Abstract: A pressure ventilator of a type carried by firefighters and rescue workers to a burning building to provide either positive or negative pressure ventilation is characterized by a modular structure in which one module comprises a fan and electric motor unit mounted together in a frame on wheels with a handle and another module comprises a battery pack, motor controller, circuits, and connectable electric cables mounted together in a carrying case. The ventilator module has a carrying structure for mounting and holding the battery module, and the battery module has enough capacity to drive the fan in the ventilator module for at least twenty minutes without falling below a minimum voltage threshold. The motor controller in the battery module compares actual voltage to the threshold and disconnects the batteries from the motor in the ventilator unit when the actual voltage falls below the threshold.