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Electric vehicle

Release time:
2018/07/02 09:47
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[Abstract]:
An electric-vehicle battery (EVB) is a battery used to power the propulsion of battery electric vehicles (BEVs)

An electric-vehicle battery (EVB) is a battery used to power the propulsion of battery electric vehicles (BEVs),which can directly influence the performance of the electric-vehicle. Lithium-ion battery is one of the most popular types of rechargeable batteries for electric-vehicle battery pack, with a high power output and long operating life. However, lithium-ion battery is sensitive to the change of temp, especially the high energy density and power battery used in EV.

As the battery demand of VE is big and the install space is limited, the battery is lined up tightly. The temp of battery operating environment is complex because when the VE runs in the exchange of high speed、low speed、speed up or speed reduction, the battery will produce immense heat with different multiplying power discharging and heat production rate. As the battery is lined up tightly, the heat in the middle area is more than the side area, which increases the heat imbalance if individual battery, and makes the difference of battery module、resistance and capacity. Also, the accumulation of time and limitation of space will gather more inhomogeneous heat. If the heat piles up for long time, it may cause safety problem and affect the performance and life time of the battery because its overcharge and over discharge. If the batter pack can't get effective heat dissipation, it can lead the excess temp or temp maldistribution and lower the battery cycle efficiency, even can lead thermal runaway and influence the safety and reliability of the battery. The other situation is in low temp, the electrochemical reaction can't work, so we need to warn the battery pack. In a conclusion, in order to make the performance and life time to the best, we need to optimize the structure of battery pack and deal with the thermal management, control the environment temp.

 

1.The warn up and heat dissipation of battery park: active system and passive system

It makes big difference choosing active or passive way to warn or heat dissipation. The passive system requires relatively lower cost and the equipment is easy. Active system’s structure is relatively complex and it needs bigger additional power, but it gets better heat dissipation. Take the cost、quality and the arrangement of the space into consideration, the vehicles run in moderate climate do not get unit for warning or heat dissipation, they depend on air. (As picture 1)  At present, hybrid electric vehicle also use air to warn/cool battery park passively. Although air warm or cool the system through air condition, it still considered as passive system. (As picture 2) The environment air temp must be during 10℃~35℃ if we want to apply this passive system to normal thermal management. If in extremely cold or hot environment, the battery park may get even worse heat distribution. Relative experiment also improves that in passive system, the battery park may get even worse heat distribution because of the different temp of environment air.

 

The below picture shows the air cooling and liquid cooling system

1:warm passively—external air circulation 2:passive warm and cool---internal air flow
3:active warm and cool—external air flow 4:passive liquid cooling-liquid circulation
 
5:active cooling-- liquid circulation  

 

2.Heat dissipation system

According to heat transfer theory, the contact of solid and air, solid and liquid will have the phenomenon of heat transfer. The air convective heat transfer coefficient is bigger than liquid, and it is even bigger of liquid and solid. The convective heat transfer coefficient bigger, the performance of heat exchange is better, so we need choose suitable heat-transfer medium. The heat transfer coefficient of all heat transfer phenomenon are as showed below:

 

table 1

Types of convective heat transfer  h/[w /(m² k)]
Natural-convection heat transfer : air  2-25
Liquid  50-1000
Forced-convection heat transfer: air  25-250
Liquid  50-25000
Phase-change Convection heat transfer: boiling  2000-50000
Congeal  2000-100000

 

If we use liquid as heat-transfer medium, we need to consider the electrical conductivity, safety and leakproofness, also the service conveniences and the weight of the whole battery pack. The capacity of heat transfer and thermal storage of phase change material (PCM like liquid paraffin) is the best, and when it reaches the phase-change temp, it can absorb or release the heat largely without cooling or warming. The individual battery can effectively get heat balance through choosing suitable phase-change material, also can control the temp limitation of the battery and avoid too hot or too cold. But take material research and cost into consideration, the most effective and common cooling medium is air.

There are parallel and serial two ways of air cooling, as is shown picture 7 and 8. It requires relevant air guide on battery park to reduce the airflow resistance to ensure the uniformity.

 

 

 

In parallel situation, the air goes through one side to the other side of the battery pack to get the result of heat dissipation. The air will take the heat from side to side, which leads the big temp difference.  In serial situation, the air is rising, which can distribute the air more uniformly to ensure the uniformity of heat dissipation.

When designing the battery pack heat dissipation and warming system, we should take all the factors into consideration. We can see that adopting air as heat transfer medium, the main advantages are: simple structure, light weight, low cost and gets effective ventilation if gets harmful gas; the main disadvantages are: the heat transfer coefficient with the battery low, cooling speed low, low efficiency. Adopting liquid as heat transfer medium, the main advantages are: the heat transfer coefficient with the battery high, cooling and warming fast; the main disadvantages are: the leakproofness high, heavy weight, service and maintain is complex, the structure is complex. The advantages of PCM(phase-change material) are: the heat transfer coefficient with the battery high, cooling and warming speed fast, and can control the limitation of the temp; disadvantage is the research and development cost high.