Lithium-ion batteries can be categorized into three application scenarios: consumer, power, and energy storage. Their earliest applications were in consumer products such as mobile phones, laptops, and digital cameras, currently accounting for about half of global lithium-ion battery shipments. With the increasing global demand for new energy vehicles, the proportion of power lithium-ion batteries has been rising year by year, currently accounting for over 40%, and power batteries will become the main application scenario for lithium-ion batteries in the future. According to standards, power batteries with a capacity below 80% cannot be used in new energy vehicles, while the requirements for ordinary energy storage batteries are not as stringent. Power batteries, after retirement, can be slightly modified and used in energy storage systems. Energy storage lithium-ion batteries are also gradually gaining attention as an emerging application scenario. Energy storage is one of the important means to solve the intermittent fluctuations of new energy wind power and photovoltaics, achieving the function of "peak shaving and valley filling."
Currently, the mainstream energy storage lithium batteries are ternary lithium and lithium iron phosphate, both of which have much higher power density than lead-carbon batteries. Relatively speaking, ternary lithium has a slightly higher power density than lithium iron phosphate.
In energy storage systems, lithium batteries, lead-carbon batteries, and lead-acid batteries all store electrical energy and are not fundamentally different; their battery capacity and charging/discharging current design selections are the same. Compared to lead-acid batteries, lithium battery energy storage is a newer technology, and currently there are no standard products, unlike lead-acid batteries which have many specifications and models. Manufacturers generally define specifications based on capacity. 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-ion batteries have advantages such as light weight, large energy storage capacity, high power, no pollution, and long lifespan. However, they are very sensitive to overcurrent and overvoltage. Large-capacity batteries are composed of many small-capacity individual cells (such as 18650 cells) connected in series and parallel. With many parallel-connected cells, current imbalances in the various branches can easily occur, necessitating the introduction of a battery management system (BMS) for control. Lead-acid batteries possess numerous advantages, such as good high-current characteristics, low self-discharge, stable performance, and safety. Currently, routine maintenance of lead-acid batteries is mainly performed manually, primarily checking the battery connections and terminal voltage for fault diagnosis, without requiring a BMS.
A Battery Management System (BMS) is a device composed of microcomputer technology, detection technology, and other components. It dynamically monitors the operating status of the battery pack and battery cells, accurately measures the remaining battery capacity, provides charge and discharge protection, and ensures the battery operates at its optimal state. This extends battery life, reduces operating costs, and further improves the reliability of the battery pack. An electric vehicle battery management system must achieve the following functions:
1---Accurately estimate the State of Charge (SOC) of the power battery pack.
This refers to the remaining battery capacity, ensuring that the State of Charge (SOC) is maintained within a reasonable range to prevent damage to the battery due to overcharging or over-discharging, thereby providing real-time forecasts of how much energy or state of charge the hybrid vehicle's energy storage battery has remaining.
2---Dynamically monitor the operating status of the power battery pack
To ensure battery safety, during the charging and discharging process, the terminal voltage and temperature of each battery in the electric vehicle battery pack, the charging and discharging current, and the total voltage of the battery pack are collected in real time to prevent overcharging or over-discharging of the battery.
3---Balance between individual cells
This refers to the equalization charging of individual cells, ensuring that all cells in the battery pack reach a balanced and consistent state. Equalization technology is a key technology in battery energy management systems that is currently being researched and developed worldwide.
Lithium-ion battery selection and design
Energy storage systems include bidirectional converters and battery systems. For example, a 21kW/42kWh energy storage system means the bidirectional converter has a power output of 21kW and the battery system stores 42kWh of energy. Lithium-ion battery systems include battery cells and a battery management system (BMS), which are provided by the manufacturer. The following key points should be considered during the design process:
1. Energy storage lithium batteries have a BMS system, which needs to communicate with the inverter or bidirectional energy storage converter PCS. Therefore, it is necessary to select equipment with lithium battery function and corresponding communication interface function.
2. Compared with lead-acid batteries, energy storage lithium batteries have different charging and discharging currents, which should be carefully considered during the design process.
3. There are currently no standardized specifications or models for lithium batteries; each manufacturer has different specifications and BMS communication protocols. Customization is required based on the specific requirements of the project.
