28 Interconnections To date, Fellten has usually opted to buy overstocked (but still new and unused) modules from OEMs, resulting in the use of the VDA 355 module in the XP-1 packs. These are made by DFD Energy with NMC cells of either a pouch or prismatic design, depending on request. Those in the XP-1 each feature 12 pouch cells (with two parallel-connected strings of six series-connected cells) and housed in aluminium. More energydense options were available, but the 6S arrangement was paramount to Fellten’s voltage requirements for compatibility with the inverter. “We’d like to push to LFP or LMFP, but the energy density we want isn’t available with those chemistries just yet, though when we start manufacturing our own modules, we’ll be able to pick and choose from various cell options,” Hazell says. “That’s a big motivator for us: conversion specialists like us can only get modules that have been commissioned to be built in volume by large OEMs, so we’re limited to use what the OEMs want to use. Our Porsche systems, for instance, use an overstocked module from XPENG, and our Universal Battery Packs use the Volkswagen modules built for the ID.3 and ID.4, which use LG cells.” Overall, the XP-1’s pack contains 14 modules arranged in a 2P84S configuration, resulting in a 320 V nominal bus voltage. The nickel-plated, copper busbars connecting the modules are treated with a special powder coating for HV insulation, giving what Hazell describes as a thick, orange coat that is extremely resistant to removal attempts and stays robust against any accidental impact during assembly. Acetone or a similar chemical is applied onto the faces of the module connections to protect against any contaminants that could cause high resistance points to form on the busbars. “Even oils from your fingers can cause high resistance points to form, so the interconnections must be made as cleanly as possible,” Hazell notes. Nord-Lock washers are typically used when bolting the busbars onto the modules. While more costly than other washers, including the far more commonly chosen Loctite washer, Fellten has found the latter to be lacking in thermal performance. “You heat Loctite washers to break them away; that’s the primary issue with how they’re designed. If you do repeated charge-discharge cycles in close contact with them, in which you’re frequently hitting 60-70 C temperatures, with your busbars heating up and cooling down, then there’s a decent probability of your Loctites getting to the point where they soften or give way,” Hazell says. “Nord-Lock washers consist of a two-part, serrated, interlocking design. As you tighten them, they twist and lock July/August 2024 | E-Mobility Engineering A lock pin prohibits the charge port from being pulled out during charging, thus preventing arcing hazards
RkJQdWJsaXNoZXIy MjI2Mzk4