22 “The plates then get bolted top and bottom, and at the sides where possible, to the steel, structural walls of the pack. With the modules bolted firmly to the plates, and the plates fastened securely to the housing, the whole pack becomes incredibly rigid,” Hazell says. “Since using the cooling plate as a structural member has such a huge benefit, we adapted the configuration into our Universal Battery Pack, and then into the 33 kWh design that is being used for the Morgan project.” Thermal management Hazell notes, however, that the original Porsche kit’s pack contained coolant that flowed in and out of the plates, making for a largely self-contained system with just two coolant access outlets. When transitioning the architecture to its 55 kWh standard product, Fellten moved away from storing coolant within the pack, placing it and its ancillaries externally. “That’s visible in the XP-1 integration also,” he says. “On the bottom of the cooling plate there are five holes: the centremost one functions as a bleed hole, running to the top of the plate to pull out any air that’s trapped up at the top. As the coolant enters at the bottom, there’s a risk of air lock at the top unless the air up there can be drawn down and out of the plate.” Of the remaining four holes per plate, two function as coolant inlets and the other two as outlets. This dual, inletoutlet approach helps to maximise the volumetric coolant flow, as the plate’s 16 mm thickness (being a thin span for a cooling plate) limits the channel width and hence the top flow rate per channel. To facilitate this, Fellten has developed a specialised, coolant line fitting with integrated O-rings for pushing up into (and then bolting into) the bottom of the plate. As Hazell explains: “The benefit of this system, which is similar to what GM and Vauxhall have done with their packs, is that it’s an external fitting, so you can’t get a coolant leak internal to the pack through a pipe or tubing malfunction, because everything’s pushing and bolting on from the outside. “That’s one of the biggest risk factors we came across while doing failure mode and effects analysis: in our opinion, the biggest risk towards failure of a battery pack is coolant leakage, because it completely destroys the modules. It can cause dead shorts and a huge number of other issues.” While a larger producer of battery packs might be tempted to invest billions into studying the probability of a coolant leak, and the breadth of quality control options available for stopping such an event from ever happening, Fellten’s strategy as a lower-volume manufacturer has been to move the source of the issue from the internals to the outside of the pack. To ensure effective conduction of heat between the modules and coolant plates, to cool or heat the packs depending on the effect of the surrounding environment, Fellten applies a specialised, thermal paste, glue or pad. Glue is used in the case of the Morgan pack, as many pastes come with a certain viscosity and would drip downwards when applied into Fellten’s verticallyoriented modules, although thermal pads and some caulk gun-applied pastes remain an option for future builds. “We’ve gone for a glue with a 4 U temperature-transfer capability. Most OEMs go for 2 U, but we’ve opted for a more expensive substance to be sure we stay ahead of EVs’ and powertrains’ thermal requirements,” Hazell notes. “Although we call it a cooling system, in most of our system arrangements we will actually scavenge the motor heat to warm the battery pack through a heat exchanger – the Morgan XP-1 doesn’t run that, but our other builds do. “We’ve taken significant inspiration from the pre-2017 Tesla Model 3’s heatrecirculation system and reformatted it for this conversion-aftermarket space, because we knew it worked very well, and it allows us to first cool the glycol for rapid-cooling during DC fast-charging, but we can also use the thermal mass of the battery to rapidly cool the motor and inverter. “Through the BMS, we might, for instance, set the battery pack to a temperature of 15 C, because then a sudden spike of heat in the motor or inverter can be taken care of Dossier | Fellten Morgan XP-1 July/August 2024 | E-Mobility Engineering The design concept in the XP-1’s 33 kWh battery pack vertically orients the modules and integrates liquid-cooling plates, which double as structural members
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