Design Considerations for High Cell-Count Battery Packs

High cell-count applications present unique battery management and thermal management challenges

Battery pack design is a complex, multi-step process with many decisions. After cell chemistry, perhaps the two most important decisions in battery pack design are the number of cells and the arrangement of the cells.

Chemistry determines, among many other properties, the nominal voltage of a cell. The size of the cell in turn determines the capacity. To double battery capacity, put two cells in parallel. To double battery voltage, put two cells in series. Pick the proper arrangement to meet the design needs.

Arranging cells in series and parallel to fit the application is battery pack design 101—uncomplicated, at least at small scales.  But big-ticket applications like electric vehicles and power tools are high-voltage and high-power, and they need a lot of cells. As cell count goes up, the need for effective battery pack design increases. Likewise, the difficulty of battery pack design increases with size.

There are several reasons why design difficulty increases with cell count. First and foremost, the addition of cells increases danger to users in the event of failure. As we discussed in our article on battery chemistry, many common cell chemistries contain corrosive or flammable materials. Additionally, more cells create more opportunities for failures such as charge imbalance and over-discharge.

Preventing safety hazards and operational failures becomes more difficult with high cell counts because increased size frequently requires battery management systems and thermal management systems to become more complex.

In this article, we’ll take a closer look at the challenges involved in battery management and thermal management for large format battery packs.

Aved ensures safety and performance in your high cell-count and high-voltage applications by carefully designing and testing custom battery packs to a high standard of performance. Contact us for more information or request a quote today.

Battery management with large cell counts

Many smaller battery applications don’t require sophisticated management circuitry. But it’s different for high cell-count applications. More cells add more points of failure, which means more opportunities for safety hazards and for the battery to be ruined by malfunctions.

Thus, it is critical to have precise cell monitoring and control in large battery packs with high cell counts. Simultaneously, the increase in scale makes it proportionally more difficult to achieve these goals. As cell count goes up, the circuitry needed to monitor voltage and balance cells becomes more complex and expensive.

The challenge of monitoring and control

For instance, large battery packs are especially prone to unbalanced cells because of uneven heat distribution. It’s very easy for a cell, especially if it was already “weak,” to reach a lower state of charge relative to the other cells, even after the battery is fully charged. Then, it is prone to over-discharge, which can cause irreparable damage to the cell and battery if repeated over many cycles.

To ensure battery longevity and user safety, high cell-count batteries need voltage monitoring and SOC calculations for every cell, plus cell balancing capabilities. These functions are provided by a battery management system.

BMS printed circuit boards (PCBs) with all the necessary functionality for a BMS—FET switches, voltages and current sensors, balancing circuitry, and more—are certainly available for large-format battery packs, and can be very economical. But they can be complicated. Many (often long) wires are needed to connect all the cells to the control and monitoring circuits of the PCB.

These diagrams show some alternatives to a centralized BMS layout, including modularized, distributed, and decentralized models. Image source: Reindl et al. in Architecture of Computing Systems

Alternatives exist to this centralized BMS layout, however. A distributed BMS layout has one separate controller circuit and multiple PCBs installed at the module level. A modularized BMS layout is very similar to a distributed layout, but instead of having a separate controller, one BMS PCB serves as the “master,” performing calculation and control functions in addition to monitoring. These decentralized BMS topologies decrease circuit complexity, but can add cost.

Hardware limitations

High cell-count batteries are often high-voltage batteries, and they are often discharged in quick, high-current bursts. It’s important to evaluate the demands that will be placed on BMS circuitry by high-voltage and high-current applications and make design choices accordingly.

For instance, field effect transistors (FETs) used for circuit isolation have amperage limits. It is common to have two FETs, one for charge control and one for discharge control, placed in series. This allows charge and discharge through a single set of contacts. However, in high-current applications, it can be more cost-effective to have separate charge and discharge FETs in parallel. This saves costs by allowing the charging FET to have a lower amp rating than the charge FET.

Some hardware, including microcontrollers and digital monitoring equipment, does not work well with high voltages—yet another reason that single-chip BMS systems might not be a good idea for high cell-count battery packs. It’s important to avoid exposing sensitive equipment to high voltages. 

Finally, hardware redundancies play an important role in BMS. Redundant voltage, current, and temperature sensors insure the battery against failure. Hardware-only shutoffs set just above the threshold of the main protection mechanisms provide additional safety.

Thermal management for large cell-count batteries

As discussed in our article on thermal management, the consequences of temperature on battery performance can be profound, affecting voltage, self-discharge, lifespan, and more. Effective thermal management is essential for battery safety and longevity, and doubly so in high-cell count batteries.

For one, temperature differentials create challenges for high cell-count batteries by leading to SoC imbalance across cells. This can, of course, be detected and managed by a BMS. Mitigation of temperature differentials with a thermal management system is also critical for high cell-count battery applications.

Additionally, the high voltages and high temperatures often seen in large-format battery packs make thermal management a matter of safety. Avoiding thermal runaway, when a battery releases more and more heat as its temperature increases, is paramount. Thermal runaway is a big enough problem when it’s just a lithium-ion phone battery that fails. In a high-voltage application like an electric vehicle, thermal runaway could be catastrophic.

This video of the aftermath of a battery fire in a Tesla electric vehicle following a crash shows the importance of avoiding thermal runaway:

Balancing space with heat exchange

As with battery management, this need for effective thermal management and even heat distribution in large-format applications comes with its own challenges. Efficient thermal management for high cell-count batteries is often at odds with the efficient use of space.

Space is in high demand in all kinds of battery applications, and packing cells closely together can save space. One of the most space-efficient ways to arrange cells is to use a honeycomb pattern. But tight spacing can impede passive heat shedding.

Large modules or battery packs can also challenge active cooling systems with unidirectional fluid or air flow. The fluid can absorb so much heat before it completes its circuit that parts of the system receive inadequate cooling.

Fortunately, battery designers and engineers have multiple tools at their disposal to overcome the challenge of thermal management in large-format systems. For example, reciprocating fluid flow can help prevent the uneven cooling that can arise in unidirectional fluid flow.

Heat modeling (shown here), along with thermal imaging and stress testing, can help designers manage temperature in large-format battery packs. Image source: National Renewable Energy Laboratory

Additionally, careful selection of the number, shape, and layout of battery modules can optimize cooling. By striking a balance between use of space, efficient passive heat shedding, and active thermal management, it’s possible to strike a balance between cost, volume, and heat management.

Most important, though, is effective design, modeling, and testing. Through the use of computer-assisted design technologies, heat modeling, and rigorous testing, designers can anticipate and identify the thermal properties of a battery prototype and design an adequate thermal management system. 

Large battery packs need effective battery management and thermal management solutions

Battery pack design involves many variables. One of the most important decisions in the design process is the number and layout of the cells. Arranging cells in parallel and series to achieve the required voltage and capacity involves simple calculations. But when a battery pack has many cells, careful consideration is needed to ensure safety, functionality, and longevity.

Monitoring and balancing SoC in large-format batteries is essential, but the BMS circuitry required to do it can be complex. Additionally, different hardware may be needed to handle high currents and high voltage. Hardware redundancy is always valuable.

Temperature control is also essential, but high cell-count battery packs present unique challenges for thermal management systems. Efficient use of space is often at odds with heat exchange, and large blocks of cells can impede cooling even with active temperature management measures.

For your large-format battery needs, turn to Aved. Our expert team of engineers and technicians can design, test, and manufacture custom battery packs with BMS and TMS to meet your specific needs. Contact us to learn more about the design process or request a quote today