You are refering to emergency lighting, to illuminate the egress path to the public way. Commonly via battery, some on a emergency generaator, occasionally both.
Keep in mind the OP is in Canada,
As the hall is described as a 'back door' it may be considered as a 'exit' which would require an exit sign & em. Lighting. Depending on the use/occupancy general illumination may be required. (USA general regs)
Battery units are powered 24/7, may be on the nite light circ., may be on the local lighting circuit before switch, or may be on a dedicated circuit if 3 or more lighting circuits in one open area.
I do not know the Canadian codes, but they may be similar.