The official explanation runs like this:
The passive features that increase range should be standard in all models, we think. But we were more curious about the battery charge situation. How do you increase a maximum? And is it a good idea to do so? The configurator includes this disclaimer, after all:
A suite of options to further increase your driving range includes added insulation of the doors and roof for to increase climate-control efficiency, along with an electrically heated windshield and a range-extending charge function. By pressing a button on the console prior to charging, the maximum charge level for battery will increased for the next charge cycle. The higher-capacity charge can provide up to 17 additional miles of range.
How much is excessive? We investigate below.
Range extender should only be used on a limited basis, and could shorten battery life if used excessively.
First, let's understand what this "temporary range extender" is all about. The B-Class ED has, according to the specs, a 28-kWh battery. But Terry Wei, from the Mercedes-Benz USA product and technology communications department, confirmed to AutoblogGreen that the B-Class ED is actually hiding a 36-kWh battery, but the automaker is calling it a 28-kWh battery because that's how much energy capacity is used in day-to-day use. Most automakers publicly claim the actual capacity and then admit they use a percentage of it. The Chevy Volt, for example, has a 16.5-kWh battery pack, but a "full charge" only fills up around 65 percent of that.
The B-Class ED has, according to the specs, a 28-kWh battery.
In the B-Class ED, the 28 kWh of useable energy provides an EPA-certified 87 miles of range. But, since there are eight kWh of reserve, the temporary range extender (we think of it as a software update accessed by a button) can access some of that and offer the aforementioned 17 miles. Now that we know what we're dealing with, this reminds us of an evolved version of the "remote wireless charging" feature that was touted in the Reva EV.
Wei said that the reason the feature is optional is because Mercedes doesn't think most people will need it. Eighty-seven miles is plenty for your average EV driver, but when you want to have 100+ in the tank, you can. It will take an extra 1.5 to two hours to charge to the new "full" on an EVSE with between 24-40 amps, Wei said, adding that, "Basically what you are doing is using, early in life, any excess capacity in the battery," so the efficacy of the temporary range extender may change over time. For all these reasons and more (the EPA tests EV range using default settings, for example) the car's official range will remain at 87 miles.
Now, is this a good idea? Apparently, yes. The "limited basis" mentioned in the disclaimer might even be more than you think. "We don't recommend that folks do this all the time," Wei said, "and we can't really make a suggestion as to how often." But it sounds like using this feature pretty regularly, even once a week, will not be all that damaging to the battery pack. So, in our mind, if you're already paying $41,450 for the B-Class ED, then another $600 for some extra range when you need it seems like a easy addition to the options list.
Head on over to the configurator to see for yourself. The range extender is in the Performance & Safety section.