"But stick the B to a 400V charger and it'll be full in one and half hours."
Yet another journalistic mistake, or did Mercedes decide to add DC fast charging?
GSP
There is not a "400 volt" charging option for the Mercedes B Class. In the USA, it will charge at up to 40 amps of up to 250 volts of AC electrical power (10kW) to the onboard Tesla charger. It's precisely the same charger used in the Tesla Model S, future Model X, Toyota Rav4 EV and even the Tesla produced private network of "Superchargers" charge stations themselves (there are 12 each of these 10kW chargers, 4 in series on each of the 3 phases to produce 120kW total DC power into the battery).
In Europe, Mercedes will likely use a slightly different onboard charger so that it can charge on 3 phase 230 volt AC power with a Menekkes Type 2 plug that is common there, just as the Tesla Model S does in the European market.
Mercedes Benz and all other German auto makers, in addition to General Motors and Ford in the USA, have started a consortium to introduce a competing high powered DC quick charging standard to counter the several other world standards already in existence, and designed to be slower than the Supercharger network from Tesla.
That new German quick charge standard (it is not interchangeable with the USA version) is not going to be an option in the Mercedes B Class ED, however it is optional in the BMW i3 and GM Spark EV. Currently, only about 100 compatible cars and a dozen or so of those actual charging stations exist in the anywhere world.
In contrast, the CHAdeMO quick charge DC standard has 3500 stations, with 1000 in Europe, 550 in the USA, and well over 130,000 compatible cars:
*Nissan LEAF - over 45,000 in the USA and over 100,000 worldwide
*Nissan e-NV200 (coming 2014)
*Citroen C-Zero - not sold in USA
*Mitsubishi i-MiEV - over 30,000 worldwide with its variants C-Zero & iON
*Mitsubishi Outlander PHEV (coming to USA in 2015)
*Peugeot iON - not sold in USA
*Kia Soul EV (coming 2014)
*ZERO motorcycles
*Tesla, except Roadster, with adaptor (available April 2014)
In addition, Tesla also has its very fast DC quick charge network called Supercharger with approximately 500 charging stalls built in the USA and Europe. Tesla has delivered over 35,000 cars.
The final western world quick charging network is used only by Renault and called Chameleon. It can charge from 3 phase AC sockets at either 22kW or 43kW. They have sold tens of thousands of cars.
Unfortunately, since Mercedes will predominately use the B Class ED to meet California Air Resources Board (CARB) mandates for Zero Emission Vehicles (ZEV), they don't need to offer any logical electric vehicle features like quick charging to sell their "compliance vehicle" in the expected extremely low volume just like the following:
Honda Fit Electric Vehicle (EV) and Fuel Cell Electric Vehicle (FCEV), Toyota Rav4 EV and FCEV, Fiat 500e, GM Spark EV, Ford Focus EV, VW eGolf, Hyundai FCEV, Kia Soul EV, and Mazda EV.