BMS Design for Energy Storage Nickel Metal Hydride No. 5 battery in DC Power Distribution System

The BMS power supply part integrates DC/DC charging module and power supply module to handle high current charging and discharging, current collection and charging and discharging control of energy storage equipment. The energy storage equipment is directly mounted on the 48V DC bus to provide uninterrupted power when the AC/DC power supply equipment is powered off or the bus voltage fluctuates.

1. Introduction

The development of the energy management system (Battery Management System, BMS) of the DC power distribution system energy storage battery has the following technical difficulties: 1) The actual use environment is often in high altitude areas, and the risk of breakdown of BMS circuit chips and other electronic components in high altitude and low pressure environments cannot be ignored; 2) The use of ternary lithium Nickel Metal Hydride No. 5 battery as energy storage components can effectively reduce the curb weight of energy storage equipment, but due to the high activity of the battery material, it is easy to thermally decompose at high temperature, causing battery fire and explosion to threaten the power system and personal safety; 3) The use of 18650 Nickel Metal Hydride No. 5 battery with safety valves and steel shell jackets can To reduce the risk of battery fire to a certain extent, but after multiple cycles of charge and discharge, the consistency of the battery cells of the battery pack composed of multiple series-parallel monomers gradually deteriorates, or due to inconsistent battery pack PACK processes, the voltage balance of each series battery cell group is unstable, affecting the life of the battery pack, and also bringing safety hazards such as over-voltage charging; 4) The effective charge capacity of the lithium battery pack gradually decreases with the increase of battery charge and discharge times and the extension of the battery pack factory time, and the battery cell material technology is in a rapid development stage, and the replacement speed is fast. After the original battery is inactivated, it is difficult to find a battery cell with the same parameters (same voltage platform, same discharge curve, same capacity, etc.) for replacement.

The above problems put forward the following technical difficulties and requirements for the BMS design of energy storage Nickel Metal Hydride No. 5 battery in DC distribution systems: 1) The voltage resistance of BMS circuits and components needs to be corrected for high altitudes, and low-voltage and low-temperature sensitive components should be avoided or used with caution; 2) Strengthen battery thermal management, reasonably increase the number of temperature sampling sensors in various parts of the battery cells and PACK, effectively collect the temperature status of key positions of the battery cells, and take into account the battery heat dissipation and thermal control under high/low temperature conditions; 3) Introduce the battery cell balancing link, and efficiently design the battery cell balancing system in an active software/passive hardware manner under the premise of safe work; 4) Adopt a flexible design concept, and carry out compatible design for different numbers of battery cells in series, single cell capacity, platform voltage, etc. For different single cell discharge curves, adopt a software customized design method to ensure the versatility of BMS; 5) Reasonably manage the battery charging and discharging status, integrate the charging/discharging circuit from a global perspective, and take into account the actual needs of small current charging/rate discharge and uninterrupted power supply.

According to the above technical requirements, the BMS of energy storage Nickel Metal Hydride No. 5 battery in DC distribution systems is developed, and the NXP/Freescale main control chip and LAPIS communication chip are used as the core hardware architecture.

2. Energy storage battery BMS hardware design

The energy storage battery system is shown in Figure 1. The BMS of the energy storage equipment consists of a strong current part and a weak current part.

2.1 BMS strong current part

The BMS strong current part integrates the DC/DC charging module and the strong current module to handle the high current charging and discharging, current collection, and charging and discharging control of the energy storage equipment. The energy storage equipment is directly mounted on the 48V DC bus to provide uninterrupted power when the AC/DC power supply equipment is powered off or the bus voltage fluctuates.

Figure 1 Circuit principle of energy storage equipment in DC distribution system

Figure 2 Hardware part of BMS of energy storage battery system

The bidirectional DC/DC current limiting charging module is directly mounted on the 48V DC bus, with parameters: DC500W (48V/10.4A), and the charging voltage and current can be dynamically adjusted according to the lithium battery voltage. The DC/DC charging module is a two-stage charging form. When the battery pack voltage is lower than 47.5V, it is charged with a large current. When the battery voltage is higher than 47.5V, it is trickle charged. The current gradually decreases to 0, and the battery terminal voltage gradually increases to 48V.

2.2 BMS weak current part

The weak current part is the core part of the BMS, which is connected to the strong current part and the battery pack and powered by the battery pack. As shown in Figure 2, the BMS weak current part consists of a main control chip module, a battery management chip module, an RS485/RS422 serial communication module, a battery balancing module, and peripheral circuits, etc., which are used to collect the voltage and temperature of the lithium battery pack, balance the battery charging, monitor the working status of the lithium battery in real time, and feedback to the host computer. Considering the wide compatibility requirements of the hardware, the BMS weak current part is designed for universal management of 6~16 strings of 18650 Nickel Metal Hydride No. 5 battery.

As shown in Figure 3, the temperature probes are set in five temperature-sensitive positions in the battery pack, namely the communication port, the charging and discharging port, the center of the box, the chip board of the weak current part, and the control board of the strong current part, so as to fully understand the temperature status of each sensitive position of the battery PACK.

Figure 3 Temperature sensor position

The hardware part of the balancing module adopts the passive balancing mode (lossy balancing), which has a simple circuit and low cost. A balancing module consisting of a shunt resistor and a switch MOS tube is connected in parallel to the battery stack; the software part of the balancing module is controlled by the main control chip and executed by the power management chip. When the highest voltage Vmax and the lowest voltage Vmin of the battery are collected, they meet.

