Putting Electric Vehicle Batteries In Car Doors: Possible But Not Practical

By · October 23, 2013

Fitting battery cells into a carbon fiber trunk lid

Fitting battery cells into a carbon fiber trunk lid

Volvo last week unveiled an electric car prototype, in which its battery cells are embedded in the body panels. Nobody knew this was possible before Volvo's announcement, but there's a drawback to that technology. It adds complexity.

When BMW built its ActiveE prototypes, the base model wasn't conceived for an electric propulsion, so the engineers had to divide the battery into three separate elements: one below the back seat, one in the center tunnel and a third one under the front hood. These cars were hand-built prototypes, but when BMW designed the i3, a true production EV right from the start, it chose to do it with one single battery pack under the floor. That's easier.

Batteries are made of hundreds, or sometimes thousands, of little cells grouped together. Logically, it should be relatively easy to assemble them in several small packs instead of a larger one—but more packs mean more wiring and more casing, adding weight to give proper insulation and controls to each sub-pack.

Three battery packs inside the BMW ActiveE

Three battery packs inside the BMW ActiveE

Working with British engineers, in a team work that involved eight companies, the Volvo researchers were smart enough to avoid the weight issue by using existing parts. Every car has doors, so they fit battery cells inside the doors. The weight penalty is offset by the use of carbon fiber, and the door with a few cells included inside is hardly any bigger than a regular door. Engineers were smart to use very thin pouch cells, although aging may be a worry because that packaging is less robust than classic cylindrical cells. And of course, the batteries get jostled every time passengers slam the doors.

Safety might also be an issue, although Volvo has a stellar reputation in that field. A quick disconnect system can be built in, but even a minor parking mishap where a door get dented could result in reduced range.

Four motors inside the Mercedes SLS AMG Electric Drive

Four motors inside the Mercedes SLS AMG Electric Drive

There's a bigger concern: thermal management. Volvo showed a prototype with cells built into the roof, which I see asa crazy idea. The roof is the part of a car which endures the widest variation in temperature. A roof can receive several inches of snow in the winter, while it's possible to fry an egg on one in Arizona. How are the cells inside expected to cope with that?

Nissan learned the hard way that the heating and cooling of a battery is one of the most important feature of an EV, and that stressing cells with extreme heat will shorten service life. Of course, a system could cool or heat the cells at all times to keep them safe, but that consumes a lot of energy.


Who remembers the first prototypes of the Bolloré Bluecar? It had two in-wheel motors, but when they decided to make a production model, they quickly ditched those, and fitted one single motor. The Exagon Furtiv e-GT and the Mercedes SLS AMG Electric Drive are two technological marvels, with two motors and two gearboxes for the first one, and four motors for the second, but they each cost half a million bucks.

All EVs that sell in decent numbers have the same architecture. One motor and one battery in one single case built into the floor. The future may change that sometimes, but we should not hold our breath for car bodies with batteries squished all over the place. Not when today's architecture are already proven to work well.

New to EVs? Start here

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