Electric vehicles are vehicles that are powered in whole or in part by
electric motors. At present, there are three main types of pure electric
vehicles, hybrid electric vehicles and fuel cell vehicles. The commonly used
power sources for electric vehicles currently come from lead-acid batteries,
lithium batteries, nickel-metal hydride batteries, etc. Lithium batteries have
high cell voltage, high specific energy and high energy density, and are
currently the batteries with the highest specific energy.
However, precisely because of the relatively high energy density of lithium
batteries, safety accidents will occur when misuse or abuse occurs. The battery
management system can solve this problem. When the battery is under charging
overvoltage or discharging undervoltage, the management system can automatically
cut off the charging and discharging circuit, and its power balancing function
can ensure that the voltage difference of a single battery is maintained within
a small range.
1Battery management system hardware consists of system-specific hardware
circuits, which can be divided into MCU modules, detection modules, and
balancing modules. 1.1MCU module MCU is the core of system control. The MCU used
in this article is the GZ16 model of the M68HC08 series. All MCUs in this series
use the enhanced M68HC08 central processing unit (CP08). This microcontroller
has the following characteristics: (1) 8MHz internal bus frequency;
(2) 16KB built-in FLASH memory; (3) 2 16-bit timer interface modules;
(4) Clock generator that supports 1MHz~8MHz crystal oscillator;
(5) Enhanced serial communication interface (ESCI) module. 1.2 Detection
module In the detection module, the voltage detection, current detection and
temperature detection modules will be introduced respectively. 1.2.1 Voltage
detection module In this system, the microcontroller will detect the overall
voltage and single cell voltage of the battery pack. There are two methods for
detecting the overall voltage of the battery pack: (1) using a dedicated voltage
detection module, such as a Hall voltage sensor;
(2) Use precision resistors to construct a resistor voltage dividing
circuit. Using a dedicated voltage detection module is more expensive, requires
a specific power supply, and the process is complicated. Therefore, a voltage
dividing circuit is used for detection. The voltage variation range of a 10-cell
lithium manganate battery pack is 28V ~ 42V. Use 3.9M? and 300k? resistors to
divide the voltage. The variation range of the collected voltage signal is 2V ~
3V, and the corresponding AD conversion results are 409 and *. For the detection
of single cells, flying capacitor technology is mainly used.
The current sampling circuit in this system is shown in Figure 2. When the
battery is discharged, a constantan wire is used to detect the current signal,
and the detected voltage signal is amplified by a differential mode amplifier
and converted into a voltage signal of 0~5V and sent to the microcontroller. If
the discharge current is too large and the voltage signal detected by the
microcontroller is relatively large, it will drive the transistor to change the
gate voltage of the MOS tube and turn off the discharge circuit. For example,
for a 36V lithium manganate battery, the protection current is set to 60A.
The resistance of constantan wire is about 5mΩ. When the current reaches
60A, the voltage of the constantan wire reaches about 300mV. In order to improve
the accuracy, the voltage is amplified 10 times through the amplifier and sent
to the microcontroller for detection. 1.2.3 Temperature detection During the
charging and discharging process of the battery pack, part of the energy is
released in the form of heat. If this part of the heat is not removed in time,
it will cause the battery pack to overheat. If the temperature of a single
nickel-metal hydride battery exceeds 55°C, the battery characteristics will
deteriorate, and the charge and discharge balance of the battery pack will be
disrupted, resulting in permanent damage or explosion of the battery pack. In
order to prevent the above situation from happening, the battery pack
temperature needs to be monitored in real time and heat dissipated. A thermistor
is used as a temperature sensor for temperature sampling.
Thermistor is a heat-sensitive semiconductor resistor whose resistance
value decreases as the temperature increases. The resistance temperature
characteristics can be approximately expressed by the following formula: 1.3
Balancing module Commonly used balancing methods in battery packs include shunt
method, flying capacitor balancing charging method, inductor energy transfer
method, etc. In this system, more I/O ports are needed to drive the switch
tubes, and the I/O ports of the microcontroller are limited, so the charging
balancing method of converting full charge to single charge is adopted. The
schematic is shown in Figure 3. Q4 is the switch that controls the overall
charging of the battery pack, and Q2, Q3, and Q5 are the switches that control
the charging of a single battery.
Taking a 10-cell lithium manganate battery pack as an example, the voltage
at both ends of the main coil of the transformer is 42V, and the voltage of the
secondary coil is the battery's rated voltage of 4.2V. At the beginning, Q4 is
turned on, Q2, Q3, and Q5 are turned off. The voltage of a single battery
continues to rise. When it is detected that the voltage of a certain battery
reaches the rated voltage 4.2V, the voltage detection chip sends a driving
signal, turns off Q4, and turns on Q2. , Q3, Q5, the entire system enters the
single charging stage, and the battery that is not fully charged continues to be
charged, so that the battery that reaches the rated voltage keeps the rated
voltage unchanged. After testing, the voltage difference will not exceed 50mV.
2SOC power detection In lithium-ion battery management systems, commonly used
SOC calculation methods include open circuit voltage method, Coulomb calculation
method, impedance measurement method, and comprehensive table lookup method
[3].
(1) The open circuit voltage method is the simplest measurement method,
which mainly determines the size of the SOC based on the size of the battery's
open circuit voltage. It can be seen from the operating characteristics of the
battery that there is a certain corresponding relationship between the open
circuit voltage of the battery and the remaining capacity of the battery. (2)
The Coulomb calculation method measures the charging and discharging current of
the battery, integrates the product of the current value and the time value, and
then calculates the amount of electricity charged and discharged by the battery,
and uses this to estimate the SOC value. (3) The impedance measurement method
uses a certain linear relationship between the internal resistance of the
battery and the state of charge SOC, and calculates the internal resistance of
the battery by measuring the voltage and current parameters of the battery,
thereby obtaining an estimate of the SOC. (4) In the comprehensive look-up table
method, the remaining capacity SOC of the battery is closely related to the
voltage, current, temperature and other parameters of the battery. By setting up
a related table and inputting parameters such as voltage, current, temperature,
etc., the remaining capacity value of the battery can be queried.
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