I was looking for a 12V battery system to mount in a kayak to power a fish finder. I found the existing solutions to both too bulky and too expensive for my needs, so I set out to design a battery using 26650 lithium cells.
The 26650 lithium cells have a cell voltage of 3.7V and a capacity of around 5Ah, so if I put 3 of them in series I get a 11.1V 5Ah battery. The 11.1V is just a nominal voltage, the actual voltage will fall between 12.6V when fully charged and 9.8V when fully discharged. This will generally be sufficient to replace a 12 lead acid or alkaline battery. For my application the minimum allowable voltage was 10V.
Bill of Materials
I designed a case to hold the batteries in place and protect them. The case features a set of rails that allow the battery holders to slide into place. A spacer then slides in on top of the batteries to keep them in place. A slotted, sliding top piece goes above that and snaps into place. I also designed an insert to fit into the one blank spot in the battery holder and provide a channel for the wiring.
While designing the part, I added an access hole at the bottom that allows battery holder to be pushed out in the event it gets stuck.This hole features a threaded cap which mounts flush to the bottom surface. Once I refined the dimensions of the model, I found that this hole was not necessary and the batteries would slide out quite easily. I recommend printing the version without the hole in the bottom to save on material and print time.