Hi-Rel Li-Ion Battery Management System for Electric Vehicles
As the chemistry for rechargeable batteries has progressed, from lead-acid (PbA), to nickel metal hydride (NiMH), and most recently to lithium-ion (Li-ion), the complexity of the Battery Management System (BMS) has grown dramatically. While the safety and reliability of small batteries are dominated by chemistry, large-format batteries for plug-in hybrid electric vehicles (PHEV), hybrid electric vehicles (HEV), and electric vehicles (EV) are inherently dependent upon the BMS to control and balance a large array of cells.
Designing a system that keeps cells within a safe operating envelope and shuts down the battery pack in the event of a BMS or cell failure is a challenging task in and of itself. However, keeping the pack operating safely and reliably in spite of detected BMS failures is a more complex problem—one that must be addressed before large-format Li-ion batteries can be used successfully and economically in electric vehicles.
Led by Larry Yount, President and CEO of LaunchPoint Energy and Power (LEAP) LLC, a spin-off of LaunchPoint Technologies, engineers are developing a High-Reliability (Hi-Rel) BMS for use in EVs. This Hi-Rel system is based on engineering techniques originally developed for the Fly-By-Wire (FBW) electronic systems used in the aerospace industry, where reliability is of the utmost importance (Figure 1).
Figure 1. Fly-by-wire 787 cockpit with hi-rel electronics.
The means by which safety and reliability are attained include designs with features such as detection and isolation of faults, reconfiguration via redundancy management, and integrated design of the combined hardware/software system to accomplish these functions. Moreover, the design processes include fault hazard analyses, failure mode and effect analysis, fault tree analysis, and common mode fault analysis.
With a Hi-Rel BMS design, the Mean Time to Failure (MTTF) can be increased by a factor of 10 to 100 over conventional BMS architectures, depending on the level of sophistication applied (Figure 2 below).
Recommended Requirements for BMS Safety and Reliability
In developing any engineered system, market and technical requirements are the starting points for a coherent design process. For a vehicle application, we suggest the following specifications for the BMS:
- Support a critical-fault rate of <10-8 hr-1 in compliance with ISO 26262 ASIL-D;
- Maintain >96% probability of successful operation over a 10-year battery life, assuming normal use and maintenance; and
- Monitor for faults in all electronics, wiring, and sensors internal to the pack and reconfigure the system when a fault is detected such that the required functionality and level-of-safety is retained.
Figure 2. Conventional vs Hi-Rel BMS Mean-Time-to-Failure (MTTF).