KR20220067890A - Cell Balancing of Lithium Battery Pack - Google Patents

Abstract

Provided is a method for controlling a battery management system (BMS), in which when a dangerous situation occurs during charging and discharging of a battery cell, cell balancing is performed by separating a specific cell to bypass a current with an identical internal resistor, and when the dangerous situation is released, returning is performed. Cell balancing in a typical charging mode is performed by charging a battery cell while applying a voltage to the battery cell, and discharging the battery cell with a balancing resistor. However, according to the present invention, cell balancing is performed more rapidly and accurately by separating a battery cell from which a danger is detected to allow the cell balancing to be performed while a charging power is cut off. Thereafter, when a cell voltage becomes uniform, a cell cut-off switch is closed, and a switch of a current bypass circuit is opened, thereby returning to a normal BMS operation.

Description

Cell Balancing of Lithium Battery Pack

The present invention relates to a method for cell balancing of a lithium battery pack.

The technology behind the invention is a technology that uses electricity output from a battery cell assembly connected in series or parallel as a main/auxiliary power source to improve mileage/fuel efficiency in eco-friendly vehicles such as HEVs, PHEVs, and EVs. In the case of cells of a battery to which a plurality of cells are connected, when charging and discharging are repeated, a difference in charging capacity of each battery cell occurs.

A lifespan difference is generated due to a voltage difference between battery cells, and cell balancing is used as a method of reducing this.

It is important to prevent deterioration of battery life in a vehicle used for a long period of time, and accordingly, cell balancing is performed to minimize the occurrence of cell-to-cell deviation, which is one of the means of preventing deterioration. Passive cell balancing that lowers the value is mainly used.

In this method, it is important to note that in the method of balancing cells by discharging with a resistor, if the balancing resistor is 27Ω and 15W is used, the cell balancing is performed based on the operating environment of 75 degrees. The resistance temperature rises to over 200 degrees within 30 minutes, and when the temperature rises above 200 degrees, cracks occur in the resistor if the standards guaranteed by the resistor supplier are exceeded.

Therefore, cell balancing must be performed to prevent resistance cracks from occurring.

Another method is active cell balancing.

The difference is that the passive discharges with a resistor while the active charges up, which can increase the low battery cell voltage.

Although it is a good method, the unit cost is high because the circuit is complicated and there are many devices.

The method of insulating each cell using a transformer and charging each cell individually charges all cells individually, so there is no restriction on the charging time and there is no problem with the charger balancing.

A simpler active cell balancing method is to transfer the current of the lower cell where the high voltage is generated among the battery cells. As a result, the high voltage battery cell is bypassed and transferred to the next cell. It is more effective than balancing passive cells that discharge resistors.

In high-voltage batteries currently used mainly in cell balancing, a voltage difference between battery cells may occur as charging/discharging is continuously performed. Alternatively, over-discharge may occur and cause catastrophic damage to the high-voltage battery.

The matters described as the background art above are only for improving the understanding of the background of the present invention, and should not be accepted as acknowledging that they correspond to the prior art already known to those of ordinary skill in the art.

KR1020147001389

KR1020150165325

KR1020130076959

There are dedicated ICs for monitoring and managing battery cells in high-voltage battery packs, but due to malfunctions caused by dangerous situations in the vehicle, re-entering the charging mode without narrowing the cell gap with sufficient cell balancing, or dangerous situations of specific battery cells It is a method to reduce the risk of the battery pack such as thermal runaway that may occur in the worst case by blocking a specific cell in an overvoltage state where a cell gap has occurred and bypassing the current with a similar internal resistance when detected.

In addition, the separated cell performs its own cell balancing until it becomes equal to other cell voltages, and when the cell voltage difference is released, the cell cut-off switch and the current bypass switch are restored to operate the BMS in the usual way.

And even in the discharge mode, when a specific individual cell has low voltage or a dangerous situation is detected, the individual cell is blocked and the current is diverted through the internal resistance circuit to prevent overdischarge of the individual cell.

In addition, the blocked cell performs its own cell balancing, and when the cell voltage becomes equal with other cells, closes the cut-off switch of a specific isolated cell and reopens the switch of the current bypass circuit to return to normal BMS operation.

Lithium battery cell voltage inequality is caused by capacity error, self-discharge rate, and capacity gap according to the number of times of charge/discharge occurring during the process.

The lifetime of a lithium battery cell is mainly affected by voltage and temperature. When a cell voltage imbalance occurs, a gap occurs in the lifespan of a battery cell.

In particular, in the case of a high-voltage battery for electric vehicles in which many cells are connected in series and parallel, if voltage imbalance occurs, not only the lifespan gap of individual cells, but also unintentional overvoltage above the rated voltage may be charged or overdischarged.

In particular, it is said that the use of lithium batteries in an enclosed space can rapidly increase the temperature.

As a result, the temperature rise of the adjacent battery cell is also affected, and the temperature rise of the battery cell is accelerated, which may lead to thermal runaway.

