Talking about the consistency of battery pack and its influence

by:CTECHi     2021-07-31

The power and driving range of electric vehicles depend on the performance of the battery. Battery inconsistency is the main factor that affects the performance of the entire electric vehicle. According to the different ways in which the inconsistency of the battery pack affects the battery performance and the different working principles, the inconsistencies of the battery can be divided into inconsistent capacity, inconsistent resistance and inconsistent voltage.

Capacity inconsistency

The influence of capacity inconsistency can be divided into three aspects:

①The driving distance of electric vehicles is the same, because With different capacity, the depth of discharge of the battery is also different. In the case that most batteries are still in shallow discharge, the battery with insufficient capacity has entered the deep discharge stage, and when other batteries are deeply discharged, the low-capacity battery may have no power discharged and become a load in the circuit.

②The same type of battery has the same optimal discharge rate, and the capacity is different, the optimal discharge current is different. In the series group, the current is the same, so some batteries are discharged at the optimal discharge current, but there are The battery does not meet or exceed the optimal discharge current.

③During the charging process, the small-capacity battery will be fully charged in advance. In order to fully charge the other batteries in the battery pack, the small-capacity battery will be overcharged, and the charging voltage in the later stage of charging will be high, even exceeding the maximum battery voltage , Poses a safety hazard and affects the entire battery pack charging process.

The above three reasons make the battery with insufficient capacity enter a vicious circle in the process of charging and discharging, and damage it in advance.

Resistance inconsistency

1) Series group

During the discharge process, the current in the series group is the same and the internal resistance is large The battery, the voltage drop is large, the energy loss is large, and a large amount of heat is generated, and the higher the temperature, the greater the internal resistance, the greater the energy loss. If the heat cannot be dissipated in time, the battery temperature will continue to rise, which may cause the battery to deform or even explode. as a result of.

During the charging process, due to the different internal resistance, the charging voltage of each battery allocated to the series group will be different, which will cause the battery charging voltage to be inconsistent. As the charging process proceeds, the battery voltage with large internal resistance may The maximum voltage limit for charging is reached early, so in order to ensure the safety of charging, the charging has to be stopped when most of the batteries are not fully charged.

2) Parallel group

During the discharge process, the voltage of each parallel group is the same, and the battery with large internal resistance has a small current. The battery with small internal resistance has a large current. In this way, the battery works at different discharge rates, which affects the battery life. At the same time, the battery discharges different energy, so that the battery discharge depth is different under the same working conditions. During the charging process, due to different internal resistances and different charging currents allocated to the parallel group, the charging capacity is different in the same time, that is, the charging speed of the battery is different, which affects the entire charging process. In the actual charging process, it can only be There is a compromise between preventing overcharging of fast-charging batteries and preventing dissatisfaction of slow-charging batteries.

Voltage inconsistency

The main impact of voltage inconsistency is the mutual charging of the batteries in the parallel group. When the voltage of one battery is low, the other batteries in the parallel group will Charge this battery. With this connection method, energy will be lost in the mutual charging process and the expected external output will not be achieved.

Case analysis

The inconsistency of the battery is not obvious when it is fully charged, but after a period of driving (shallow discharge stage), it is intuitive from the voltage The angle will show a certain degree of inconsistency. When the battery is deeply discharged (over 80%), the battery inconsistency will be an important factor affecting the driving range of electric vehicles.

The above figure shows the lithium-ion battery pack used in the BJD6100-EV electric bus under different discharge conditions. Voltage inconsistency. It can be seen from the figure that in the case of 80% discharge, the highest voltage of a single battery is 3.3V, which is within the normal operating voltage range of the battery (2.8~4.25V); the lowest voltage is 2.5V , Exceeding the minimum voltage of the battery, the voltage difference between the batteries is 0.8V. In this case, if the battery continues to discharge, the battery with lower voltage has no energy to discharge, which will cause permanent damage. If the bad battery is not found in time and it is used together with the normal battery, it will become the load of the battery pack, affect the work of other batteries, and then affect the life of the entire battery pack. Therefore, in the deep discharge stage when the inconsistency of the battery pack increases significantly, the discharge can not be continued. The good power of the vehicle comes from high power. The high power of electric vehicles requires high current discharge of the battery, and high current discharge will aggravate the formation of inconsistency. Therefore, the maximum discharge current of electric vehicles is limited, which affects the power of electric vehicles.

The BJD6100-EV lithium-ion battery electric bus operation test has proved the impact of battery inconsistency on the performance of electric vehicles. As the mileage increases, the inconsistency of the battery will enter a vicious circle. The most intuitive reflection is the operation The battery voltage inconsistency increased after a period of time. When loading the car, the inconsistency of the single cell voltage after charging is within 0.1V, and it has reached 0.2V after 2500km.

Source: Inconsistency of reliability and electric vehicle battery pack is connected,

Author: Wang Zhen slope share:

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