The global forecast of BloombergNEF expects the 12 million passenger electric vehicles (EVs) on the road to climb to 54 million by 2025. One strategy automotive manufacturers like Tesla, Rivian, Ford, GM and Stellantis are using to reach this level of production is to transition internal combustion engine (ICE) vehicle plants to EV plants. Production engineering and manufacturing line assembly play key roles in this transition.

ICE vehicle plants tend to have similar plant layouts and manufacturing technologies as EV plants. According to automotive industry veteran Sandy Munro, they require about the same amount of electrical power to run equipment, have foam fire safety systems, and have roughly the same plant footprint. Both ICEs and EVs involve hydraulic cylinders and approximately the same number of robotics for assembling vehicles.

One area where EV manufacturing technology is beginning to depart from ICE vehicle manufacturing is body assembly. ICE vehicles tend to have steel or aluminum body panels that are often welded to vehicles. While many EVs have aluminum body panels, the BMW i3 may point to how carbon fiber EVs may increasingly differ from that in the future. “The body [Life Module] build is all glued together. There’s no welding at all,” says Munro.

While much ICE vehicle manufacturing technology is technically capable of producing EVs, the cost of transitioning a plant and the design of the EV can pose challenges for plant conversion. ICE plants build vehicles that have a rolling chassis or unibody vehicle structure. A rolling chassis is assembled by installing the body onto a chassis, whereas a unibody is composed of a structure where the body is part of the chassis. EVs, especially the newest models, tend to have a skateboard chassis—a flat structure with space for a battery pack to which the motors, suspension and brakes are attached.

The skateboard design of new EVs is one reason that some ICE plants are not converted to EV plants. According to Munro, converting an ICE vehicle plant to a plant that produces skateboard-based EVs costs significantly more than shifting to production of unibody-based EVs. The cost of shifting an ICE vehicle plant to a unibody-based EV plant is about the same as refreshing an ICE plant, where changes are made to the body and suspension of an ICE vehicle.

There are other factors too. It would cost carmakers more to convert large production capacity plants to EV plants than it would to shut them down because of lower EV production levels compared to a plant’s ICE vehicle production levels.

ICE vehicle plants can be converted into three types of EV production systems. In a mixed production line, a single production line manufactures battery electric vehicles (BEVs) and ICE vehicles in the same plant. Carmakers adopting this model usually face many challenges, including an increase in complexity, more workers on the assembly line, more space requirements, and sequencing problems.

A second type of production system is designed to reduce sequencing problems, which occur when the order of production tasks creates too few or too many vehicles on a production line, leading to situations where workers wait for vehicles. According to a report from Boston Consulting Group, “flexible-cell manufacturing (FCM) replaces the conveyor belts that move one car after another along the same assembly line with automated guided vehicles (AGVs) that transport car bodies individually to the assembly workstations appropriate for that specific model of vehicle.” Since FCM cells are not interconnected, workers don’t need to wait.

A third type of EV production system is composed of an all-EV production line. The EV truck maker Rivian is one example of a company that successfully converted a former ICE vehicle plant into an all-EV production line.

While EVs and ICE vehicles have many similarities, one key differentiator is the battery packs in EVs. Efforts to produce battery cells, assemble them into packs, and install them on EV chassis are all key ways that EV plants differ from ICE vehicle plants.

Producing battery cells starts with sourcing some of the raw materials, including nickel and lithium. The lithium-ion batteries that EVs typically use are manufactured in production plants through the stages of electrode manufacturing, cell assembly and cell finishing. Electrode manufacturing includes using nickel and lithium to manufacture cathodes. After the battery cells are assembled through separation, stacking, winding, packaging and electrolyte filling, cell finishing includes testing, which ranges from pulse tests to leakage tests. Car companies perform simulations and tests on battery designs to validate them. Research and development (R&D) also enters the picture because efforts to improve the energy density, life span and stability of batteries affect the products.

After the cells are delivered to the vehicle plants, the work of assembling battery packs begins. The automakers gather the cells into modules. A battery management and thermal management system are in place when assembling battery packs.

The quantities of battery modules on hand can complicate things for the automakers. A robust supply chain is required for battery delivery, as large inventories of battery modules pose fire risks and the batteries are likely to degrade over time. This makes battery pack assembly something of a balancing act.

According to Munro, there are three ways that battery packs are currently installed on EV chassis. One technique in use for the structural battery packs of the Tesla Model Y is to install a battery pack with built-in seats during the final assembly stage. Another method for structural battery packs introduces the battery to the process earlier by attaching front and rear components including stampings, castings, and the suspension. A third means currently used starts with putting together a front and rear framework attached to a structural case, after which the battery pack is dropped into the case from above.

The production processes of EVs also present challenges when converting from ICE vehicles. Painting is one step that is beginning to change. The unibody and rolling chassis structures of ICE vehicles usually undergo degreasing, phosphate coat dipping, rinsing and draining to clean the steel and prepare it for paint adhesion. The BMW i3 illustrates how this may change in the future. Comparable to a unibody, the BMW i3’s Life Module is made of a carbon fiber composite that is not painted. Instead of painting the underlying structure, the thermoplastic skins covering the doors, fenders, bumpers and hood are painted. The process of preparing these plastic materials for paint differs from that of steel. The skins are flame treated for paint adhesion.

The process of assembling vehicles is also different because the process for a rolling chassis is typically different than it is for the unibody and skateboard structures of EVs. According to Munro, for ICE vehicles based on a rolling chassis, the suspension, brakes and wiring harness are installed on the chassis relatively early in the manufacturing process. Engines are also sometimes installed early because the engine compartment can become crowded with parts. “And with a rolling chassis, you don’t see the body until further along,” says Munro.

The vehicle bodies can be different too. Compared to rolling chassis vehicles, vehicles with unibodies tend to have the remaining body parts installed earlier along with skateboards.

The final assembly also departs from ICE vehicles. According to Senior Analyst of Product Lifecycle Management and Engineering Design Tools for ARC Advisory Group Dick Slansky, some Siemens controls and automation engineers expect the process, automation and controls of EV final assembly to be very different as a result of the power electronic components that aid battery packs.

Converting ICE plants to EV plants will also require different labor arrangements in plants and companies in their supply chains.

Research from BCG calculated and compared the labor hours of manufacturing a midsize ICE car in comparison to a midsize EV car by looking at components, drivetrain assembly and installation, battery manufacturing, press, body, and paint shops, and vehicle assembly. BCG concluded that current EV labor requirements are approximately 1 percent less than those for ICE vehicles.

While Munro expects layoffs to result from this transition, Slansky doesn’t foresee job losses for plant workers. According to a report written by Slansky, layoffs are “highly unlikely as the global automotive industry makes the historic manufacturing transformation to electric transportation and mobility. They will require a fully engaged and retrained workforce to make this transformation.”

Slansky expects plant workers with experience in powertrain assembly and final assembly to be retrained on electronic component assembly and new quality assurance requirements.

Munro doesn’t believe the impact on supply chain jobs will be uniform. “They don’t know how they’re going to get stuff done fast enough. The people who are making circuit boards or wire connectors—they’re definitely happy as clams and trying to figure out how to ramp up as fast as they possibly can.”