Because the internal characteristics and internal resistance of the battery cells are different, charging for high-voltage batteries

As /discharge is continuously performed, a voltage gap between battery cells may occur, and if the voltage gap between battery cells is accumulated, the battery cell may be overcharged or overdischarged, causing fatal damage to the high-voltage battery.

In addition, if the battery cell characteristics change rapidly in a dangerous situation of the vehicle, it is expected that the method of switching off the relay of the entire battery pack to switch to a stable state is necessary to manage individual cells because the power of the entire EV vehicle is not supplied.

In addition, if cell balancing is performed during charging, charging and cell balancing are carried out at the same time in a situation where the cell voltage gap has already widened, so it is expected that it will take more time. it is expected

In general, when the voltage of a specific battery cell is detected to be lower than that of other cells, passive cell balancing is used, which lowers the voltage of other cells as well.

It may cause a fatal risk to the battery as it may re-enter the charging mode without completing cell balancing and closing the cell gap.

In addition, in an actual EV vehicle, the terminal cells connected to the positive terminal of the battery pack have a higher current flow during charging and discharging compared to other battery cells, so the cells age faster.

Therefore, when charging, the voltage rises faster than other cells, so even when adjacent cells are not fully charged, the terminal cell is fully charged and charging may be interrupted.

In this case, the number of times of charging and discharging of the cells is different, which may cause a difference in the lifespan of the cells.

In a dangerous situation in which the characteristics of a specific battery cell rapidly change due to danger, it is necessary to cut off the high voltage of the battery pack with a relay, etc., but if this is cut off, power is not supplied to the entire electric vehicle (EV) and operation is not possible. may create a hazard. Therefore, it is sometimes used as a method of gradually decreasing the battery power.

In this case, the voltage difference between the battery cells is not resolved, and the battery cell continues to have a risk.

In order to prevent this, it is necessary to cell-balance only the relevant cell in a state in which the battery cell in which the risk is detected is separated from other cells.

To this end, a device that protects the cell by isolating the cell in which the danger is sensed and bypassing the current with a similar internal resistance circuit is required.

If only individual cells that are simply connected in series with a sensed danger are bypassed, the number of cells decreases and the charging voltage affected by individual cells rises, and the rated shock voltage of other cells may be exceeded, which may endanger the cells.

Even if the cell is bypassed, normal charging and discharging should be performed without affecting other cells, so an internal resistance bypass circuit with similar capacity is required.

Cell balancing in the normal charging mode is charged while voltage is applied to the battery cells and the battery cells are discharged with the balancing resistor. This enables faster and safer cell balancing. After that, when the cell voltage becomes equal, the cell cutoff switch is closed and the internal resistance current bypass switch is opened again to return to normal BMS operation.

In the present invention, even in a situation where the voltage difference of the battery cells remains, it enters the charging mode again to aggravate the cell voltage difference or when an individual cell encounters a dangerous situation, it bypasses the dangerous detected battery cell by bypassing it with a similar internal resistance. By performing charging and discharging, the risk of thermal runaway can be prevented in advance.

It is expected that the present invention can alleviate the phenomenon that the aging rate of the battery pack is accelerated due to a phenomenon in which the voltage is charged and discharged quickly because the battery cells connected to both ends of the battery pack are rapidly aging.

In addition, the battery cells at both ends connected in series, which rise quickly during charging and discharging, are bypassed with a similar internal resistance circuit to increase the charging capacity of the battery by reconnecting it after the other cells are fully charged. can

It is the cause of the danger of the vehicle due to the invention that has not yet been able to find a clear cause, such as the danger in the vehicle that is caused by continuously charging/discharging the high voltage battery of the currently used vehicle battery, to the malfunction that can affect the battery pack. It is expected that fatal damage to the battery due to thermal runaway caused by overcharging can be reduced by reducing the unknown cell voltage gap.

The present invention is an emergency operation when a battery cell abruptly approaches the limit value or a dangerous situation occurs in a dangerous situation of a vehicle, and in the worst case, thermal runaway, etc. Separated battery cells perform cell balancing on their own, and when the voltage becomes equal with other cells, close the switch and reconnect. It is a method to relatively extend the lifespan of a cell by charging the battery pack while the individual cells are separated without consuming the overvoltage of the cell to equalize other cells.

