the opening 6b to define a refilling passage sealed airtight to the interior of the driver 39 s cabin 6 the rubber element 31 has one end thereof fitted on the refilling port 3a and secured thereto by a clamping band 37 thereby the rubber element 31 being rigidily connected to the refilling port 3a in a leakproof manner and the other end thereof including a tubular periphery defining a peripheral groove 31a which is engaged by a peripheral edge of the opening 6b a cover 33 is hinged as at 34 to the side wall 6a to open and close the opening 6b the described construction may be modified as follows the cover 33 may be removably attached to the rubber partition element 31 the cover 33 will serve the purpose if it can close the opening 6b and therefore the opening 6b is named herein an opening having an opening and closing cover the rubber partition element 31 may be replaced by a partition element formed of a synthetic resin material these elements are collectively named a resin partition element herein the lid 15 of the battery case may be attached to the side wall 6a of the cabin 6 to permit access to the battery 13 thus the opening 19 is just called an access opening having an opening and closing lid the battery 13 may not be contained in the battery case 16 but instead may just be covered at inner portions thereof by the cover 17 thus the term cover herein used will be understood to include the.