Nowadays, the application scope of battery energy storage systems is increasingly wide, but the market conditions they face are also becoming harsher. The operation and maintenance of battery energy storage systems require the use of battery management systems (BMS), which combine electronic technology and software and serve as the brain of battery energy storage systems. The most basic function of a BMS is to ensure the balance and safety of batteries and to transmit important information to users or connected battery energy storage systems.
Status of Battery Energy Storage Systems
Batteries come in different sizes, which directly relates to their capacity. For example, lithium-ion batteries are usually the smallest in size. The minimum voltage of a lithium-ion battery can be as low as 2.5V (such as for iron phosphate lithium batteries), while the maximum voltage can go up to 4.3V (such as for NMC ternary lithium batteries).
Batteries can be connected in parallel to increase the maximum current of the battery pack. Parallel or series-connected batteries are usually referred to as super batteries. Generally, the voltage of a parallel-connected super battery can self-balance, and further management is unnecessary. Exceptions may include new types of batteries such as lithium-sulfur batteries, or batteries that operate under extreme charge and discharge rate conditions, such as iron phosphate lithium batteries that exhibit a flat state of charge and voltage curve.
Series-connected super batteries will increase the voltage of the battery pack, which is mainly applied in high-power application scenarios. When adding batteries to the battery pack configuration, the energy storage capacity of the battery energy storage system will increase. Therefore, adding parallel-connected battery packs to super batteries will increase the energy storage capacity of the battery pack, just like connecting additional super batteries in series.
Methods of Balancing Battery Energy Storage Systems
Passive balancing dissipates energy (as heat) through resistors to achieve consistent battery voltage after the charging process is complete, which is supposed to enter the fully charged battery. The advantage of this method is low electronic component cost. Its disadvantage is that the current flowing through all batteries is consistent, which means the performance of the worst battery in the serial-connected battery pack will affect the energy, power, life, and safety of the entire battery pack. As the current of the worst-performing battery is relatively high, its degradation rate will accelerate, which may also cause local hotspots and lead to a drop in battery pack capacity, or even generate safety issues. In addition, energy will be wasted during the charging process. A passive BMS can only monitor the battery pack current and interrupt the battery pack current through a circuit breaker in case of failure.
If bidirectional information flow is implemented, system-level parameters (such as operating settings) may be modified to determine the priority of battery energy storage system life or performance. Increasing battery life requires reducing charge and discharge operations at the cost of available capacity or power, while improving performance resulting in a reduction in battery life requires increasing charge and discharge operations.
Active balancing is generally implemented through low-current shunts that provide lower charging current to batteries that have not yet been fully charged, rather than dissipating the energy as heat. The main benefit of this method is that it improves charging efficiency, which may be important if available charging energy needs to be efficiently utilized. However, for most application scenarios, active balancing cannot demonstrate the benefits that increased component cost can bring. Similarly to passive balancing, the higher current of the worst-performing battery accelerates battery degradation and may form hotspots.
Battery Energy Storage System State Estimation
The estimation of charging state and health status is based on the combination of battery models and estimation algorithms. The complexity and accuracy level that state estimation and battery underlying models can achieve largely depend on hardware; here hardware is used to distinguish different methods.
A circuit integrated for state estimation is used for most traditional battery management systems (BMS). Integrated circuits are “hard wired” with specific battery models and estimation algorithms. The advantages of integrated circuits are low cost. The disadvantages are limited system design flexibility and accuracy. These disadvantages tend to get worse over time. The limited design flexibility is due to the fact that integrated circuits are usually created for specific batteries with specific specifications.
Information Flow Design for Battery Energy Storage System
One-way information flow is common in most battery energy storage systems: parameter information flows from the battery management system (BMS) to higher-level systems and user interfaces. If battery manufacturers provide a BMS, less low-level information is available, as this information may be considered sensitive. However, the most important information is related to safety and performance, including charging status and health indicators.
If a microcontroller is used, the BMS can process input parameters, such as changing operational settings (such as maximum and minimum allowable battery voltage or SoC), and even update battery models or estimation algorithms to maintain their accuracy, making bidirectional information flow possible.