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I like to go cave diving. In cave diving, lights are important, and we carry a bunch of them. The rule in cave diving is to carry at least three lights for redundancy: one primary and two or more backups.

I use these lights as backup lights: https://www.divegearexpress.com/dgx-600-twist-handheld-light

They are ruggedly built, simple, and so far quite reliable. They do take rechargeable batteries. There is debate on whether non rechargeable batteries are better suited for use here, centered on discharge characteristics and the human factors involved in remembering to charge a light that you intentionally never use. Nevertheless I do currently use them. I had the pleasure of meeting Bill Main in the parking lot of Little River springs one day; he uses them as well. Consider that a celebrity endorsement. Bill also uses a straight-through manifold without an isolator valve, but that is for another wiki page.

It's important to know how long your main lights and backup lights can burn for. Therefore, I did some burn tests.