Keychain lights

The sort of flashlights that fit right onto a key ring, though some might surprise you for being a little bigger than others. However, others are so small that conventional batteries are not able to fit in them.

This is where a majority of the button cell flashlights go, as those are compact enough to fit, and power, these items. Having one of these is a sure way to not be without a light source, no matter how long you need it.