12V Battery Box for 26650 Lithium Ion cells

thingiverse

I was searching for a 12V battery system to install in a kayak to power a fish finder. The existing solutions were too bulky and too expensive, so I decided to design a battery using 26650 lithium cells. The 26650 lithium cells have a cell voltage of 3.7V and a capacity of approximately 5Ah. Placing three cells in series gives me an 11.1V 5Ah battery. The nominal voltage is 11.1V, but it will actually fall between 12.6V when fully charged and 9.8V when fully discharged. This setup should be sufficient to replace a standard 12 lead acid or alkaline battery. In my application, the minimum allowable voltage was 10V. I purchased [these 26650 batteries on Amazon](https://www.amazon.com/TUOFENG-rechargeable-Applicable-Flashlight-Household/dp/B0773BYHWY/ref=sr_1_7?crid=ID9AKAIX45KX&keywords=26650+battery&qid=1552760480&s=gateway&sprefix=26650%2Caps%2C142&sr=8-7) for $27 per pack of 4. I also purchased a pair of [battery holders from Amazon](https://www.amazon.com/BCP-Plastic-Double-Battery-Storage/dp/B074GVPWSH/ref=sr_1_11?crid=1XHG3BLF897WP&keywords=26650+battery+case&qid=1552759602&s=gateway&sprefix=26650+battery+%2Caps%2C135&sr=8-11), which can be attached together using a pair of 10mm M3 bolts. I designed a case to hold the batteries in place and protect them. The case features rails that allow the battery holders to slide into place. A spacer then slides in on top of the batteries to keep them secure. A slotted, sliding top piece goes above that and snaps into place. While designing the part, I added an access hole at the bottom for emergency situations where the battery holder gets stuck. However, after refining the dimensions, I found this hole was unnecessary and the batteries would slide out easily.