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.

Comments

· · 4 years ago

Fitting batteries into body panels will become far more practical when we finally see solid state batteries . . . which will be lighter, probably not requiring a thermal management system because of better high heat operation and being far less prone to fire or poisonous gas venting if punctured. Agree that a roof is probably the worst location to place a battery, regardless of the cell technology.

Hub motors seem to have made some strides these past few years.
Protean seems to be on schedule for their 2014 launch . . .

http://www.proteanelectric.com/en/

· · 4 years ago

Sure . . . you CAN put them anywhere. But is that a good idea? You have to run wires to everywhere you stick them. And ideally, you want a decent thermal management system which will be most cost-efficient if the battery cells are grouped together. And then there are the safety issues . . . you want them protected from impacts because they are expensive and could short out causing a fire. You also want them low to the ground so that you get a low center of gravity.

So I just don't see sticking them in door panels as a good choice. I think Tesla has definitely done the best job with that battery pack and that really is their core technology. They've them armored, they've got thermal management, they've got them isolated to reduce fires spreading, they've got them low to the ground, and they are even replaceable with a battery swap system!. Tesla took what they learned from the Roadster and built a better system for the Model S.

· · 4 years ago

Its a wonder that none of the engineers at Volvo have thought of any of the issues raised!

I don't know if the difficulties will be overcome, but it is not simply a matter of dismissing the technology with a wave of the hand and generalisms.

Added complexity? Have another look at the video.
One of the early objectives is to do away with the starter motor battery.
Less parts tend to reduce complexity.

Dual use materials, which contribute to structural strength at the same time as storing energy, are inherently efficient - if they can be made to work.

As for the temperature difference, battery chemistries vary enormously in how sensitive they are to it, and also to punctures, impact etc.
One imagines that the Volvo engineers are focussing on chemistries which have good performance in these regards.

I have no idea whether they will be able to make this work economically, but I am glad Volvo are trying.

Give them credit that they are no doubt aware of what the issues they have to solve are, and think that they have solutions, at least potentially.

· · 4 years ago

I think the latest Tesla fire pretty much squashes the idea of putting batteries in any impact zone. Would you put a fuel tank in the door?

· · 4 years ago

The title may be true at this moment in time and given the assumptions that the particular investigators have used, but inventions are not predictable. No knows how to make something work, until the guy that knows how to make it work comes along with the idea that makes it work. I know in this world of controlled thought and managed people my statement may sound weird, but it is the way it works. No matter what the government wants or the fat lazy managers want, inventions come from ideas that cannot be controlled or predicted or managed. Too bad for the people that have scheduled invention, it's doesn't work that way.

· · 4 years ago

Here again, folks, present day lithium ion batteries with liquid electrolyte are a poor choice for in-panel placement. It will be a very different scenario when (not if) we'll get solid electrolyte batteries in a few years.

· · 4 years ago

@Benjamin:
Yep, that is pretty much the way development works.
The guys doing this project will be concentrating on making the idea of combining batteries and structural components work at all, and the type of battery etc comes later.
It is pretty obvious that solid state batteries are on their way, and Volvo are no doubt also involved in research on that.
You don't put the two ideas together until later though, then after that comes the whole process of trying to make your product mass producible and cost effective.

· · 4 years ago

Agreed, Davemart. But until we have cells that aren't so volatile and fragile, building them into a secure case and fitting that case between frame members still seems to be the most logical way to go. I've been hearing about the integration of batteries into panels for years, but it seems to be an exercise in futility until lithium batteries move beyond foil packages that contain liquid, which may burst into flames if punctured and typically require thermal management just retain a reasonable lifespan.

What I'm trying to express here, for those who might not be even be thinking about solid state batteries just yet, is that the idea has merit when we finally see those batteries.

· · 4 years ago

What this thread is ignoring (though the issue was raised in another discussion about this Volvo concept) is the vulnerability of this sort of configuration to collision impacts, and the possibility of some very expensive repairs. Laurent makes a passing reference to a parking lot ding possibly reducing range - but a $3000 bill for that ding is at least as relevant. As some posters have noted, this will become less important with less expensive batteries, but I'm not sure putting batteries in the most vulnerable locations will ever be optimal.

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