How to heat the power battery of electric vehicle

The car battery selection of, mainly according to the following standards, as shown in figure 2.General hybrid electric vehicle (HEV) use is higher than the power of the battery, and pure electric vehicles (EV) is higher than the energy of battery.

Different types of cells work only in a certain temperature range, can guarantee the performance and life, such as lead-acid battery temperature between 35 to 40 ℃, nickel-metal hydride batteries between 0 ~ 40 ℃,Lithium ion batteries between 20 ~ 75 ℃.The discharge of the battery efficiency under different temperature is shown in figure 3.

As the main energy storage element in the electric vehicle batteries, are key components in electric cars, directly affect the performance of the electric car.And the battery thermal management is essential for modern electric car, the reason is that

(1) the electric car battery long time work in relatively poor thermal environment, it will shorten the battery life, reduce the battery performance;

(2) the battery long time will cause uneven distribution of temperature field in the uneven performance of each battery module, monomer;

(3) battery thermal monitoring and thermal management is of great significance to the vehicle running safety.

The choice of car battery cooling solutions

Different cell types, its heat generating reasons vary, but mainly includes the following four aspects: the battery chemical reaction, battery im very hot, charging side reaction heat, resistance of joule heat.After the determine the life of the battery heat, through reasonable choice cooling and heat dissipation structure can guarantee the working temperature of the battery.

The choice of heat transfer medium

The choice of the heat transfer medium has a great influence on the operating temperature of the battery pack.Classified according to heat transfer medium, battery cooling system can be divided into air cooling, surface and three kinds of cooling phase change materials.Air cooling is the most simple way, simply let the air through the surface of the battery.Liquid cooling can be divided into direct and indirect contact in two ways.Mineral oil can be used as direct contact heat transfer medium, water or antifreeze can be as a typical non direct contact heat transfer medium.Surface must be through the water jacket to the battery for cooling, heat exchange facilities such as this to a certain extent, the lower thermal efficiency.Cell wall and the rate of the heat transfer between fluid and fluid flow pattern, flow velocity, fluid density and fluid thermal conductivity and other related factors.

The choice of the heat dissipation structure

Battery temperature differences between different battery module in the cabinet, to intensify the battery internal resistance and capacity of inconsistency, if long time accumulation, will cause the part of the battery overcharge or overdischarge, affect battery life and performance, and cause potential safety hazard.Battery in the battery module of temperature difference and battery pack arrangement has much to do, in general, the middle of the battery easy accumulation quantity of heat, the edge of the battery cooling conditions better.So in the battery pack structure arrangement and heat dissipation design, to ensure the uniformity of battery pack cooling.In the air cooling heat dissipation and ventilation way generally has two kinds of serial and parallel, as shown in figure 4 and 5.

As shown in figure 4 serial ventilation mode, the cold air from the left side of the blow blow out from the right.Air heating are constantly in the process of flow, so on the right side of the cooling effect is bad, than on the left side of the battery in the battery temperature rise from left to right in turn.The first generation of the Toyota Prius and Honda Insight are adopted serial ventilation mode.

Shown in figure 5, parallel way ventilation air flow is more evenly distributed between the battery module.Parallel ventilation way need to exhaust channel, position of battery arrangement are well designed.Toyota's new Prius is parallel ventilation structure, the wedge into the exhaust passage makes the gap between different modules, and the pressure difference between the basic consistent, to ensure that the air flow of the blowing different battery module of consistency, thus ensuring the consistency of the stack temperature field distribution.

From the Angle of improving battery performance, parallel ventilation mode is better than that of serial ventilation way, more commonly used in the thermal management system.

A HEV battery CFD thermal solution case analysis

The performance of the hybrid car is subject to the performance of the battery pack and battery performance is closely related to temperature.Hybrid cooling system with nickel metal hydride batteries goal is: the working temperature of the battery is controlled in the range of the best 20 ℃ to 40 ℃, the temperature difference between the module under 5 ℃.

We consider the different arrangement of the battery pack air duct, the influence of battery pack cooling system.With side air duct is refers to the order of battery module configuration within the battery box body, the external side of cooling air flow from the battery pack into the, through internal air duct into the gaps between the battery module, finally in the lateral position of the air duct with intakes in the gas discharge.Different side duct is refers to the order of battery module configuration within the battery box body, outside into the side of the cooling air flow from the battery pack, through internal air duct into the gaps between the battery module, finally with intakes in the opposite direction position of the air duct in the gas discharge.

Ipsilateral in and out of the air duct, air from the air inlet pipe into the bottom of the battery pack, arriving at the right back into the left side, the backflow phenomenon will lead to thermal cycle, is not conducive to the top and bottom battery cooling.Relative to the same side in and out of the air duct, opposite side in and out of the air duct scheme from the left side of the imported into the bottom of the battery pack, directly to the right, can reduce the degree of battery thermal cycle, can lower the highest temperature of the upper battery;At the bottom of the opposite side in and out of the air duct flow field velocity distribution more uniform.

From the above picture shows that with the side in and out of the air duct cell surface top speed and minimum airflow velocity is large, can lead to cell surface uneven heat dissipation, temperature rise is bigger, affect battery performance.Side in and out of the air duct scheme improve the battery on the left side of the airflow velocity, is advantageous to the battery on the left side of the upper radiator.Compared with the same side in and out of the air duct, opposite side in and out of the air duct cell surface air velocity distribution even, has a better solution.

, through the analysis of the flow field along the axial velocity flow is the basic module is uniform, but because the battery is working, the battery internal temperature rising.Airflow from import to export temperature rise gradually, make the upstream battery cooling condition is superior to the downstream.So suggest to minimize the area of the downstream air duct, the flow velocity in the downstream increase, improve battery internal flow velocity, take internal heat.

The simulation results verify the

Ipsilateral test temperature of 11 ℃ in and out of the air duct, constant temperature placed 1 day.Side in and out of the air duct scheme test temperature of 22 ℃, temperature placed 1 day.Trial begins, the test plan for test, when the temperature of the battery pack is at 30 ℃, battery pack fan to start.

Placed inside the battery pack, 36 temperature sensor temperature acquisition module, inside the battery pack, as shown in diagram battery temperature sensor are arranged from left to right sequence for increasing Numbers 1, 2, 3, etc.Require batteries in the range of SOC30%70%, through to the battery pack uninterrupted for constant current charging, constant current discharge continue to test.Battery monomer on the temperature measurement points shown below, plan an ipsilateral in and out of the air duct, plan 2. Opposite side in and out of the air duct.

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