Although the present invention has been illustrated and described with reference to specific embodiments, it is recognized in the art that the present invention can be variously improved and changed without departing from the spirit of the present invention provided by the following claims. It will be obvious to one with ordinary knowledge

1 is a view showing a battery cell balancing system according to an embodiment of the present invention;
2 is a flowchart of a method for balancing battery cells during charging according to an embodiment of the present invention;
3 is a flowchart of a method for balancing battery cells during discharging according to an embodiment of the present invention;
2 is a flowchart illustrating an operation of a battery cell balancing device related to the present invention in a charging mode.
First, the battery cell voltage and temperature are measured through the cell voltage/temperature measuring means (S100).
Measurement result 1. In case of re-entering the charging mode without narrowing the cell voltage gap or
2. When it is determined that a dangerous situation of an individual cell is detected (S200).
Instead of proceeding with the conventional cell balancing method, a specific cell in which a cell voltage gap remains or a danger is sensed is blocked and the current is bypassed to the internal resistance circuit (S400).
In a state in which the dangerous cell is separated and the current is bypassed, the battery pack is charged, and the overvoltage cell is self-balancing in the separated state until the voltage becomes equal with other cells (S500).
When the cell voltage becomes equal to other cells, the cell cut-off switch is closed, and the internal resistance bypass switch is opened again to return to the normal BMS operation (S600).
And Figure 3 is a flow chart showing the operation of the battery cell balancing device related to the present invention in a discharging mode.
First, the battery cell voltage and temperature are measured through the cell voltage/temperature measuring means (S100).
Measurement result 1. When it is determined that an individual cell has low voltage or 2. When a dangerous situation of an individual cell is detected (S200).
It cuts off dangerous individual cells and bypasses the current to the internal resistance circuit (S400).
The battery pack is discharged in a state in which the current is bypassed (S500).
When the cell voltage becomes equal to other cells, the cell cut-off switch is closed and the internal resistance bypass switch is opened to return to the normal BMS operation (S600).
When it returns to normal operation, it returns to normal BMS operation.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art to which the present invention pertains can easily practice the present invention.

As a BMS circuit including a battery cell balancing device, when cell balancing is performed and re-entering the charging mode without narrowing the cell gap, or when a dangerous situation of individual battery cells is detected, the switch is closed to bypass the current with a similar internal resistance It includes a current bypass circuit and a cut-off switch that cuts off a cell in which a hazardous situation has occurred.

In addition, it is a method to return to normal operation by performing self-celling in a state in which each dangerous cell is separated and closing the switch again if there is no cell gap with other cells.

In order to achieve the above object, the method for balancing battery cells according to an embodiment of the present invention uses the existing cell balancing circuit when the state of the battery cell is within a specific limit, and when the state of the battery cell is higher than the limit, the battery cell is separated from itself After balancing the battery cells, the current is bypassed with a similar internal resistance circuit. After that, when the cell voltage becomes equal, the separated cells are reconnected and the current bypass circuit is blocked to return to normal BMS operation.

For example, if Cell 1 of Figure 1 enters the charging mode again without completing cell balancing in the overvoltage state, the cell voltage is higher than other cells, which increases the cell gap and increases the risk, resulting in thermal runaway in the worst case. There may be risks.

Therefore, when cell balancing is not completed or when re-entering into charging mode without narrowing the cell gap, or when an individual cell short or open, or a dangerous situation such as heat generation is detected, a similar internal resistance circuit bypasses the current to charge the battery and It can reduce the risk of cells.

When a specific cell is separated and the cell voltage gap with other cells is resolved by self-cell balancing or when a dangerous situation of an individual cell is resolved, close the cell cut-off switch and reopen the bypass switch to return to normal operation.

Similarly, if an overvoltage occurs in Cell 2 or a cell voltage gap occurs and the cell enters the charging mode again without completing cell balancing, the cell voltage gap gets worse and the risk is aggravated if charging and discharging are repeated, and there is a risk of thermal runaway. can

ES?: Cell Emergency Bypass Switch
EM_R?: resistance similar to cell internal resistance
CD? : Cell balancing switch
CB_R?: cell balancing resistor

Claims (6)

It is a cell balancing device of BMS that measures the individual cell voltage of a high-voltage battery in which a plurality of battery cells are connected in series with each other, discharges the high voltage with a resistor and charges the low voltage to make the cells equal. A cell balancing method equipped with a cell bypass circuit that diverts current with a resistance similar to that of If the battery re-enters the charging mode without narrowing the cell voltage gap after performing cell balancing, the battery cell is cut off with a cell cut-off switch and the current is diverted with the cell bypass switch to reduce the cell voltage gap. When individual battery cells are judged to be in a dangerous situation such as open, short, or overheat (S200). A cell balancing method that cuts off a specific battery cell with an individual cell cut-off switch and closes the bypass switch to bypass the current. BMS operation method of balancing cells in the usual way by closing the cell cut-off switch and opening the bypass switch again if the dangerous situation is relieved or the cell voltage gap is resolved by rechecking the state of the battery cells after blocking the battery cells and bypassing the current. [Claim 4] The battery cell bypass circuit according to claim 2 or 3, wherein, when balancing the cells by diverting the current with the cell bypass switch, the circuit bypasses through a resistance value similar to that of the internal resistance of the battery cell so as not to cause overvoltage due to the bypassing of the battery cell. . The method according to claim 2 or 3, wherein the battery pack is charged in a state in which the individual cells are separated without consuming the overvoltage of the individual cells as a resistor to equalize the other cells, thereby extending the lifespan of the cells relatively.

Images