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Boost cell monitoring accuracy in energy storage BMS

Posted: 27 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:battery management systems? energy storage? Battery Monitor IC? cyclic redundancy check?

The use of large-scale battery arrays for backup and carry-through energy storage is gaining increasing attention, as evidenced by Tesla Motors' recent announcement of their Powerwall system for homes and offices. The batteries in these systems are continually charged from the power-line grid or other source, and then deliver AC-line power back to the user via a DC/AC inverter.

Using batteries for power backup is not new, with many systems spanning basic 120/240Vac and several hundred watts for short-term desktop-PC backup, to thousands of watts for speciality vehicles, such as ships, hybrid cars, or all-electric vehicles, up to hundreds of kilowatts for grid-scale telecom and data-centre backup. Yet while advances in battery chemistry and technology get much of the attention, an equally critical part of a viable battery-based installation is its battery management system (BMS).

There are many challenges when implementing battery management systems for energy storage, and their solutions do not simply "scale up" from small-scale, lower-capacity battery packs. Instead, new and more sophisticated strategies and critical support components are needed.

The challenge begins with the need for high accuracy and confidence in the many measurements of key battery cell parameters. Further, the design must be modular in its sub-systems to enable tailoring the configuration to the specific needs of the application, along with possible expansion, overall management issues, and necessary maintenance.

The operating environment of larger-scale storage arrays brings other significant challenges, as well. The BMS must provide precise, consistent data within an extremely noisy electrical and often hot environment despite the high voltage/current inverters and resultant current spikes. In addition, it must provide extensive "fine-grain" data on internal module and system temperature measurements, which are critical for charging, monitoring, and discharging, rather than just a few broad-brush aggregate values.

Due to the basic role of these power systems, their operating reliability is inherently critical. To translate that easily stated objective into reality, the BMS must ensure data accuracy and integrity, along with continuous health assessments so it can take the needed actions on an ongoing basis. Achieving a robust design and safety is a multi-level process, and the BMS must anticipate problems, perform self-test, and provide failure detection on all sub-systems, then implement appropriate actions while in standby and operational modes. As a final mandate, due to the high voltage, current, and power levels, there are many stringent regulatory standards that the BMS must meet.

System design translates concepts to the real-world results
Although monitoring rechargeable batteries is simple in concept C just place the voltage- and current-measurement circuits at the cell terminals C the reality of a BMS is quite different and much more complicated.

Robust design begins with comprehensive monitoring of individual battery cells, which places significant demands on analogue functions. The cell readings need millivolt and milliamp accuracy, and voltage and current measurements must be time-synchronised to calculate power. The BMS must also assess validity of each measurement, as it needs to maximise data integrity, while it must also identify errors or questionable readings. It cannot ignore unusual readings which may indicate a potential problem, but at the same time, it should not take action based on data which has errors.

A modular BMS architecture enhances robustness, scalability, and reliability. Modularity also facilitates use of isolation where needed in the data links between subsections to minimise impact of electrical noise and to enhance safety. In addition, advanced data-encoding formats including CRC (cyclic redundancy check) error detection and link-acknowledgement protocols assure data integrity, so the system management function has confidence that the data it receives is what was sent.

An example of a BMS that incorporates these principles is the scalable and customisable battery management system developed by Nuvation Engineering (Waterloo, Ontario, Canada and Sunnyvale, California).

Figure 1: Nuvation Engineering battery management system is the interface between the AC power grid and an array of battery cells; it provides both sophisticated battery charging/discharge oversight as well as the DC/AC inverter function.

Figure 2: The three major sub-systems of the Nuvation BMScell interface, stack controller, power interfacecomprise a modular, hierarchical design which results in scalability, robustness, and reliability across a wide range of power levels.

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