I find the Photon is the perfect keychain light. It does not weigh much, and is small enough to disappear on keychain. I carry two spare batteries in my wallet and are much smaller than trying to carry a spare AAA.
I also EDC a L1P next to my wallet and use that more than the photon. I do carry a spare for that as well.
Great combo if you ask me.
Ready
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"Do what you can, with what you have, where you are" Theodore Roosevelt