The configuration of lithium-ion battery packs, particularly the total number of cells connected in series and parallel, has a great impact on the performance, thermal management, degradation, and complexity of the Battery Management System (BMS). While selecting suitable form factors and cell voltage/current specifications can mitigate some issues, the essential design decisions around cell arrangement remain critical.
When cells are connected in series, the voltage of the battery system increases. This is essential in applications where a higher operating voltage is necessary. Increasing the series count introduces several challenges. Voltage imbalances can occur more easily in series configurations, as any small difference in cell capacity or state of charge (SoC) gets magnified across the series string. These imbalances can lead to under-utilization of capacity and accelerate degradation as cells are cycled at different depths, leading to varied states of health. The BMS must, therefore, incorporate sophisticated balancing algorithms and perhaps even active balancing systems to mitigate these effects, which adds to the complexity and cost.
Another consequence of high Series configurations is their heightened sensitivity to temperature variations. Cells in a series may not heat or cool uniformly, causing localized hot spots that can degrade those cells more rapidly. The BMS and thermal management system must be designed to account for these discrepancies, often requiring complex thermal models and active cooling or heating strategies, such as liquid cooling systems that ensure a more uniform temperature distribution.
In contrast, when cells are connected in parallel, the capacity and discharge current capability of the battery increase. This is suitable for applications that require high power and energy density. However, paralleling cells also introduces challenges. If one cell in a parallel group fails, it can affect the entire group's performance and safety. A failing cell can become a current sink, overheating and potentially leading to thermal runaway. The BMS must be capable of detecting such faults early and isolating the faulty cell if possible.
The impact on degradation in parallel configurations is somewhat mitigated since cells support each other, leading to more uniform discharge and charge cycles, reducing the stress on individual cells. However, it also means that any variation in cell quality or performance can be more impactful, as weaker cells are forced to keep up with stronger ones, potentially accelerating their degradation.
The complexity of the BMS scales with the number of cells and the series/parallel arrangement. A higher number of cells increases the monitoring and control points for voltage, temperature, and current. This complexity means more wiring, more precise communication protocols, and advanced algorithms for cell monitoring and balancing.
Selecting suitable form factors and cell specifications can address some of these issues. For instance, larger form factors can reduce the total number of cells needed for a given capacity, which can simplify the BMS design. Likewise, cells with higher individual voltage and current ratings can reduce the number of cells in series and parallel, respectively, to achieve the desired battery pack specifications. However, form factor selection is often a compromise between available space, thermal performance, and mechanical robustness.
In terms of cell specifications, selecting cells with closer performance tolerances can reduce some of the challenges associated with cell balancing and mismatch. However, this may come at a higher cost. Additionally, the cell's internal chemistry and construction can be optimized for better thermal stability, reducing the thermal management burden but again potentially increasing costs.
The impact of the configuration of lithium-ion cells in series and parallel on battery performance is significant and multidimensional. While intelligent design and selection of cell specifications can reduce some issues, the inherent complexities associated with series and parallel configurations cannot be entirely eliminated. Designers/engineers must carefully consider these trade-offs when designing lithium-ion battery systems to balance performance, life, safety, and cost. The BMS and Thermal Management System must evolve together with these design decisions to ensure the reliable operation of increasingly sophisticated and powerful battery systems.