58 May/June 2024 | E-Mobility Engineering Nick Flaherty looks at strategies to get more power out of battery cells, which brings thermal management challenges Packing more punch There are many ways to boost the energy density of an e-mobility platform’s powertrain. The materials in a battery cell can be improved to produce more energy, from silicon in the anode to new electrolytes. Solid state batteries under development, for example, can provide higher energy densities, stacking more cells closer together by addressing the materials and the packaging. There are strategies to include more cells in packs to improve energy density with structural designs that eliminate much of the overhead of the packaging. Then there is the challenge of getting all the available energy out of a cell to provide the maximum possible power across its lifetime. Here, the modelling and testing of cells is key, with implications for the design of the battery management system (BMS). There are even opportunities to integrate elements of the chargers and inverters into the battery pack to improve the energy density of the entire powertrain. However, increasing energy density brings challenges in thermal management as smaller cells and more compact packs have more power. Faster charging also generates more heat, which can be more difficult to dissipate. Volumetric energy density The amount of energy that can fit into a given physical volume is most important for applications that are volume-constrained, such as passenger vehicles. To add more range in an EV with conventional lithium-ion batteries, the battery pack must be made physically larger, which means the car itself must also be larger. As the vehicles cannot easily be made bigger to accommodate more batteries, the cells need to pack more energy into the available space. While weight is important, better volumetric energy density is a bigger priority, as it allows passenger vehicles to be smaller and lighter, with more interior space for legroom or cargo. Gravimetric energy density Weight is at an absolute premium for applications such as aerospace for electric vertical take-off and landing (eVTOL) air-taxi designs. In such cases, a bulkier battery can be accommodated by good design, but increased weight constrains performance, so the lighter, the better. In heavy-duty trucking there are legal maximum weight limits to avoid damaging roadways. For an electric truck, the less weight taken up by batteries, the more payload can be hauled, enabling it to earn more per trip. In cases such as high-performance sports cars, lighter models can handle better and accelerate faster. For these types of applications, gravimetric energy density, also known as specific energy, may be the bigger pain point. Battery energy density has slowly but steadily increased over the past few decades, and lithium-ion batteries are now so energy dense that they can power EVs for hundreds of miles. Energy density “You can break the battery issues down into three elements. There’s the cell chemistry, dominated by lithium NMC, and in the short term, for five to seven years, that will continue to dominate,” says Peter Freedman, chief commercial officer at Brill Power. “Over the last couple of years, the companies we deal with in automotive have seen advances in cell-to-pack technology that have really helped improve the overall energy density. In electric bus design everyone wants to put the batteries on the roof or at the rear, as you need a low floor, so that’s where you find the biggest advancements. So, it’s about improving efficiency over the lifetime to reduce the size of the packs.” Brill Power is developing technology across three areas, with a combination Increasing energy density is a key driver for eVTOL aircraft (Image courtesy of Amprius)
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