E-Mobility Engineering | September/October 2023 25 Nyobolt EV | Dossier inboard lamp units with five functions: high beam, low beam, a position light, an indicator and a side light, covering what is required by road authorities.” New 21st-century driver subsystems include wing-mirror cameras that are housed in machined, solid, transparent ABS that leave the PCBs visible for aesthetic purposes. Further back, the charging connector input is installed at the B-pillar. In addition to these external changes, the IC powertrain and fuel system has of course been replaced with Nyobolt’s battery technology and an electric powertrain. Adam Donfrancesco, engineering director and co-founder of CALLUM, notes that from an engineering perspective, using the Series 1 Elise as a resto-mod platform wouldn’t have been the right starting point because CALLUM needed to be able to focus on the engineering and integration of the powertrain and the HV architecture. That took precedence over more resto-mod oriented objectives, such as developing the brakes and steering to mimic the experience of driving an Elise. Iterations throughout much of the design and integration process were carried out using an Exige as the donor car, although the Elise remained the primary design inspiration. The Exige was the last of the Series 1 Elise’s line, and carried more mass than the others. “That was useful when we were deltaing the HV components in and out,” Donfrancesco notes. “We spent much of the project aiming for around 50 kg incrementally over the original vehicle, so now the Nyobolt EV is about 1246 kg, roughly 40 kg heavier than the Exige with a full tank of fuel.” That though is far below the weight of many comparably sized EVs, and was achieved thanks partly to the lightness of the battery pack, the exact weight of which was undisclosed at the time of writing. It also achieves the aim of retaining the lightness of the Elise, a key factor behind its performance. Given the central aim of demonstrating the battery technology, the powertrain has been designed to be as simple as possible, and hence follows the trend of central, single drive motor configurations seen increasingly across the passenger EV industry. The motor itself is a unit from Helix (formerly Integral Powertrain) in the UK, which provides a peak power of 364 kW (488 bhp) at the wheel through a single-speed gearbox with a limited-slip plate differential. It is located under the battery in the rear of the car, just ahead of the rear axle. Nyobolt adds that the inverter is supplied by Cascadia Motion, but declines to specify the exact model. “It’s as simple, traditional and lightweight an arrangement as you can get in an EV powertrain,” Donfrancesco adds. “We wanted the car to feel still and stable but also be capable of considerable performance, being a sportscar, so the e-motor itself is capable of up to 364 kW, resulting in a peak torque of 457 Nm. “Couple that with the limited-slip differential and it should put power down very well, and we gain a little extra traction thanks to the width of the Exige. The downside of that is the car’s tendency to push on, so the differential is relatively open, but having to tailor the front-end grip while driving was part of the Exige’s original charm, so we don’t necessarily want to mitigate that too much. We’ve also gone for slightly larger diameter wheels for design aesthetics and improved traction over the original. “Overall, the resulting traction might be a little excessive from a demonstration point of view, but proving the battery’s discharge capabilities is obviously very important, and we SiC EVSE DC charger In addition to the fast-charging battery technology in Nyobolt’s EV, the company has also showcased a separate project consisting of power conversion technology developed for off-board DC fast chargers. The project has resulted in an 800 V charger module, intended to enable highly efficient charging stations to be set up as part of an ecosystem of materials and power usage in the future EV industry. “Rather than force OEMs to design a highly efficient charger themselves, we’ve made one compatible with our fast-charging battery, which we’re going to be manufacturing and supplying ourselves,” says Iain Mosely, Nyobolt’s director of power electronics. At the time of writing, the 800 V EVSE DC charger module prototype was configured for throughputs of up to 30 kW, which Mosely says equates to replenishing 120 miles of range per hour. Multiple units of the charger can be connected in parallel to increase the power level in steps of 30 kW. Nyobolt proposes that a charging station and connector capable of 300-400 kW would hence be appropriate for a 6 minute charge of the 35 kWh pack in the Nyobolt EV. “Our modules use silicon carbide, which gives really high performance with the lowest heat losses of any commercially available power electronics,” Mosely says. “Over the vast majority of cycles, accounting for temperatures and other variables, the charger transmits power from charging input to battery pack with greater than 99% efficiency. “When we started the custom design, we had three target parameters: the voltage capability, the current capability and the power output characteristic. We had to tailor our system and these values in a way that would maximise our efficiency across all feasible environmental conditions and states of charge and health. “Often, charger manufacturers try to make the power output characteristic as wide as possible, so that it can charge anything from a Class 8 EV to a small scooter, but that means a lot of power efficiency and performance is lost. Our 30 kW module could certainly be used to charge a small e-scooter; it just wouldn’t be very power-efficient or cost-effective to do so.” To explore which architectures and components worked best, Mosely and his team used rapid prototyping to produce numerous different charger modules they could test, connect in parallel, and gather performance data from. Each module also ran embedded, real-time control software that the power electronics team could tune for different parameters in their pursuit of maximum efficiency.
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