60 work with OEMs to understand the right level of granularity, depending on the use case. This allows you to extract more energy from the pack, especially as vehicles get older and the cells degrade,” says Freeman. Tardelli says: “It’s always a trade-off of how granular you want the control and weight of the system,” says Tardelli. “You can do clever stuff with granular control. You can effectively integrate the power conversion needed in the powertrain. Within stationary storage we already integrate DC-DC conversion in the module, but we could do power conversion from DC to AC that can be flexible.” This could reduce the overall weight of the powertrain by integrating elements of the inverter or charge into the pack. “Another one is to scale up the battery pack with basic building blocks at a much lower voltage than the full pack. You still need to monitor all the cells in the pack, and we leave a lot of control at the module level,” says Freeman. “If you keep fast-charging a battery, regardless of the chemistry, it will degrade, so the integration of the power conversion gives you the opportunity to match the operation of the charging station. By properly interfacing the battery to the charging station you can make sure you have the maximum charging power, and we can charge batteries 8-10% faster. In the commercial space this is interesting for DC fast charging,” he says. However, a key challenge is the accuracy of the algorithms in the BMS and how the performance of the cells changes over time. “One of the biggest challenges is that the embedded algorithms massively diverge from reality over time, and the way we are addressing that is being able to have this firmware platform, taking the data to the cloud and updating the algorithms. This reduces the size and weight of the module,” says Freeman. “If the BMS is giving you inaccurate SoH and SoC readings, you cannot use all the energy in the pack; it will stop you. But if you can update those algorithms, you get more energy out.” This will be key for new chemistries, including sodium for smaller city-car designs. “Sodium is interesting from a commercial and cost perspective, rather than energy density. We are chemistry agnostic, and we have our BMS operating with sodium ion batteries, and they will have a part to play in automotive,” says Freeman. Sizing a pack “People are sizing their battery packs for the power requirement in years six, seven and eight for commercial platforms, which is different from car designs that are about capacity on the first day,” says Freeman. “So, if you can minimise the size of the pack by increasing the energy density of the cell, and use a cell-to-pack architecture, this minimises oversizing the pack. This gives you the smallest pack on day one that will still provide what you need in year seven.” The cell-to-pack technology developed by CATL, for example, has significantly boosted the volumetric use efficiency of the battery pack, which has risen from 55% for the first-generation CTP battery to 72% for the thirdgeneration battery, called Qilin. The energy density of Qilin using lithium NMC cells can reach 255 Wh/kg, while that of the LFP one amounts to 160 Wh/kg. Cell-to-chassis (CTC) technology integrates the battery cell with the vehicle body, chassis, electric drive and thermal management, as well as various high and low-voltage control modules. This extends the driving range to over 1,000 km. It optimises power distribution and reduces power consumption to less than 12 kW/h per 100 km. Lithium-metal, solid state batteries Lithium metal poses some unique challenges that require innovations in cell architecture. For example, QuantumScape’s solid state, lithiummetal battery technology uses FlexFrame, which is a cross between Deep insight | Battery energy density May/June 2024 | E-Mobility Engineering Building a battery management and control system (Image courtesy of Brill Power) The FlexFrame improves the energy density of a solid state battery cell (Image courtesy of QuantumScape)
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