Large battery arrays are gaining increasing attention as energy storage systems for backup and continuous power supply, as evidenced by Tesla's recently launched Powerwall system for both home and commercial use. In these systems, the batteries are continuously charged by the grid or other energy sources, and then AC power is supplied to the user via a DC/AC inverter.
Using batteries as backup power is not new; many battery backup power systems already exist, such as basic 120/240V AC and hundreds of watts of power short-term backup power systems for desktop PCs, thousands of watts of special vehicle and ship backup power systems for ships, hybrid vehicles, or all-electric vehicles, and grid-scale hundreds of kilowatts of backup power systems for telecommunications systems and data centers (see Figure 1), etc. While advancements in battery chemistry and battery technology have garnered significant attention, another equally crucial element for a viable battery-based backup system is the battery management system (BMS).
Based on battery backup power, it is ideal for stationary and mobile applications ranging from kilowatts to hundreds of kilowatts, providing reliable and efficient power for a variety of uses.
There are many challenges in developing a complete battery management system for energy storage, and the solution is not simply to "extend" the management system of small, low-capacity battery packs. Instead, new, more complex strategies and key support components are needed.
The challenge begins with requiring high accuracy and reliability in the measurement of many critical battery parameters. Furthermore, the subsystem design must be modular to allow for customization to meet specific application needs, while also considering potential expansion requirements, overall management issues, and necessary maintenance.
The operating environment of larger storage arrays presents other significant challenges. With high inverter voltage/current and resulting current spikes, the BMS must provide accurate and consistent data in noisy and often high-temperature electrical environments. Furthermore, the BMS must provide extensive, detailed data on internal module and system temperature measurements, rather than a limited set of rough summaries, as this data is crucial for charging, monitoring, and discharging.
Because of the crucial role these power systems play, their operational reliability is inherently critical. To realize this easily stated goal, the BMS must ensure data accuracy and integrity, as well as continuous health assessments, so that the BMS can consistently take the necessary actions. Achieving robust planning and reliable safety is a multi-stage process; the BMS must anticipate potential problems in all subsystems, perform self-tests and provide fault detection, and then select appropriate actions in standby and operating modes. Finally, due to the high voltage, high current, and high power, the BMS must meet many stringent regulatory standards.
System planning transforms concepts into real-world outcomes.
While monitoring rechargeable batteries may seem simple in concept—simply placing voltage and current measurement circuits at the battery terminals—the reality of a BMS is quite different and much more complex.
Robust planning begins with comprehensive monitoring of each battery cell, which places several important demands on the simulation circuitry. Battery readings need to be accurate to the millivolt and milliampere levels, and voltage and current measurements must be synchronized in real time to calculate power. The BMS must evaluate the validity of each measurement, as it needs to maximize data integrity, while also identifying erroneous or problematic readings. The BMS cannot ignore unusual readings, as these may indicate potential problems, but at the same time, it cannot take action based on erroneous data.
