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BMS system between electric vehicle batteries

BMS system between electric vehicle batteries

Electric power batteries require high power density, high energy density, long life, environmental protection and other requirements, and lithium batteries have the above advantages, so in the electric vehicle is widely used, today to talk about lithium batteries and management of their systems.

What are the commonly used battery types and their application requirements?

Car lithium batteries have the following:

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And other types, the battery discharge temperature of -20 ~ 55 ℃. The charging temperature is 0 ~ 45 ℃. If the Li4Ti5O12 / LTO as the anode material, the charging temperature can reach -30 ℃, usually lithium battery voltage range of 1.5V ~ 4.2V (including C / NCA, C / NCM, C / LMO 2.5V ~ 4.2V; LTO / C / LMO is 1.5V ~ 2.7V; C / LFP is 2.0V ~ 3.7V).

Usually the temperature is 90 ~ 120 ℃, SEI film began to enter the heat decomposition (Figure 1).

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Figure 1 Battery safe working area

Some of the electrolyte even in a very low temperature decomposition; when the temperature exceeds 120 ℃, SEI membrane can not protect the carbon anode and organic electrolyte side reaction to produce gas; when the temperature exceeds 130 ℃, the diaphragm began to melt and cut off the battery reaction. When the temperature is higher, the cathode material begins to decompose:


When the temperature exceeds 200 ℃, the electrolyte begins to decompose to produce combustible gas.

Decomposed combustible gas and oxygen will undergo a violent chemical reaction and lead to thermal runaway. A charge temperature of less than 0 ° C causes the metal lithium to deposit on the surface of the carbon negative electrode, thereby reducing the battery cycle life. In the extreme case of low temperature, will lead to the battery negative piercing caused by the occurrence of short circuit conditions. If the voltage is too low or the battery is overdosed, the phase transition causes the battery lattice to collapse and thus affect the performance of the battery. And even cause the negative current collector to dissolve in the electrolyte. Extreme overrunning can also lead to a reduction in electrolyte and produce flammable gases and thus a potential safety risk. High voltage and overcharge will destroy the composition of the positive electrode and cause a lot of heat generation. The same will lead to metal lithium deposition in the negative surface and accelerate the capacity of attenuation and lead to internal short circuit and lead to safety problems, the battery voltage of about 4.5V electrolyte began to break down.

Lithium battery in the application of electric vehicles

There are many types of power batteries used in electric vehicles, the widely used power battery generally LMO, LFP, NCM, NCA as the cathode material, while the use of carbon anode material, while LTO has also been developed to improve the battery mileage And fast charge capacity.

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Table 1 electric vehicle lithium battery applications

BMS function and its key technology

BMS function

At present commercial battery must have BMS. Through the BMS to control and manage the battery more efficient, each battery work in the range of running, to avoid the battery overcharge and overheating and thermal runaway problems occur. The capacity of a single cell is relatively low, requires a lot of batteries integrated into a module, a battery system contains multiple modules. Usually a battery system contains hundreds or even thousands of batteries. How to keep batteries working in the right interval, BMS plays an important role.

BMS function to monitor the battery status, the establishment of battery status, protection of the battery, reported data, such as balance. BMS in the vehicle in the main tasks are:

1, the protection of batteries and battery packs are not damaged;

2, the battery work in the appropriate voltage and temperature range;

3, to keep the battery in the right conditions after the operation to meet the needs of the vehicle.

Of course, BMS must meet the relevant standards and regulations. BMS basic hardware architecture shown in Figure 2.

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Figure 2 BMS basic hardware architecture

4, battery parameter detection: including total pressure, total current, single voltage detection, temperature detection, insulation detection, collision detection, impedance detection, smoke detection and so on.

5, the battery state to establish: including SOC, SOH, SOF.

6, online diagnosis: failure, including sensor failure, network failure, battery failure, battery overcharge, over discharge. Overcurrent, insulation failure and so on.

7, battery safety protection and alarm: including temperature control system control and high pressure control, when diagnosed failure, BMS reported failure to the vehicle controller and charger, while cutting off the high voltage to protect the battery from damage, including leakage protection.

8, charge control: BMS slow charge and fast charge control.

9, battery consistency control: BMS collect single-voltage information, the use of balanced way to make the battery to achieve consistency, the battery balance of dissipation and non-dissipative.

10, thermal management function: the battery pack at all points of the collection temperature, in the charge and discharge over, BMS decide whether to open the heating and cooling.

11, the network features: including online calibration and health, online program download. Usually using CAN network.

12, information storage: BMS need to store key data such as SOC, SOH, charge and discharge hours, fault code and so on.

BMS Key Technologies

BMS key technologies are accurate measurement of battery voltage, the establishment of battery status, the battery consistency of the balance, the battery fault diagnosis technology.

1, single voltage measurement

Difficulties in measuring single voltage:

a, the battery system has a lot of batteries in series together, the need for multi-channel battery voltage collection. The voltage of each battery may be different, which makes the hardware circuit design difficult.

b, the cell voltage measurement needs to have a high collection accuracy, especially the establishment of the battery SOC state needs to have a high collection accuracy requirements.

The following to C / LPF and C / NCM as an example: Figure 3 reflects the different open circuit voltage and SOC correspondence, we can see from the figure C / NCM OCV cancellation slope comparison shaking, the maximum per mv voltage corresponding to the change Rate of 0.4% (except 60 ~ 70%), if the battery measurement accuracy of 10mv, then SOC according to the corresponding relationship between the OCV established state error does not exceed 4%. For C / NCM batteries, the measurement accuracy of the batteries is within 10mv, but the OCV curve for C / LFP is relatively flat, and the corresponding change rate of the frequency of the voltage is more than 4%. Therefore, the accuracy of the single voltage is high , But most of the acquisition chip accuracy can only reach about 5%. The current single-voltage acquisition is mainly used to collect the chip chip, in Table 2 lists some integrated chips.

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BMS state establishment algorithm framework

3, Soc algorithm are:

1) discharge test method;

2) cumulative an hour;

3) open circuit voltage method, according to the relationship between OCV and SOC one by one, the accuracy is relatively high, but need to have a long battery of standing (also need to consider the phenomenon of voltage hysteresis, as shown in Figure 5);

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