The shift to sustainable energy systems has made electric vehicles (EVs) and renewable energy systems technological drivers of innovation. At the heart of the performance, safety and lifespan of these systems is the battery pack, a complex assembly with its full potential only demonstrated through a complete Battery Management System (BMS). Yet in it's entirety, the design of the BMS itself, and most importantly its PCB, has a huge effect on the overall robustness of the system. An ill-conceived PCB is likely to cause disastrous failures such as thermal runaway, unreliable state-of-charge, and electromagnetic interference, ruining safety and efficiency. This post explores some of the core tenets of BMS PCB design and what engineers must take into account in order to operate reliably within the electric mobility and grid-scale energy storage harsh environments.
Thermal Management and High-Current Handling
Battery Management Systems (BMSs) are responsible for controlling energy ingress and egress to and from the battery pack with high currents that can create a lot of heat. Thus, proper thermal management does become a design necessity in a good design. The PCB layout also needs to be optimized to keep resistive losses — another direct source of heat — to a minimum. For high-current paths, this means designing wide, thick copper traces — typically 2-ounce or heavier copper weights. In addition, the location of thermal vias underneath power components (for example, MOSFETs for cell balancing or contactors) is critical. These vias serve as heat pipes, moving heat from the component side of the board to large copper pours on the other side, which can in turn be attached to a heat sink outside of the system or the chassis of the system itself.
In addition to the PCB itself, you also need to ensure that all passive components are rated for high temperatures. Automotive-grade parts that are qualified for 125°C or 150°C junction temperature operation are the norm. It depends on the PCB material used as well since standard FR-4 is sufficient for lower-power applications, but high-temperature laminates like polyimide or ceramic-filled substrates provide better performance under extreme thermal stress. Good designs will not only have them on the cell but also far away on the PCB and will monitor the conditions of the BMS electronics and enable safe mode if overheated.
Quality of Signals and Electromagnetic Compatibility (EMC)
The BMS PCB provides a mixed-signal environment where sensitive analog measurements of cell voltages and even temperatures have to coexist with noisy digital processors and communication interfaces. For precise battery monitoring, the signal integrity is critical. This has a need to the careful partitioning of analog and digital ground planes to minimize corrupting of voltage readings by noise. Analog sense lines need to route from the battery cells to the monitoring IC with the shortest and most direct route, often diffential paired to reject common-mode noise, shielded from noisy digital signals.
At the same time, it should not affect other electronic systems (or be affected by them), so there are very stringent Electromagnetic Compatibility (EMC) standards that the BMS needs to comply with, particularly in the automotive industry. Good PCB design defines things like grounding schemes, decoupling capacitor placements near IC power pins to cut high-frequency noise, and filtering on all I/O lines. In the case of certain communication interfaces such as the CAN bus that are essential for the operation of the system, impedance-controlled routing and common-mode chokes are required to protect the data in an electrically noisy environment. The fundamental building block of good EMC performance is also a well designed multilayer stack-up with dedicated power and ground planes.
Robustness Against Environmental Stressors
Beyond temperatures, the other severely-harsh condition for all vehicles is obviously the EVs and renewable energy installations BMS units are under for arranged long-term life cycles of 5-10 Years or longer, Reliability is seriously threatened by vibration, shock, humidity, and the risk of contamination (e.g., dust or condensation). In order to reduce mechanical stress, large and heavy parts such as a large capacitor or inductor should not be placed in vibrating and high vibration area in PCB layout. Rather, these components should be situated close to board supports. Conformal coating is also an important finishing process, coating the entire assembled PCB with a thin polymeric film to protect against moisture, dust, and chemical contaminants that could lead to short circuits and corrosion.
In addition, it should have a design that is applicable to high-voltage safety. In hundreds of volt systems, creepage and clearance distance (the distance between the conductors along the surface and through air) must be calculated and followed in the PCB layout. This typically necessitates the inclusion of slots or grooves into the board to provide a larger surface separation between high voltage traces. Higher than a certain level of the comparative tracking index (CTI) in the used material increases the resistance to electrical breakdown. Part of these design choices are not simply best practices, but actually safety standards which should always protect the system and the users too.
Modularity, Diagnostics, and Functional Safety
For larger vehicles or grid storage where the battery packs are larger, a single BMS PCB becomes unwieldy as the pack scales up. Strong designs conform to a modular architecture with a master-slave layout, where a master unit and multiple slave modules are tumbled end-to-end along the battery stack. This makes the wiring easier, increases scalability, and provides the capacity to monitor individual cells for increased accuracy. Due to the key concept of daisy-chaining the modules, the PCB design of these slave modules needs to be as small as possible, while also being able to effectively isolate themselves to support the high common-mode voltages present between modules at different nodes in the battery stack.
Lastly, robustness comes from the system's capability to identify faults and to fail safely. The PCB does need to include circuitry to monitor its own health — watchdog timers for the microcontroller, voltage supervisors for the power supplies, and redundant measurement paths for critical parameters such as total pack voltage. The entire design process — component selection, PCB layout, etc. — is driven by functional safety standards, such as ISO 26262 for automotive applications, to identify potential failures, safely downgrade the system state, and to prevent hazardous events.