Electric vehicles must have electrical energy storage devices, and
currently lithium-ion batteries are the first choice and mainstream for power
batteries. When lithium-ion batteries are used in series and in groups, there
are problems such as overcharge, over-discharge, over-current, and excessively
high or low temperatures, which can cause rapid damage to the lithium-ion
batteries. Therefore, a battery management system is required to manage
them.
The emergence of electric vehicles is driven by global warming,
environmental pollution and energy crisis. In 2015, global electric vehicle
production and shipment exceeded 500,000 units respectively, of which China
exceeded 370,000 units. Electric vehicles must have electrical energy storage
devices, and currently lithium-ion batteries are the first choice and mainstream
for power batteries. When lithium-ion batteries are used in series and in
groups, there are problems such as overcharge, over-discharge, over-current, and
excessively high or low temperatures, which can cause rapid damage to the
lithium-ion batteries. Therefore, a battery management system is required to
manage them.
1. Lithium-ion battery
Lithium-ion batteries refer to batteries made of four main materials and
casings: positive electrode, negative electrode, separator, electrolyte. The
positive and negative electrode materials must be able to reversibly insert and
deintercalate lithium ions, the separator must conduct lithium ions and insulate
electronics, and the electrolyte must be a lithium ion solution.
Usually in the positive electrode material, a transition element undergoes
a redox reaction, while in metallic lithium and carbon negative electrodes,
metallic lithium undergoes a redox reaction. During the charging and discharging
process, lithium ions transfer back and forth between the positive and negative
electrodes inside the battery, and the battery moves in the external circuit.
Some people vividly call this lithium ion transfer process a rocking chair, and
lithium ion batteries are called rocking chair batteries.
Figure 1 Illustration of the working process of lithium-ion battery
Lithium-ion battery cathode materials generally use lithium-intercalated
transition metal oxides, such as lithium-intercalated oxides of Ni, Co, and Mn.
The negative electrode material must choose a lithium-embedded compound with a
potential as close as possible to metallic lithium, such as various carbon
materials, SnO, SnO2, silicon alloys, etc.
The electrolyte commonly uses LiPF6 solutions, and the solute is organic
matter. Commonly used ones include ethylene carbonate (EC), propylene carbonate
(PC), and low-density diethylene carbonate (DEC). The separator is mainly made
of olefin polymers. Porous composite membrane; shell materials include steel,
aluminum, plastic, aluminum-plastic film, etc. The typical structure of
lithium-ion battery is as follows:
Figure 2 Typical structure of prismatic battery
Typical parameters of lithium-ion batteries include: capacity, internal
resistance, voltage; lithium-ion battery characteristic parameters include:
cycle life, discharge platform, self-discharge rate, temperature performance,
storage performance, etc. Lithium-ion battery safety tests include: overcharge,
short circuit, needle puncture, drop, water immersion, low voltage, vibration,
etc.
Lithium-ion batteries are relatively delicate, and their charging and
discharging are multi-variable, nonlinear and complex electrochemical processes.
If the conditions for charging and discharging cannot be met, it is prone to
rapid decline in life, performance degradation, fire, explosion and other
events, because lithium Ion batteries are very sensitive to temperature,
voltage, current, etc.
2. Development of battery management system
Early battery management systems include: the BADICHEQ and BADICOaCH
systems designed in Germany in 1991, the battery management system used by
General Motors EV1 in the United States, and the high-performance battery
management system called BatOPt developed by the American AC Propulsion
company.
At the earliest in China, some universities relied on their own scientific
and technological advantages to jointly carry out some research work with some
large automobile and battery manufacturers. Tsinghua University provided a
battery management system for the EV-6568 light electric bus, and Tongji
University and Beijing Xingheng jointly developed Lithium-ion battery management
system, Chunlan Research Institute developed HEV-BMS system, Beijing Institute
of Technology and Northern Jiaotong University and other major electric vehicle
sub-projects of the National 863 Plan have also developed distinctive battery
management systems. With the launch of the electric vehicle market, many
commercial products have been applied in large quantities.
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