Bijoy Mehta

13th January 2020

Selecting a Battery Management System (BMS) for High Voltage Li-Ion Batteries

A Battery Management System (BMS) is the key to safe, reliable and efficient functioning of the lithium-ion batteries. It is an electronic supervisory system that manages the battery pack by measuring and monitoring the cell parameters, estimating the state of the cells and protecting the cells by operating them in the Safe Operating Area (SOA).

A BMS is an essential component of all lithium-ion battery packs. These battery packs can be classified into Low Voltage (LV) or High Voltage (HV). In automotive engineering, “high voltage” is defined to be within a range of 30 – 1000 VAC or 60 – 1500 VDC (UNECE 2013). Voltages under 30 VAC and 60 VDC are defined as “low voltage.” LV 112-1 presents three voltage classes, which are based on ISO 6469-3 class A and B: Low voltage class 1: ≤ 30 VAC and ≤ 60 VDC; High voltage class 2: ≤ 600 VAC and ≤ 900 VDC; High voltage class 3: ≤ 1000 VAC and ≤ 1500 VDC.

LV battery packs are typically used in light electric and hybrid vehicles, two and three wheelers. HV battery packs are typically used in traction applications for electric automotive and stationary applications in Energy Storage Systems (ESS). HV battery packs have a large number of lithium ion cells connected in series and parallel to build up the total voltage and capacity of the pack. For example, a HV battery pack of a hybrid bus rated for 600V, 100kWh built of 18650 NMC cells will have about 160 cells in series and 55 cells in parallel, taking the total cell count to 8800.

Irrespective of the voltage, all lithium ion battery packs require a BMS. Several off-the-shelf BMSs are available in the market and they are of different topologies. Selection of the right type for the battery pack is important. These BMSs can be fundamentally classified into – Centralized and Decentralized.

Centralized BMS Architecture

Centralized BMS Architecture

centralized BMS is one central pack controller that monitors, balances, and controls all the cells. The entire unit is housed in a single assembly, from which, the wire harness (N + 1 wires for N cells in series and temperature sense wires ) goes to the cells of the battery. These wires are used for cell voltage, temperature measurements and balancing.

The board is commonly powered from the battery output and does not require an external power supply. It consists of multiple Analog to Digital Converters (ADC) channels as part of the cell monitoring circuitry. The voltage on each cell is referenced to the BMS ground and this voltage grows with the number of cells and provides high voltage at the ADC channels that are measuring the top most cells in the stack. The cell monitoring circuitry is also coupled with an intelligence circuitry. The intelligence circuitry is responsible for internal communication with the cell monitoring circuitry for data acquisition, computing the battery’s State of Charge (SoC) and State of Health (SoH), controlling the Power Distribution Unit (PDU) and for external communication.

Decentralized BMS Architecture

Centralized BMS Architecture

A decentralized BMS, fundamentally, does not have the entire cell monitoring and intelligence circuitry on a single assembly. This architecture can be implemented through various topologies as explained below:

Modular: The BMS is divided into multiple, identical modules, each with its bundle of wires going to one of the batteries in the pack. Typically, one of the modules is designated as a master, as it is the one that manages the entire pack and communicates with the rest of the system, while the other modules act as simple remote measuring devices. Readings from the other modules to the master module are transferred via a communication link.

Master-Slave: This architecture comprises of the Master and Slave BMS units. The slave unit monitors, balances and controls a group of battery cells within the battery module. It communicates with the master unit through a communication interface. The Master unit is responsible for state estimation, control of Power Distribution Unit (PDU) and external communication. A master-slave BMS is similar to a modular system, in the sense that it uses multiple identical modules (the slaves), each measuring the voltage of a few cells. However, the master is different from the modules and does not measure voltages. It only handles computation and communications.

Distributed: A distributed BMS is significantly different from the other topologies. While the electronics are grouped and housed separately from the cells in other topologies, a distributed BMS has the electronics contained on cell boards that are placed directly on the cells being measured. Instead of many tap wires between cells and electronics, a distributed BMS uses just a few communication wires between the cell boards and a BMS controller, which handles computation and communications.

Importance of Decentralized BMS Architecture for HV Battery Systems

HV battery systems consist of a large number of cells. This implies that there are also a large number of wires originating from these cells to the BMS. This makes the assembly, management, and maintenance of these HV battery packs more complex. Decentralized BMS architecture offers the following advantages in this context:

Measurement Precision – The quality of measurement of fundamental parameters such as voltage, current and temperature is critical to the functioning of the BMS. These measurements are made over the cell monitoring wires and are prone to perturbations from disturbances such as noise. The short wires have high electromagnetic compatibility which decreases the noise susceptance.

Connection Reliability – It enables the cell monitoring circuitry to be placed in close proximity to the cells. As a result, the wires from the cells to the BMS are of short length. Short wires are less prone to be affected by mechanical shocks and vibrations that can cause a disconnection. Hence, having short wires ensures better connection reliability.

Expansion Versatility – Typically, the number of cell monitoring inputs in a centralized BMS is fixed. But a decentralized BMS allows for multiple cell monitoring units to be stacked. Therefore a decentralized BMS is more versatile in the sense that it can be used even if the number of cells in the pack is increased or decreased, just by changing the number of cell monitoring units.

ION’s FS-XT BMS

FS-XT is a decentralized BMS designed for high voltage applications. It has a Master-Slave topology, with Battery Monitoring Unit (BMU) as the BMS slave and Slave Monitoring Unit (SMU) as the BMS master. The BMUs consist of cell voltage, temperature measurement, and balancing channels. The SMU communicates with BMUs to collect measurements and to send control commands over isolated SPI channels. SMU uses those measurements to control cell balancing, perform state estimation calculations and control the PDU to ensure battery operation within the safe operating area defined through the BMS configuration. Additionally, the SMU is also equipped with HV battery management features such as interlock check, Insulation Monitoring Device (IMD) interface, weld detection, etc.

Conclusion

BMSs are extremely vital in ensuring the safety of battery packs. With the increased adoption of Lithium-ion battery technology in automobiles and energy storage, the design and integration of a good BMS for these high voltage batteries becomes paramount. Decentralized BMS architecture is especially suited for these high voltage battery packs.

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