Alkaline batteries used in Ni-Cd battery rechargable power outage LED emergency light
Cadmium is obsolete due to toxicity for disposal.

NiCd is 1.2V/cell while Alkaline is 1.5Vcell thus 4.5V vs 3.6. Meanwhile White LED,s are around 3.1V so they have internal resistors selected to limit the current ( and brightness) and extend the duration.

Since the luminaire is obsolete due to NiCd availability, you have nothing to lose by trying it in the dark with no power.  It is safe, but the internal resistors will see a bigger power loss and get hot inside. But this is still very low power, but may fail if poorly designed.

Portable flashlights are cheap now and some come with Lipo cell charger and some using Alkaline.These much brighter than what you have. 

My preference is LiPo hand torch 5W.