The application scenarios of lithium batteries can be divided into three types: consumption, power and energy storage. Early applications are in consumer products such as mobile phones, laptops, and digital cameras, which currently account for about half of the world's shipments of various lithium batteries. With the increase in global demand for new energy vehicles, the proportion of power lithium batteries has increased year by year, currently accounting for more than 40%, and power batteries will become the main application scenarios of lithium batteries in the future. According to the standard, the capacity of the power battery is less than 80% can no longer be used in new energy vehicles, and the requirements of ordinary energy storage batteries are not so high. Energy storage is one of the important means to solve the intermittent volatility of new energy wind power and photovoltaic power and realize the function of "peak shaving and valley leveling".
At present, there are two kinds of mainstream energy storage lithium batteries: ternary lithium and lithium iron phosphate, and the power density is much higher than that of lead-carbon batteries.
In the energy storage system, lithium batteries, lead-carbon batteries, and lead-acid batteries are all stored electric energy, and there is no essential difference, and the design and selection of battery capacity and charge and discharge current are the same. Compared with lead-acid batteries, lithium battery energy storage is a new battery thing, there is no standard product at present, unlike lead-acid batteries, there are many specifications and models, and general manufacturers are based on the amount of electricity to set specifications. The biggest difference between lithium batteries and lead-acid batteries is that lithium batteries must be equipped with a battery management system.
BMS battery management system
Lithium battery has the advantages of light weight, large energy storage capacity, high power, no pollution, long life, etc., but lithium batteries are very sensitive to overcurrent and overvoltage, and large-capacity batteries are made up of many small-capacity single batteries (such as 18650), which are formed by a large number of series and parallel, and there are many batteries in parallel, which is easy to cause the current of each branch to be unbalanced, so it is necessary to introduce a battery management system to join the control. Lead-acid battery has many advantages, such as good high current characteristics, small self-discharge, stable performance, safe and clean, the current daily maintenance of lead-acid battery, mainly through manual completion, mainly to the battery connection status, terminal voltage, etc. troubleshooting, do not need BMS battery management system.
The battery management system (Battery Management System, BMS) is a device composed of microcomputer technology and detection technology, which is to dynamically monitor the operating status of the battery pack and battery cells, accurately measure the remaining power of the battery, and at the same time protect the battery for charging and discharging, and make the battery work in the best state, so as to extend its service life, reduce the operating cost, and further improve the reliability of the battery pack. The battery management system of electric vehicles should achieve the following functions:
1--- Accurately estimate the state of charge (SOC) of the power battery pack
That is, the remaining power of the battery, to ensure that the SOC is maintained within a reasonable range, to prevent damage to the battery due to overcharge or overdischarge, so as to predict how much energy is left in the energy storage battery of hybrid vehicles or the state of charge of the energy storage battery at any time.
2--- Dynamically monitor the working status of the power battery pack
To ensure the safety of the battery, in the process of battery charging and discharging, the terminal voltage and temperature, charging and discharging current and total voltage of the battery pack of each battery in the electric vehicle battery pack are collected in real time to prevent the battery from overcharging or overdischarging.
3--- Equilibrium between single cells
That is, the single battery is charged equally, so that each cell in the battery pack reaches a balanced and consistent state. Equilibrium technology is a key technology of battery energy management system that is currently being researched and developed in the world.
Selection and design of lithium batteries
The energy storage system includes a bidirectional converter and a battery system, such as a set of 21kW/42kWh energy storage, which means that the power of the bidirectional converter is 21kW and the power stored in the battery system is 42kWh. The lithium battery system includes a battery cell and a BMS battery management system, which is provided by the manufacturer. There are a few key points to keep in mind when designing:
1. The energy storage lithium battery has a BMS system, which needs to communicate with the inverter or the two-way energy storage converter PCS, and the equipment with the function of the lithium battery and the corresponding communication interface function should be selected.
2. Compared with lead-acid batteries, the charging and discharging currents of energy storage lithium batteries are different, so special attention should be paid to the design.
3. At present, there is no unified specification and model of lithium battery, and the specifications of each manufacturer are different, and the BMS communication protocol is also different. It should be customized according to the specific requirements of the project.