Battery Capacity of Mercedes EV is 67% Bigger than for BMW i3

By · July 18, 2014

BMW i3 and Mercedes B-Class Electric Drive

The two newest electric cars on the market—both luxury models from Germany—make for a fascinating study of contrasts. The BMW i3 and Mercedes B-Class Electric Drive carry nearly identical price tags of $42,300 and $42,400 respectively. They are both beautifully designed and well engineered. But it’s their differing approaches to overcoming limitations in driving range—arguably the biggest limiting factor for battery-powered cars—that is most instructive for the future of EVs.

The official EPA range estimates hide these essential differences. The window sticker indicates a driving range for the BMW i3 at 81 miles, while the Mercedes B-Class’s driving range this week was pegged by the EPA at 85 miles. That’s a negligible difference. Unfortunately, the estimates are wrong.

Efficiency Is Good

It’s well known that EPA efficiency and range numbers are consistently misleading. For electric cars, I look past those numbers and instead laser-focus on the size of the battery pack. I use this rule-of-thumb: every kilowatt-hour of capacity provides between three and four miles of range.

Great aerodynamics and lightweight materials—and a gentle touch with the accelerator pedal—can mean four miles or more per kilowatt-hour. Drive like a demon in a heavier blunter car, and efficiency drops closer to 3 miles/kWh.

The size of the BMW i3’s battery pack is commonly called out as 22 kilowatt-hours. Actually, we should focus on the “usable” portion of the pack—because engineers don’t allow the full potential of energy storage to be depleted. (Running a battery pack down to zero reduces the longevity of the battery.) The usable capacity of the i3’s pack is 18.8 kWhs.

Tom Moloughney, who contributes to this site and puts a lot of miles on his i3, tells me that he gets about 80 miles per charge in his car. (That’s purely on electricity, and for the sake of simplicity, let’s leave aside the issue of range-extending gas engines.) Eighty miles from the i3's pack represents phenomenal efficiency. In fact, it breaks my rule-of-thumb because it means the i3 is capable of closer to 4.5 miles per kilowatt-hour in the pure EV version of the car, which goes further than 80 miles purely on electricity.

This extraordinary level of efficiency—and the use of a relatively smaller pack—is only possible because BMW built the i3 from the ground-up as a dedicated EV with superior aerodynamics and lightweight carbon fiber materials.

But Size Counts

Mercedes, on the other hand, used an existing platform for the B-Class Electric Drive. Nonetheless, it’s small and efficient—although not groundbreaking. However, the Mercedes EV has a significantly larger battery pack: It’s 64 percent bigger!

Michael Schweizer, program manager for e-mobility at Mercedes-Benz USA, informed me by email yesterday that the B-Class Electric Drive’s total battery capacity is 36 kilowatt-hours. The number usually ascribed to the B-Class is 28 kWhs—and that’s what the EPA used for its 85-mile driving range estimate.

Yes, under “normal” charging, the Mercedes EV fully charges to 28 kilowatt-hours. But when the “range plus package” is employed, it pushes utilization to about 31.4 kilowatt-hours. Using that number compared to the i3’s usable capacity of 18.8, and the delta between the two is actually 67 percent.

Those 31.4 kilowatt-hours, with a mid-range level of efficiency of 3.5 miles/kWh delivers a genuine 100-mile EV—to be exact, 110 miles. Compare that with the superlative efficiency of 4.5 miles per kWh on the i3. Its18.8-kWh pack yields an impressive 85 miles of range.

Yet, all things being equal (including final price of the car), it looks like a bigger battery strategy wins for total range. I know that all things are not equal, just as I know that EV drivers love a good argument. In fact, the idea for this post was inspired by a discussion I had this week with Martin Eberhard, founder of Tesla, who said that nobody except Tesla is building EVs with big enough packs. Okay, let the arguments begin.

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