive balancing device, it is actually a combination of relevant software and hardware to implement the above method. The hardware mainly includes: Figure 1 The circuit includes a computer program stored in a memory, and the above method can be implemented when the master control and the slave control execute the computer program.

[0035] In this embodiment, two processors, a master controller and a slave controller, cooperate to implement the above method. As another implementation method, the method can also be completed by one processor.

Claims

1. A passive equalization method for lithium iron phosphate batteries, comprising an equalization marking stage and an equalization stage; in the equalization marking stage, when the maximum value of the cell voltage Vmax ≥ V3 and the charging current is within a predetermined range, if Vmin ≥ V1, no marking is performed; if Vmin < V1, all cells in the battery system with a voltage exceeding V2 are marked; wherein V 3 is a set value close to the upper limit of the single cell voltage; V1 is the upper limit of the voltage during the charging platform period, V1<V2<V3; in the balancing stage, the marked single cells are passively balanced.

2. The passive equalization method for lithium iron phosphate batteries according to claim 1, characterized in that: Passive equalization is performed by periodically turning on / off the equalization circuit.

3. The passive equalization method for lithium iron phosphate batteries according to claim 2, characterized in that: When the maximum value of the single cell voltage Vmax≥Vm, the open balance time is T1, the closed balance time is T2, and the duty cycle = T1 / (T1+T2); when the maximum value of the single cell voltage Vmax<Vm, the open balance time is T3, the closed balance time is T4, and the duty cycle = T3 / (T3+T4); T1 / (T1+T2)<T3 / (T3+T4).

4. The passive equalization method for lithium iron phosphate batteries according to claim 2, characterized in that: When the total equalization opening time reaches the preset time, the equalization ends.

5. The passive equalization method for lithium iron phosphate batteries according to claim 4, characterized in that: During the balancing process, the temperature of the balancing circuit is detected in real time. When the highest temperature is greater than the set temperature upper limit, the balancing is interrupted, and the balancing mark and the time when the balancing is turned on are retained; When the maximum temperature is lower than the set temperature limit, the balance is restored.

6. The passive equalization method for lithium iron phosphate batteries according to claim 1, characterized in that: It also includes a wake-up phase; in the wake-up phase, all monomers are balanced. If after the balance, the voltage drop of a monomer is lower than the preset value, the fault of the equalization circuit corresponding to the monomer is reported.

7. A passive equalization device for a lithium iron phosphate battery, comprising a processor and a memory, wherein the memory stores a computer program, and when the processor executes the computer program, the method according to any one of claims 1 to 6 is implemented.

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