The rise of electric vehicle (EV) technologies holds great promise to provide consumers with vehicles that are quieter, less expensive to operate and offer reduced emissions. Combined with sophisticated advanced driver-assistance systems (ADAS) capabilities, next-generation EVs can offer enhanced safety features along with improved comfort and convenience. For all their possibilities, however, acceptance of these advanced vehicles depends fundamentally on their ability to reduce “range anxiety” for consumers who are worried that an EV might suddenly leave them stranded with a depleted battery. Arrayed against this fear, advanced battery management systems (BMSs) built with a flexible printed-circuit assembly (FPCA) provide features able to optimize battery efficiency and maximize range.
Battery management system design challenges
In any battery-powered product, some form of BMS technology is involved in various aspects of battery operation. BMS circuits play a fundamental role in charging batteries, ensuring their operation within safe limits, monitoring “state-of-charge” and more. Designing BMS solutions able to deliver these capabilities reliably is a difficult task in any application area. In the automotive arena, BMS design is particularly challenging because of the diverse technologies employed in advanced vehicles and because of the ubiquitous role that vehicles play in daily life. Unlike consumer products, where cost is typically the overriding issue, automotive products require very aggressive reliability performance because of human safety concerns and associated regulatory requirements.
The technological requirements alone can be daunting. Automotive BMS circuits need to operate with precision in charging high-voltage battery packs, balancing voltages of individual cells or cell strings and monitoring their ongoing performance. Furthermore, they must continue to operate reliably and accurately, while withstanding harsh conditions, including mechanical shock and vibration as well as temperature extremes. Working in coordination with other vehicle safety mechanisms, BMS circuits must help protect the vehicle and its occupants with features designed to identify battery failures, isolate high voltage levels, and safely dump high current loads in the event of vehicular accidents or catastrophic battery failure.
Besides these functional requirements, the limited space available within a battery pack itself adds challenges for the physical design of these systems. Circuitry needs to conform to the volume of space available within the battery-pack package without compromising the mechanical design or operation of the battery pack itself.
Moving to flex circuits
The emergence of advanced flex circuit technology provides an effective solution for this broad range of requirements. Manufactured on various thermoplastic polymer substrates, flex circuits are built with layered Copper interconnects. These individual layers are in turn encapsulated with thermoset adhesives and formed into the precise three-dimensional FPCAs required by the application. Using this approach, an engineer can create an FPCA able to conform exactly to the available volume in a battery pack. In practice, effective application of this approach needs to overcome multiple hurdles. Besides addressing the many functional and mechanical requirements related to their operation, BMS FPCAs must accommodate the battery pack manufacturing process itself: They must remain intact under the stresses encountered during delivery, preparation and final assembly in battery packs.
As a further complication, automotive-specific BMS design is a relatively new field, lacking a broad base of experienced engineers building on knowledge accumulated through many practical applications. Worse, experience in non-automotive BMS functional design and conventional printed-circuit board (PCB) physical design does not translate reliably to automotive BMS FPCA design. Even the most experienced PCB designers can find themselves struggling to deal with the unique 3D shape required for each application. The process of bending, folding and twisting the substrate into the required shape introduces stresses that conventional PCB designers have simply not encountered. Sometimes, designers new to flex circuit design overcompensate by using layers that are thicker than actually required. They also might use Copper that is thick enough for electrical connectivity in conventional PCBs but not thick enough for mechanical stability in FPCAs.
The Molex Advantage
For BMS manufacturers, access to highly experienced design organizations such as Molex Printed Circuit Solutions has become essential for creating effective design solution on tight schedules. In engaging with manufacturers, Molex engineers work closely with customer teams to optimize designs and identify optimal materials and manufacturing methods. For the design of the FPCA itself, Molex engineers are able to accurately account for the effects of shaping circuits into 3D configurations, using detailed simulation analysis to confirm their work.
Experience in flex circuit design becomes even more critical as automotive systems move to more powerful primary drive motors supplied by denser lithium-ion battery packs. Within these packs, flex circuits will need to provide more layers with denser interconnections for managing more battery cells. In this environment, the ability to rely on engineers well versed in the details of flex circuit design for automotive BMS solutions will be essential.
