Lithium battery application scenarios can be divided into consumption, power and energy storage. The earliest applications were in consumer products such as mobile phones, notebook computers, and digital cameras. At present, it accounts for about half of the global lithium battery shipments. With the increase in the demand for new energy vehicles worldwide, the proportion of power lithium batteries has increased year by year, currently accounting for more than 40%. The future of power batteries will become the main application scenario of lithium batteries. According to the standard, the capacity of the power battery is less than 80% and can no longer be applied to new energy vehicles. The requirements for ordinary energy storage batteries are not so high. After the power battery is decommissioned, it can be modified slightly and can also be used in the energy storage system. As an emerging application scenario, energy storage lithium batteries are gradually gaining importance. Energy storage is one of the important means to solve the problem of new energy wind power and photovoltaic intermittent volatility and realize the function of “cutting peaks and flats”.
At present, the mainstream energy storage lithium battery has two kinds of ternary lithium and lithium iron phosphate, and the power density is much higher than that of the lead carbon battery. Relatively speaking, the ternary lithium is higher than the li-fe phosphate.
In the energy storage system, lithium batteries, lead carbon batteries, and lead-acid batteries are all storing electrical energy. There is no essential difference, and the battery capacity and charge and discharge current are designed and selected. Compared with lead-acid batteries, lithium-ion battery storage is a new thing. There are no standard products at present. Unlike lead-acid batteries, there are many types of specifications. Generally, manufacturers are regulated according to the amount of electricity. The biggest difference between a lithium battery and a lead acid battery is that the lithium battery must be equipped with a battery management system.
BMS Battery Management System
Lithium batteries have the advantages of light weight, large energy storage capacity, high power, no pollution, long life, etc. However, lithium batteries are sensitive to overcurrent and overvoltage, and large-capacity batteries are made up of many small-capacity single cells (such as 18650), through a large number of series and parallel connection, there are more batteries in parallel, which is easy to cause the current of each branch is not balanced, so it is necessary to introduce the battery management system to join the control. Lead-acid batteries have many advantages, such as high current characteristics, self-discharge, stable performance, safety and cleanness. At present, the routine maintenance of lead-acid batteries is mainly done manually, mainly for the connection status of the battery and the terminal voltage. Troubleshooting, no BMS battery management system is required.
The Battery Management System (BMS) is a device consisting of microcomputer technology and detection technology. It dynamically monitors the operating status of the battery pack and battery unit, accurately measures the remaining battery capacity, and simultaneously protects the battery from charge and discharge. The battery is operated in an optimal state to extend its service life and reduce operating costs, thereby further improving the reliability of the battery pack. The electric vehicle battery management system must implement the following functions:
--- Accurately estimate the state of charge (SOC) of the power battery pack
That is, the remaining battery capacity ensures that the SOC is maintained within a reasonable range to prevent damage to the battery due to overcharging or overdischarging, thereby predicting at any time how much energy is left in the hybrid vehicle energy storage battery or the state of charge of the energy storage battery.
--- Dynamically monitor the working status of the power battery pack
To ensure the safety of the battery, during the charging and discharging process of the battery, the terminal voltage and temperature, the charging and discharging current and the total voltage of the battery pack of each battery in the battery pack of the electric vehicle are collected in real time to prevent overcharging or overdischarging of the battery.
---Equilibrium between single cells
That is, the single battery is uniformly charged, so that each battery in the battery pack reaches a state of equalization and consistency. Equilibrium technology is the key technology of a battery energy management system that the world is currently researching and developing.
Lithium battery selection and design
The energy storage system includes a bidirectional converter and a battery system, such as a set of 21 kW / 42 kWh energy storage, indicating that the power of the bidirectional converter is 21 kW, and the battery system stores 42 kWh. The lithium battery system includes a battery cell and a BMS battery management system, which are provided by the manufacturer. Pay attention to the following points when designing:
1 The energy storage lithium battery has a BMS system, and needs to communicate with the inverter or the bidirectional energy storage converter PCS. It is necessary to select a device with a lithium battery function and a corresponding communication interface function. 2 Compared with lead-acid batteries, the energy storage lithium battery has different charging and discharging currents, so pay special attention to the design. 3 lithium batteries currently do not have a uniform specification model, each manufacturer's specifications are not the same, BMS communication protocol is not the same. To be customized according to the specific requirements of the project.