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

bumps and debris that snowmobiles routinely hit, real-world testing was much more important than simulation for environmental ruggedisation. “The most challenging thing is how ice forms or melts across different surfaces,” Haavikko says. “Ice expands when it freezes, and contracts when it melts, which imparts quite a lot of force that can break enclosures, connectors and other surfaces. “For instance, we tested a few products that featured pressure relief vents with cup-like intrusions. Water from precipitation would seep into those nooks, freeze, expand, and melt, leaving cracks through which water could then easily access the components’ insides. “That’s a tangible risk for every layer you could think of enclosing your systems in, for any snowbound vehicle,” Autioniemi adds. “It’s just impossible to completely eliminate the possibility of snow powder trying to collect in every hole and cavity. “It’s going to happen, and it’s going to cause cracks, but we’ve resolved that by designing all our enclosure surfaces with drain-like channels throughout the chassis. These guide water ingress away from electronics and electrical systems, to outer parts of the eSled where they can’t cause harm.” Years of testing where ice will gather and how water flows has informed Aurora on how to predict the movements of moisture throughout the chassis, powertrain and suspension. “That’s something we absolutely would never have found through simulation,” Haavikko says. “It’s quite unique to Arctic conditions. It really makes you check your components in a way most electric component manufacturers don’t think of doing. “And winter testing has been key to nailing the mechanical ruggedness of the system, as we know to use things such as crash shields on some HV cables for added impact protection.” Battery thermal management Of course, the biggest problem with Arctic temperatures is the degradation that exposure to the cold causes to batteries, more so than any other component. Aurora reports that essentially no special reconfiguration of the inverter and motor was needed other than choosing a liquid-cooled (and heated) drive unit. Meanwhile, some winter-proofed cabling and connectors from a range of suppliers ensure the security of high- and low-voltage power throughout the eSled. But the thermal management needed for keeping the battery energy capacity and charging/discharging stable was by contrast quite complex. “Each battery module is constructed with its own heating element, which sits at the bottom of the module, right against the cells, so that each group of cells can be independently heated as is necessary for a consistent thermal balance across the cells,” Autioniemi says. “Then, at either end of each module is a proprietary cooling plate design, with liquid-coolant pipes running lengthways up and down the pack, so that the pack can also be cooled as needed.” The liquid coolant is propelled from a centralised pump, the flow rate of which is adjusted via the BMS. The back-and-forth piping of coolant serves to balance and average out temperatures across the modules, with any targeted balancing taken care of by the module-specific heaters. Naturally, for this part of the world, actual cooling of the battery is rarely needed outside extremely aggressive driving instances, or if starting a recharge immediately after a lengthy drive. The use of the terms ‘coolant’ and ‘cooling system’ is therefore meant largely in a technical sense, rather than a literal one. This enables a precise balancing act between heating and cooling, to target the 25 ºC optimum temperature for battery capacity and longevity. The use of an external cooling circuit – contrasting with the internal heating elements – is a safety-conscious design choice intended to prevent the risk of coolant leakages inside the pack and modules. “We’ve tried at every turn to limit the number of seals used,” Haavikko notes. “In many of the early generations of our prototypes, several tens of O-rings and other seals were used, with liquid flowing right next to the cells, and it occurred to us that if any of them broke then the entire battery pack would fail. “In our present-day design, every single O-ring in the cooling system could fail but the pack would keep on working. In fact, the transfer of heat from inside the pack to the outside would probably continue, as there is not too much material for the heat to move through before it is transmitted to the heat exchange plates.” The heating element is a foil-type resistor, known for its high resistance stability, quick response time and long lifespan. In addition to quick and precise heating of modules during operations, they can also be The front of the seat covers the battery modules, which are also made in-house Winter 2022 | E-Mobility Engineering 25 Dossier | Aurora Powertrains eSled