Controlling the massive amount of energy stored in electric vehicle (EV) battery packs is critical. Significant advances in cell design and battery management systems (BMS) are addressing this; however, there’s always a possibility of a single cell failure.
In the worst-case scenario, failure of a single cell will cause failure of the cells near it through overheating, and cells can fail in a cascade of thermal events. Recognizing this remote but plausible possibility, EV pack designers added features to limit or eliminate a single cell’s failure from propagating.
Thermal management basics
Designing a battery module or pack requires balancing several competing thermal factors. The most common strategy is to provide just-enough thermal management to achieve the battery pack’s fundamental goals. Adding additional thermal management material only adds cost, weight, and volume.
Thermally related goals are:
Maximum charge/discharge rate – How fast can you charge or discharge the battery without damaging the cells from excessive heat? An EV may have charging requirements as low as 0.5°C, as high as 2.0°C, or even higher in some newer designs.
Cell cycle lifetime – Cell lifetimes range from 1,500 to 2,500 cycles. Cell chemistry, cell temperature, and cell temperature gradient are determining factors.
Driving range per charge – The heavier the pack, the lower the driving range. The type and amount of thermal management material are significant factors in specific energy (Wh/kg) and energy density (Wh/L).
Desired safety level – Do you need to prevent propagation from cell to cell, or will it be acceptable to contain the fire’s heat within the pack housing for a set period of escape-time?
Four primary methods prevent thermal propagation in prismatic and pouch cell packs, and each method has significant consequences for cell cycle lifetime, the ability to fast charge, and driving range. Used alone or combined, each manufacturer adds its experience to produce the best results.
1. Isolation – Modules are placed within a heat-resistant pack housing. If a cell fails, there will be sufficient time for the EV to stop and for passengers to safely exit. The most common escape- time requirement is five minutes.
Since failure of an individual cell may generate a hot spot on the pack housing, various thermal insulation and heat-spreading materials dissipate the heat. Even in a cascading failure, heat will be widely distributed on the housing surface, maintaining its thermal integrity throughout the failure event. Materials can include flexible graphite, mica, aerogel, air gaps, and ceramics.
The isolation method is often viewed as the least expensive way to meet escape-time requirements. These packs have high energy density and high specific energy. Construction is simple and cost-effective.
Isolation, however, often leads to inadequate thermal management for the individual cells. Cell temperature and cell temperature gradients generally aren’t tightly controlled; fast charging/discharging can overheat and damage the cells, limiting cycle lifetimes; and thermal degradation of a single cell can significantly reduce the range of the EV.
2. Insulation – Thermal insulation materials are placed between cells, preventing heat from spreading to adjacent cells if a single cell fails. Combining different insulating materials such as aerogel, fiberglass, phase-change, mica, polyimide, ceramics, and air-gaps prevents heat from transferring. A minimum of 4mm to 6mm of insulation material is typically needed between cells to stop propagation.
The insulation method isn’t complex and uses lightweight materials, typically resulting in high overall pack specific energy.
However, individually insulated cells can’t shed heat to the environment during day-to-day operations as easily as unwrapped ones. Fast charging can overheat the cells, leading to substandard cell cycle lifetimes. The added thickness between cells leads to a lower overall pack energy density, potentially making the pack too large to be practical.
3. Immersion – Individual cells are surrounded by a dielectric liquid circulated throughout the module by a mechanical pumping and cooling system. Surrounding each cell in a cooling fluid is the ultimate method of preventing propagation. If a cell fails, the liquid would carry away the heat and stop the fire from spreading.
Using a cooling liquid allows tight control of cell temperatures and cell temperature gradients, leading to excellent fast charge/discharge and cell cycle lifetimes.
The immersion system’s size, weight, and complexity must be taken into account. There may be pumps, chillers, valves, baffles, and piping that aren’t part of other systems. The pack’s baseplate size may be significantly reduced, but the system may have a higher overall weight due to cooling liquid.
The cooling liquid may also be hazardous. Special maintenance, handling, and disposal rules may apply. The system’s increased complexity generates a higher initial cost, and maintenance may be higher than with other system types.
4. Spreading – Heat from a cell failure will spread across a thermally conductive material to a cold plate or heat sink, shedding into the environment.
In day-to-day operations, spreading allows cells to be fast-charged without heat buildup. Heat spreading material will also maintain a low thermal gradient across the cells, extending cell cycle lifetimes.
Flexible graphite cooling fins (0.25mm to 1.00mm) or aluminum plates (1mm to 3mm) are the most common heat spreading materials. Using graphite instead of aluminum improves pack energy density and specific energy, resulting in smaller, lighter packs with greater driving ranges.
A layer of polyurethane foam and a layer of dielectric material are typically added between the cells to maintain physical contact of the heat spreader against the cell and for additional thermal and electrical insulation.
Graphite vs. aluminum
In lower-performance battery packs, aluminum has been the primary material, often used for mechanical structure and heat spreading. For higher-performance battery packs, the amount of aluminum needed for safe, efficient operation may result in a pack that is too heavy and bulky.
Aluminum is dense and has poor thermal conductivity (200W/mK), but graphite is lightweight and has high thermal conductivity (400W/mK to 1,100W/mK). A graphite heat spreader would be half the size and one-third the weight of an equally potent aluminum one.
Flexible graphite consists of hundreds of thousands of graphene layers, mechanically stacked to form a continuous sheet. A layer of thin polyethylene terephthalate (PET) plastic adds toughness and dielectric protection. The graphite is presented as a finished die-cut piece with a pressure-sensitive adhesive applied to one side for simple application.
Flexible graphite is widely used for heat spreaders in mobile phones, notebook computers, and large-screen televisions. Its thin, lightweight, easy-to-apply properties have made it the standard heat spreading material in the electronics industry.
Spreading is the best way to prevent thermal propagation in pouch and prismatic cell battery packs because it prevents propagation while extending cell cycle lifetime and fast charging while cutting size and weight. Flexible graphite heat spreaders outperform aluminum and can support high-performance, small, lightweight battery packs.
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