Two verified functional spare independent light sources should be carried and accessible to exit the cave in case of primary failure. Since Cavers visit multiple caves during an average caving day, lighting safety issues must be considered prior to each cave entry. Adequate supplies must be carried to support the lighting requirements. If you remove your pack for any reason you must insure that your spare lighting sources requirement are not compromised as a result. Fresh new batteries in a pack sometimes are not.
Verification is critical to insure adequate supplies indeed are adequate.
This can be verified by direct testing with a voltmeter or active lighting load. Your very next step or handhold could be in darkness in case of primary light failure. This possibility should always be ever present in your mind as you maneuver through a cave. Consider how you would acquire your secondary lighting if needed.