Abstract: A mobile power supply housed within a wheeled trailer which can be towed to various locations, particularly remote locations, for providing electrical power as needed for operating equipment or for charging of batteries or other electrical supply devices at such remote locations. The apparatus includes a unique trailer design including venting means for preventing the accumulation of hydrogen normally associated with high power battery operation, particularly charging thereof and therewith, by monitoring hydrogen at all times within the trailer interior and selectively passively and actively venting the interior environment when hydrogen levels are high or whenever necessary. A unique roof design is included for gathering hydrogen near the upper portion of the interior roof construction adjacent to a venting outlet to facilitate expelling thereof.