Rechargeable LED Battery Pack(s)

I am just wondering with the various types of rechargeable batteries available for LED lights, is lithium-ion clearly the way to go, or are there any reasons to select either nickle cadmium or nickle metal hydride?

It depends on the application. If you want to use a device for fifteen minutes each morning and leave it plugged in the rest of the time (like an electric shaver, for example), NiCad is the way to go. If you want long runtimes and a safe chemistry, try NiMH (there are a few models with low self-discharge for use in infrequently-used devices). If you want bigger current, voltage, and risk, look for info on Li-Ion.

Let's use the Fenix L0D-CE (a AAA LED light) as an example. With NiMH, you get really solid runtime with good output. With a Li-Ion of the correct size, you'll get enormous output, minuscule runtimes, and the responsibility of watching your cells while they charge (just in case).

You didn't mention this chemistry, but if you're building a bike light or something like that, you can use sealed lead-acid (SLA) batteries. They're kind of heavy, but they're simple, compact, and not too expensive.