At the heart of the electric version of the B-Class is a lithium-ion battery pack that can hold 28 kWh, enough for 200 kilometers of driving (115 miles). The maximum speed is electronically limited to 100 mph (160 km/h), which should be plenty. The B-Class Electric Drive can be recharged at any standard domestic power outlet. In the USA, the charging time for a range of about 60 miles (100 kilometers) is less than 2 hours at 240-volt/40 Amps, but Daimler doesn't say how long for a full-charge (hopefully it'll be compatible with fast-charging station standards).
As you can see, the battery pack is in the vehicle's floor; this saves space, and gives the B-Class Electric Drive a very low center of gravity.