Rechargable Battery Systems

I need help to choose a battery system for a Cree XP-G flashlight. It's a home made one I'm building and will have a 20 mm reflector and will use a single cell to be compact and bright.

The cell size is either C, AA or something between but I'm totally lost with the rechargeable lithium system.

What do numbers like 1855xxx... mean and what's the advantage of lithium ion, lithium polymer, iron phosphate and things like that?

Who makes reliable good quality batteries?

Thanks!!

I imagine it will be very exciting building a homemade light, without any understanding of the power systems, when that system has the capability of causing pretty fireworks Hopefully, "exciting" in a good way!

The numbers for Li Ion batteries refer to size and shape of the battery. A Li Ion battery that is roughly the size of an alkaline AA is called a 14500: roughly 14 mm wide, 50 mm long, and the final 0 means it's cylindrical in shape. I say "roughly", because the sizes of Li Ion batteries can and almost always do vary from the numbering system, depending on the manufacturer, whether there's an added protection circuit, etc. But, roughly speaking, those are the sizes and shapes. We call it a 14500 rather than a AA because Li Ion rechargeables have different voltages than alkaline batteries, and if you mistake one for the other, bad things can happen if you put a 14500 in a light that's only made for Alkaline or NiMH AAs. Of course, good things can happen too, if you put a 14500 in a light that's made for 14500s. The no-brainer approach, if you're going to be using a Li Ion rechargeable in a small size (14500/16340/etc) is to get an AW Protected battery. There are other options if you go larger, and different chemistries with different advantages/disadvantages if you have some special requirements (e.g., capable of sustaining super high discharge rates), but we can't know if you need those until we hear more about what you're building and how you're driving it.

This thread will probably be moved to the Battery Sub-forum, where you'll get a lot more detail...

Thanks Joe

I didn't know there was a sub battery forum.

I actually build flashlights previously based on rectangular RC "lipos".

They work great but are square shaped and don't look a lot like a normal flashlight.

So this would be my first round flashlight, and it seems confusing because like you said there are so many sizes and they don't necessarily match the standard AA and C size.

Common Li Ion batteries that can drive an XP-G to nice high performance include 14500 (AA sized, roughly) and 16340 (123A-sized, roughly). If you can live with a good deal bigger size, but a lot more *ooomph*, 18650 is another common battery size. No reason to not stick with the more common sizes, for now.

If you're picking your driver carefully, you can probably make a light that will work on a standard C cell battery, and on a li-ion rechargeable 26500. I think I'd be partial to the one-C-cell size, but such lights have proved out of my budget so far, due to limited production runs and small batches.