E-Mobility Engineering 016 l Aurora Powertrains eSled dossier l In Conversation: Thomas de Lange l Automated manufacturing focus l Torque sensing insight l Battery Show Europe 2022 report l Sodium batteries insight l User interfaces focus

Dossier | Aurora Powertrains eSled controlled (via the BMS) to preheat the battery pack and hence prep it for efficient charging before a tour starts, and to maintain a stable temperature while slow, efficient AC charging is taking place. “Also, the foil resistor naturally has a very flat and thin shape, meaning it’s great for the constrained space and volume that we can dedicate to each part,” Autioniemi notes. “And of course, we don’t want to accidentally send the heat we’re generating into other electric or electronic components, or suffer parasitic effects from the resistors. So the battery compartment is lined with a cellular foam typical for insulating buildings and vehicles in the Arctic.” To ensure quick transmission of heat from the resistors to the cells, and from the cells to the cooling plates, thermally conductive pads are installed above the heating plates and on either side of the cooling plates. Each pad is made from a silicone gap filler, extruded during the battery construction process. The team experimented with several materials before settling on silicone for its simplicity and workability. Haavikko adds however, “We’re currently evaluating graphene-based thermal transfer materials, which exhibit very good in-plane thermal conductivity and are therefore really useful for passively and efficiently balancing the heat between battery cells.” Battery cells and modules To ensure that the thermal management system is designed and installed optimally, the battery modules are of Aurora’s own design and made in- house. Each one weighs about 18.5 kg and measures 166 mm long, 217 mm wide, and 272 mm tall. When fully charged, each stores up to 3.5 kWh of energy and is configured to output DC power at a maximum voltage of 120 V. The modules can be strung together in parallel or series using a proprietary interconnection system, with finger- proof HV terminals situated along the tops of the modules. These sit no higher than a few millimetres above the enclosures, to prevent unnecessary usage of the limited space inside the chassis. “We’ve also designed the terminals such that each module has two negative tabs on one end and two positives on the other, so it’s really easy to connect them next to each other, even in three dimensions or in layers for any future EV designs we’ll use these batteries in,” Haavikko explains. “We designed the battery module HV connectors in-house with very flat, packageable shapes to maximise the energy density of the overall packs and to enable us to shape the pack geometry as we needed. And the modules are fastened together mechanically to keep the actual processes of modularity and maintenance as simple as possible.” This modularity enables each eSled to store energy from 7 to 21 kWh depending on the range requirement for each outing; an eSled with 7 kWh onboard will weigh just under 200 kg, compared with 270 kg for one with the maximum pack size. The 21 kWh pack has a specific energy of 190 Wh/kg, and all the modules have an IP67-rated enclosure (as are all the other enclosed onboard systems) or better. “Our early generations of battery designs were highly integrated solutions, so much so that we had to disassemble the entire pack whenever we had a problem with just one module, so we focused on developing a modular architecture that would hugely ease maintenance,” Haavikko recounts. Inside the modules, energy is stored in pouch cells with NMC cathodes, a chemistry chosen for its specific energy and energy density. Each module has 28 cells, and the structure and profile of the pouches fit well inside the rectangular profiles of the modules, Battery heating is provided through foil-type resistors in each module, while the liquid circuit (blue-to- orange) balances out temperature diɈerentials Ietween tOe modules 26 Winter 2022 | E-Mobility Engineering