A power system that is attachable to a helmet is disclosed. The power system provides power and data connections for helmet-mounted accessory devices and provides power to the accessory devices. The power system includes a base unit that is attachable to the helmet. The base unit includes a processor, an internal power source, and one or more cable interfaces. One or more accessory interfaces can each be attached to the helmet, connected to a cable interface, and connected to an accessory device. The base unit further include an interface for attaching a power module to the base unit. The power module includes one or more removable power sources. The base unit selectively provides, to an accessory device connected to an accessory interface, power from either the internal power source or from a removable power source.