1 Introduction
The main problem in the current development of power lithium battery power
systems is that the safety and service life of power batteries are significantly
reduced after they are grouped, and serious accidents such as battery combustion
and explosions even occur frequently. The main reason for the above problems is
the lag in research on power lithium battery group technology, group application
technology and equipment; realizing power lithium battery system integration and
providing users with power lithium battery system integration technology and
products is to solve the current problems and promote An important issue in the
development of power lithium batteries.
At present, single power lithium batteries have basically met the
conditions for promotion and application, but the existing group technology,
group application technology and equipment level are not yet mature. In other
words, single power lithium batteries basically solve the problems of safety and
lifespan. The key to promoting the development of power lithium batteries is
battery group technology and BMS technology.
The quality of single cells determines the quality of the power battery
pack after being assembled. In the application of power battery packs, the
performance consistency of single cells is particularly important. Based on the
current level of single cell production process control, it is currently a
realistic method to screen, group, and assemble the produced batteries according
to their actual performance. Currently, the screening and grouping methods used
in industrial production are based on a few static parameters of the battery
after formation, and the battery is grouped. The grouping parameter control
standard determines that the status of each single cell in the battery group is
consistent when the grouping is completed. sex. During the use of the battery
pack, the differences in the status of each single cell in the battery pack will
gradually appear. The battery management system (BMS) manages the use process of
the battery pack and adjusts the status of each single cell in the battery pack.
It can maintain the consistency of the status of each single cell in the battery
pack to a certain extent and avoid battery status differences. Causes
accelerated degradation of battery pack performance. The development of power
lithium batteries requires the improvement of battery production process control
levels, scientific battery screening and grouping methods, and effective battery
management systems.
2 Power lithium battery pack safety and life concept
2.1 The root cause of battery unsafety
When a certain part of the battery deviates, such as internal short
circuit, large current discharge, and overcharge, a large amount of heat will be
generated, causing the temperature of the battery system to increase. When the
battery system reaches a higher temperature, it will cause a series of side
reactions, causing thermal damage to the battery. Since the liquid electrolyte
in the battery is flammable, in the worst case, the battery will catch fire
[1].
In the case of abnormal use (such as overcharging), lithium ions are likely
to obtain electrons on the surface of the negative electrode of the battery,
precipitate as metallic lithium, and gradually gather into lithium dendrites,
puncture the separator, and connect the positive and negative electrodes, thus
causing battery failure. internal micro short circuit. Lithium-ion batteries use
high-pressure, high-boiling-point organic electrolytes. Under abnormal use
conditions such as short circuit, overcharge, and high temperature, the
temperature inside the lithium-ion battery will rise, and the electrolyte may
decompose into a large amount of gas, which will generate a relatively large air
pressure in the sealed case. Once this air pressure is large enough to break
through the battery shell, it will rupture and even cause combustion or
explosion[2]
Large-capacity lithium power battery packs with high power charging and
discharging are more likely to induce deviations in certain parts of the battery
under harsh usage conditions, thus causing safety issues.
2.2 The important role of battery management system (BMS)
The ideal lithium-powered battery has a high degree of performance
consistency. As long as it is not overcharged or discharged during use, there
will be no safety issues. However, the actual situation is that the quality of
mobile phone batteries fully meets the standards when they leave the factory,
but explosions occur during use. There are also constant reports of fires,
explosions and product recalls of battery packs used in laptop computers. There
are safety issues in batteries from the time they are produced until they are
recycled at the end of their life. Even if they fully comply with international
safety standards when they leave the factory, they will evolve from a safe state
to an unsafe state during use. The performance of a good-quality single battery
will gradually decline and reach the limit of its life. , The performance of
single batteries with poor quality will gradually experience various abnormal
degradations and declines during use, and enter the feared unsafe state. This is
a gradual and cumulative process of change. The battery management system is not
dispensable as some battery manufacturers say, but a must, and it must be
present throughout the battery pack application. There have been many studies on
battery management systems at home and abroad, and they have begun to be applied
in many fields. They have been able to meet the needs of some markets. The
application of lithium power batteries has put forward higher requirements for
battery management systems to ensure the safety of power battery packs. It is
the primary task of the lithium power battery management system.
3Battery management system
A typical battery pack charge balance management system works by charging
the connected series battery pack and at the same time detecting the terminal
voltage of each single cell in the battery pack. When the terminal voltage of a
certain single cell is higher, The battery is charged and shunted through a
balancing circuit connected in parallel with the single battery, reducing the
charging speed and terminal voltage rise speed of the single battery, so that
the terminal voltages of each single battery tend to be balanced and consistent,
thereby avoiding the risk of battery packs being connected in series. The
difference in charge and discharge characteristics of each single cell affects
the life of the battery pack. Since the balancing circuit connected in parallel
with the single battery consumes part of the charging power during charging
shunting, the balancing circuit generates heat and the balancing capability of
the balancing circuit is limited. For large-capacity, high-power batteries, when
charging with high current, the balancing capability of the balancing circuit
cannot meet actual needs.
