As an important component of the electric vehicle battery system, the
battery management system (BMS) controls the voltage, temperature, current, SOC,
SOH and other parameters of the battery pack as a whole. Among them, since the
battery pack is composed of several single cells, the BMS also plays the role of
energy balance.
The need for balance
With the current battery manufacturing level and technology, there will be
subtle differences in each cell during the production process, which is a
consistency issue. This inconsistency will make the parameters of the battery
cells very different. In order for them to fit together to form a whole, a
balanced approach must be used in the process of action. Just like the wooden
barrel effect, only by making up for shortcomings can performance be
maximized.
On the other hand, when the cells are used to form a battery assembly
vehicle, there will also be inconsistencies in the individual cells due to
reasons such as self-discharge and part temperature. The inconsistency of the
individual cells will in turn affect the charge and discharge characteristics of
the battery pack. .
The battle for balanced technical routes
The debate on the BMS technical route mainly focuses on two aspects:
passive balancing and active balancing.
As the name suggests, passive balancing is to consume the individual cells
with slightly more capacity in the single battery to achieve overall balance.
Active balancing is to transfer the slightly higher energy of the single unit to
the slightly lower energy battery through the energy storage link. What is
achieved is an active distribution effect.
Some people believe that passive balancing is more suitable for the current
development status of power batteries and has less impact on battery life;
others believe that active balancing can significantly help improve the overall
operating results and driving experience of electric vehicles.
Currently, there are many BMS systems using passive balancing on the
market. On the one hand, passive balancing appeared earlier than active
balancing, and the technology is relatively mature and simple, and is widely
used. The active balancing structure is more complex, and the design of the
transformer scheme and the switch matrix will undoubtedly increase the cost
significantly.
As mentioned before, the advantage of passive balancing is that it has a
simple structure and low cost, but because it uses resistors to consume energy,
it will generate heat, thereby reducing the efficiency of the entire system.
Active balancing has a high energy utilization rate because its principle
lies in the transfer and distribution of energy. However, it can generally only
transfer energy between two adjacent single cells. The structure is relatively
complex and the cost increases. The shortcomings lie.
BYD Qin’s BMS balancing technology inspiration
According to the official introduction, BYD Qin’s BMS not only has basic
battery energy management and battery thermal management functions, but also has
an automatic balancing function for battery cells. According to the
introduction, during the operation of the vehicle, monitoring the single
performance parameters of the entire battery pack and achieving timely and
automatic maintenance through the battery balancing function can greatly reduce
the cost of power battery maintenance, extend the service life, and improve each
stage. performance.
Qin optimizes and extends the performance and life of the battery pack
through an actively balanced BMS system. According to the introduction, Qin
specifically uses the DMⅡ generation battery management system, and the MIⅡ
generation adopts a distributed design. By collecting the temperature and
voltage of battery cells, the distributed battery management system performs
power battery energy management, thermal balance management, vehicle charge and
discharge management, and vehicle high-voltage safety management to further
improve the energy management efficiency of the battery while achieving
lightweight design. Design requirements.
Each two single cells form a unit, which transfers energy through energy
storage and balances the voltage.
summary:
Whether it is active equilibrium or passive equilibrium, it has its
application value. Balance technology is not a god-like existence. The overall
design of the BMS and its matching with the entire vehicle are the key. At the
same time, the two BMS equalization methods have obvious advantages and
disadvantages, and the main controversy at home and abroad is which method is
more reasonable. But it still depends on the characteristics of the battery.
When the consistency of the cells is strong, a small-capacity battery pack is
more suitable for passive balancing. If the consistency of a large-capacity
power battery is average, active balancing will be more suitable for use.
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