The battery management chip turns on the gate of the MOS tube connected in parallel on the battery stack with the highest voltage, and the current passes through the shunt resistor to play a balancing role. The balancing current should be selected reasonably. If the balancing current is too small, the effect is not obvious. If the balancing current is too large, the energy loss of the system is large, resulting in low balancing efficiency. Here, the design of the balancing current Iblc is 50~100mA; the design of the battery maximum voltage Vmax is 4.3V, the minimum voltage Vmin is 2.8V, and the balancing is turned on during charging. The shunt resistor Rsnt will become the bypass load of the battery, so there is:

Therefore, there is:

According to the component specifications, the shunt resistor is selected as 47Ω.

The serial communication module supports both RS485 and RS422 communication modes, which are used to transmit battery status data to the host computer. For the above two communication methods, MAXIM485/488 chips are selected respectively, and both use half-duplex working mode. The circuit uses NECR2561 optocoupler for photoelectric isolation to enhance the anti-interference ability of the communication module. The communication module and the main control chip system do not share the same ground, which effectively suppresses the generation of high common mode voltage and reduces the chip damage rate, thereby improving the stability of the system.

3. BMS software design

3.1 Software flow

Figure 4 shows the BMS software flow chart. The design purpose of BMS software is to complete the energy storage equipment control system, realize the battery voltage, current, temperature acquisition, battery charge state estimation, single cell voltage balancing, overcharge and over discharge protection, and communication with the host computer. The software design concept of BMS adopts modular and hierarchical design ideas. Modularity is to integrate the same function to be implemented into a code segment for unified management, which is convenient for modification. Multiple software functions are implemented using separate functions, and the program can be divided; in addition, separate functions can be reused to a certain extent. Hierarchy is to divide a work into multiple links during software operation, which are completed by different hardware and software modules, so as to facilitate communication, calling and maintenance between various functions within the program. When running the main program, the BMS system continuously calls each independent sub-function to collaboratively complete the data acquisition and processing.

The BMS software first performs system initialization: the task is to complete the hardware power-on check, the initialization of each functional module and the initialization of the application. The initialization module includes software timer, peripheral communication protocol SPI initialization, memory, A/D module, serial communication SCI, bus clock, indicator light setting and other initialization modules; application initialization includes setting the initial value of each variable, declaration of related variables and arrays, communication protocol, and definition of voltage, current and temperature protection upper and lower limit preset values. Among them, the upper and lower limit protection values, total battery ampere-hours and other preset values can also be set online through BMS serial communication to increase the elasticity and stability of the system. Second, obtain each initial value and make an initial value state judgment, and write it into the register of the communication chip. Then the software enters the acquisition loop, enters the interrupt by calling the subroutine, obtains multiple key information of the battery by querying, operating and interrupting and returning, and performs multiple battery operations. After the interrupt is executed, return to the main function loop. The software completes various system tasks by constantly jumping out and returning to the loop.

Figure 4 Software Flowchart

3.2 Interrupt Response

Serial communication is achieved by responding to serial interrupts. In addition to the serial port, the program also has timing interrupts and current detection interrupts. The interrupt priority of the main chip MC9S08DZ60 of this system is shown in Table 1.

Table 1 Interrupt Priority

The main program uses a timed interrupt processing method, and each interrupt response completes different tasks. After the interrupt response, the program mainly collects voltage, temperature, and current values; determines the lithium battery status; controls the FET to control the battery charge and discharge switch, balances the lithium battery cell voltage, and estimates the remaining battery capacity SOC. The communication between the system and the host computer uses serial port interrupt communication. The host computer is adapted based on the MODBUS protocol. After the chip receives the code and verifies it, it reads the data values in the register and returns it to the host computer for the monitoring personnel to observe the status in real time. The communication protocol supports reading the contents of the chip register, and also supports modifying the preset values initialized in the register, so as to modify the upper and lower limits according to different usage states.

3.3 Battery Control

Energy storage equipment is mounted on the DC bus and needs to be fully controlled to avoid the battery pack from being out of control. Table 2 shows the battery pack control table, which controls the battery pack cut-off voltage, cut-off current, high and low temperature, and battery cell balance. When the control variable exceeds the limit value, it is controlled, and the control is released when the control variable returns to the limit value.

Table 2 Battery pack control table

Among them, voltage control and charging current control are determined by the performance of battery cells; discharge overcurrent control is a control measure to avoid over-discharge of Nickel Metal Hydride No. 5 battery in the case of busbar short circuit; high and low temperature control is a protection value of -10℃~50℃ determined according to the actual use environment of energy storage equipment; battery balancing is an effective means to solve the voltage imbalance of battery cells.

4. Testing and practical application

4.1 Energy storage equipment test

After the BMS circuit board is placed in the aging test box for 48 hours for equipment aging test, it is assembled with the battery pack, box, etc. The energy storage equipment after PACK is completed is shown in the figure. The test is divided into two parts: environmental test and functional test.

According to the actual use environment of energy storage equipment, simulated environmental tests are carried out in combination with technical requirements. The test conditions are:

Table 3 Energy storage equipment test conditions

Table 4 shows the environmental test results of energy storage equipment. It can be seen from Table 4 that the energy storage equipment is in four types of low-frequency random vibration environments of low voltage and low temperature, low voltage and high temperature, normal pressure and low temperature, and normal pressure and high temperature. The charging and discharging state of the energy storage equipment conforms to the software logic, the communication state is normal, and the equipment state can be used normally.

Table 4 Energy storage equipment environmental test results

Note: A low temperature B high temperature M low pressure L normal pressure Fc vibration

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