At the end of the battery life, the internal performance of the battery
gradually deteriorates. When discharging or charging according to the original
discharge current limit value and constant current charging value,
over-discharge and over-charge may occur, resulting in further deterioration of
battery internal resistance and other parameters. Charging The battery will be
seriously heated during the discharge and discharge process. Although the
existing BMS can detect and cut off the external charging or discharging path
when the battery temperature rises abnormally, the battery itself is already in
an unsafe state at this time, and cutting off the path cannot prevent the
abnormal electrochemistry inside the battery. During the reaction process, the
risk of the battery continuing to heat up or even explode still exists. When the
battery temperature rises abnormally, the discharge path is suddenly cut off,
which may cause safety problems in the operation of equipment that require
continuous power supply, such as electric vehicles.
4Battery management system based on internal resistance measurement and
status comparison
Research shows that as the number of battery charge-discharge cycles
increases, the battery capacity decreases, the internal resistance of the
battery increases, and the internal performance of the battery deteriorates
until it eventually fails completely. The process is a gradual accumulation
process. Inconsistencies in the battery pack will accelerate the battery pack.
Enter an unsafe state.
This article proposes a battery management system for internal resistance
measurement and status comparison, as shown in Figure 1. It uses a single-chip
microcomputer as the control core and pre-stores the initial internal resistance
value and error allowable value of each single battery under different terminal
voltages in the memory. During the charging process, the terminal voltage rise
rate and error allowable value of each single battery in different terminal
voltage sections, the predetermined value of the cumulative charge amount, the
charging voltage range, the discharge voltage range, the charging cut-off
voltage value, the discharge cut-off voltage value, and the battery temperature
range , charging current value and charging current error allowable value,
discharge current protection value, balanced starting voltage value, terminal
voltage imbalance value and other parameters. Detect various parameters of the
charging and discharging process during the charging and discharging process.
Measure and record the terminal voltage, terminal voltage rise rate, and
charging current of each single cell in the battery pack before, during, and
after each charging process. , charging capacity and battery internal
resistance, and compare it with the parameters measured and recorded during
previous charging, analyze the charging capacity of the battery pack and the
changes in the internal resistance, terminal voltage, and terminal voltage rise
rate of each single cell, and determine the internal resistance of the battery.
The process and degree of gradual performance degradation provide early warning
of battery failure and possible safety issues. Control the charging and
discharging process to achieve charging and discharging termination voltage
protection, temperature protection, and overcurrent protection. During the
charging process, the series connection is achieved by balancing the current of
single cells with lower terminal voltages. The terminal voltage of each single
cell in the battery pack is consistent to avoid the impact on the life of the
battery pack due to differences in the status of each single cell.
The system was used to test 10-cell 20Ah series battery packs, using 5A
constant current charging and 2A supplementary current balancing. The system's
supplementary current balancing effect is very obvious, there is no extra heat,
and it is easy to control energy saving.
There are many factors that affect the change of internal resistance,
especially the study of the change of internal resistance during the complete
life cycle of the battery pack, which requires a lot of experiments. The
relevant experimental plan is in progress, including the entire discharge of the
power battery pack under simulated working conditions. Life process testing.
5 Conclusion
Detect and record battery charging capacity, terminal voltage, current,
internal resistance, battery temperature, charging time and other parameters
during the charging process before, during and after each charge, calculate the
change amount and change rate of each parameter, and Compare with the preset
initial parameters to accurately determine the degree of change in battery
charging performance and provide early warning for possible battery safety
issues, providing high safety guarantee for the use of rechargeable batteries.
Taking electric vehicles as an example, even if a battery safety warning occurs,
the driver has at least a few days to repair and deal with it without having to
forcefully disconnect it immediately.
During the charging process, the single battery with a lower terminal
voltage is charged with supplementary current and equalization, which not only
reduces the status difference of each single battery and improves the service
life of the battery pack, but also avoids the heating problem of the balancing
circuit during the charging and equalizing process. , thus greatly improving the
balancing ability, which has a significant effect on the application of
large-capacity and high-power battery packs.
During the charging process, the battery charging capacity, terminal
voltage, current, internal resistance, battery temperature, charging time and
other parameters are comprehensively recorded during each charging process,
thereby recording the change process of various electrical parameters throughout
the life cycle of the rechargeable battery, in order to conduct battery
research. Group failure cause analysis provides the data basis. Thanks to all
the researchers in the research team for their joint efforts to complete the
above work, and also to battery companies such as CITIC Guoan for providing new
power battery samples.
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