Do you own a C-Cell flashlight?
 

My kid gave me a cool Camo LED flashlight for Xmas 2015 and the old batteries
have finally gotten too weak to be useful.. (They weren't even alkaline cells).

Since I use a lot of rechargeable nimh AA cells, I decided to use those in my
2 C-Cell flashlight. So, I was about to order some plastic adapters, when
I started reading the reviews.. Some were not good..

So, I used the plastic nozzle parts off an old CPAP mask hose and made two
AA battery holders. AA cells work great in this low current application..

I think you could roll up thin cardboard around AA cells, to form an adapter
for C cell flashlights..

Even if you are going to use plain AA alkaline cells to replace larger C-cells,
it might be worth your time. C-cells prices are going out of sight..
AA cells are pretty good for low current LED lights.. :)

Cheers,
Rich

http://ecorenovator.org/forum/attach...-p1040727s-jpg

Been doing it since I was a kid. Nowadays it's laptop batteries. The vast majority of them are filled with 18650 lithium cells. These put out roughly the same voltage as two c cells, but have way more capacity. There is one caveat, though: battery management. Many 18650 batteries have internal battery management, many don't.

Thanks Jeff, that reminded me of the laptop battery that failed on my kid's laptop.
She got a new laptop and I got her old one. Got a pack from Amazon that had more mAh..
Anyways, I still have the old battery.. I'll have to see what's going on inside it.. :)

The battery banks in laptops have a dedicated BMS board inside. It is classified by the number of series-connected nodes (3s, 4s, 5s) inside the pack. Most boards are useless for repurposing due to the fact that they are married to the laptops they serve.

Lucky for us, there are a bajillion generic BMS boards out there. You can get them long and skinny, postage stamp rectangular, round wafers, flat like paper, whatever. All the way from a single cell (1S) to 72 Volts (20S) are pretty common. I like the round wafers for single cells because they stack on top of the cell and don't take up much space. They sell on flea bay for a dollar alone and about two dollars in a kit with some heat shrink tubing, insulation, and wires.

For size reference, the Li-Ion batteries' numbers tell what size it is. A 14500 battery is the same size as a AA cell, and measures 14mm in diameter x 50 mm long. A 25500 is the same size as a C cell (25mm x 50mm). An 18650 is skinnier and longer than a single C cell.