Abstract: The present invention is a method and system for powering a patient monitoring system. The present invention is a modular patient monitoring system utilizing a bi-directional power bus to efficiently couple a multi-parameter acquisition device, a display and any number of expansion modules. Each coupled device includes a battery, enabling each device to send or receive power through the bi-directional bus. The method and system of the present invention utilizes power from any single battery or from any number of the batteries in the system, as charge is not transferred from one battery to another. The present invention allows for the batteries to charge when it is operated from AC mains derived power.