To borrow a term from the economic circle, electric vehicle BMS products
are currently in a state of "stagflation." Although product functions are
constantly improving and market applications are expanding, the key technical
levels of the products are still stagnant, and the original pain points Still
exists. One of the main reasons for this situation is that our product design
solutions all use special battery management ICs provided by foreign
semiconductor IC manufacturers, and design based on their application solutions.
BMS products can be divided into software Algorithms and hardware architecture
are two relatively independent parts. Because software algorithms are more
professional and sophisticated and have little connection with the hardware
part, this article only analyzes and discusses the connection between hardware
architecture and product functional performance.
Let’s start with the battery management dedicated chip. The emergence and
development of battery management-specific ICs are closely related to various
problems encountered in the application of lithium batteries. The charge and
discharge protection chip for a single battery was first designed to solve the
problem of overcharge and overdischarge of lithium batteries. Later, in the
application of multi-cell lithium batteries in series, chips for multiple
strings were developed. At this time, it became the battery management chip. The
chip mainly collects the voltage data of each battery in the battery pack. In
the future, in order to deal with the problem of battery inconsistency, the
driving function of the power switch will be further integrated. This is the
battery management IC with balancing function.
Objectively speaking, battery management-specific ICs enabled the early BMS
industry and led the development of BMS products. It is precisely because of the
dedicated chip that the design of BMS can be greatly simplified, and the
miniaturization and reliability of the product have been greatly improved.
However, at the same time, we must also see the limitations of the dedicated
chip. As mentioned earlier, battery management dedicated chips also developed
with the application of lithium batteries. Early lithium batteries were mostly
used in small electronic devices, and later became widely used in notebook
computers. So far, battery management dedicated chips have been used for
low-series batteries. Number and small equipment services. When lithium battery
packs are applied to electric vehicles, the situation begins to change. Lithium
battery packs for electric vehicles are high-series, large-capacity batteries
used in series. The number of dozens or even hundreds of series is no longer
comparable to the single-digit series use of several series in laptops.
Special-purpose ICs have not been idle, and have rapidly launched products with
more string applications, but considering the voltage and application
complexity, they generally do not exceed 20 strings. The typical architecture of
a BMS designed using these ICs is a centralized architecture. There are only
connections between the BMS and the battery pack. The number of connections
depends on the number of battery strings. The number of dedicated battery
management chips on the BMS circuit board also depends on the number of battery
strings.
As can be seen from the schematic diagram, the advantages of centralized
BMS products are simple structure and low cost. When the number of battery
strings is low, such as 10 or so, the wiring is not too complicated. Moreover,
when the battery string capacity is small, the BMS can be installed close to the
entire battery string, shortening the wiring distance. The battery string -
---BMS, the entire energy system is relatively compact and integrated, and is
more suitable for electric bicycles and electric motorcycles. However, when
applied to lithium battery packs for electric vehicles, because the battery
capacity is required to be large and the physical size of the group is
relatively large, the wires will be longer and vary in length. In addition,
there are many strings and the number of wires will be large. The arrangement of
dozens or even hundreds of lines is very troublesome. Another important detail
is that the order of these connections needs to be fixed, because the pins of
the dedicated chip have pre-defined the battery series connection sequence, so
the connections on each string of batteries must be connected to the connector
pins designated by the BMS. . Although this is not difficult in the BMS design
work, it is a big trouble in the actual connection work of the BMS and the
battery pack. Generally, one end of the wire is connected to the battery, and
the other end is connected to the BMS through a plug-in. The work of connecting
to the battery is now done manually, and it will be difficult to do it by
machine in the future. The wires connected to each electrode of each battery
string are all connected. No mistakes can be made, the entire workload can be
imagined. Through the analysis of centralized architecture, we see that
dedicated ICs are more suitable for small-capacity, low-string-count situations.
In large-capacity, high-string-count situations, there are disadvantages of
complicated wiring and the need for one-to-one correspondence.
Looking at the issue of balancing again, a centralized architecture is more
suitable for passive balancing without increasing the complexity of the circuit
design. Nowadays, mainstream special-purpose ICs also have this function.
However, the current capacity is limited, at the 100 mA level. When the
consistency difference is not big in the early stage of use of the battery pack,
the problem is not big. When the consistency difference is large in the middle
and later stages, there is a risk that the battery imbalance cannot be
corrected. . If you want to add active equalization function, the existing
architecture is basically of no help. Additional wiring harnesses and switch
matrices are required, and the circuit complexity increases sharply. The switch
matrix requires a large number of electronic switches and MOSFETs. However, due
to the large number, the control circuit is quite complex. Some companies use
relays instead, which simplifies the design, but brings problems with the life
of the relay as a mechanical switch and the risk of malfunction. Of course, you
can also extend the life of the relay by reducing its switching frequency and
avoid risks by checking for malfunctions. However, doing so cannot guarantee the
mean trouble-free working time of the device, not to mention that the number of
relays is quite large, more than one. This is a compromise of last resort rather
than a solution to the active balanced switch matrix conundrum.
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