Rimac is best known for the Concept One and C_Two electric supercars, but founder Mate Rimac views his firm as a technology company first, and as an automaker second. So Rimac has farmed out its expertise to other automakers—including Koenigsegg.

In this video from Apex One, Rimac explains how his company designed the battery pack for the Koenigsegg Regera hybrid supercar. The main challenge was squeezing the power Koenigsegg wanted into a tiny space, he said, calling the Regera unit the world's highest-density battery pack.

"Our guys were fighting with the Koenigsegg guys over every millimeter, because the package was so tight," Rimac said. Engineers even had to make an indentation to allow for throttle-pedal travel for the front-mounted pack.

Rimac does not manufacture its own battery cells; it assembles commercially available cells to make battery packs. Some cells are better at producing power, while others have greater energy density, meaning they can store more energy in a given volume. The choice of cell depends on the project, and what element, from power to range to longevity, is considered most important.

Koenigsegg Regera

Koenigsegg Regera

In the Regera, the battery pack feeds a trio of electric motors, which produce a combined 700 horsepower. They work with a 5.0-liter twin-turbo V-8 to produce a combined output of 1,500 hp and 1,475 pound-feet of torque.

Gasoline and electric power are juggled by Koenigsegg's Direct Drive system, which replaces the transmission and thus avoids the inherently high energy loss associated with one. Instead, the system uses a single-speed reduction gear for the V-8, and a hydraulic coupling that connects it to the rear wheels. The Regera is able to operate mostly on electric power at speeds up to 30 mph, and reaches a claimed top speed of 248 mph after the gasoline engine kicks in.

In addition to Koenigsegg, Rimac will provide the battery pack for the Aston Martin Valkyrie hybrid. In 2018, Porsche bought a 10% stake in Rimac, while Hyundai confirmed an investment and plans for an N electric sports car the following year. Rimac's next production model will be the 1,914-hp C_Two electric supercar.