There are many Instructables on how create larger batteries from 18650s but I wanted something simple, cheap and practical. I use harvested 18650s from old laptop batteries (I work in schools and it is disgusting how much 'redundant' tech they throw away - but that's another story), I use them in various little projects but I'm always looking for ways to combine them into larger batteries.
There are many Instructables on how to create large batteries with battery management systems with all sorts of bells and whistles, I just wanted larger batteries for things like LED lights and such.
I use an 18650 charger not dissimilar to this one made from some super cheap TP4056 boards so I don't really need a BMS for what I want. It all started when I had several batteries knocking about my workbench and needed somewhere to store them. I noticed a discarded length of plastic tube that they fitted snugly inside, and that got me thinking.
Step 1: Time to Get Tubular
I popped down to my local screwfix and bought a length of 21.5 mm Overflow Pipe for the princely sum of £1.49 and set to work. Make sure you get the right tube because I inadvertently bought some 20mm conduit which is too small, I'm going to use it one day though I'm sure.
Step 2: Size Matters
I wanted to combine three 18650s together to get a roughly 12v battery. Because my batteries are harvested their ends are a bit uneven due to removing the spot welded tabs. To ensure I get a secure connection I used some little Neodymium magnets, I needed four, two to go inbetween the batteries and one for each end. You can get them from Ebay or Amazon pretty cheaply.
I measured the length and decided I needed about 205 mm. I used a tube cutter but you equally use a hacksaw or sharp blade to cut the tube to length.
When you slide the batteries in you want them to line up neatly at the ends with just the magnet outside - check the picture.
Step 3: Please Recycle Me
Now this bit is a stroke of luck. I was trying to work out different ways of fitting a cap to the ends, I looked at commercial fittings and I even considered 3D printing something. As it turns out, most fizzy drink bottles have an opening that fits tightly over the tube. Not all bottles are the right size, for a while I was this crazy guy with a short length of plastic tube going round shoving it into bottle tops.
In the end I raided my neighbour's recycling bin and found the mother-load of cola bottles. I obeyed the command on the lid and cut the top off the bottles. I then trimmed off any excess until I had neat little screw caps. I even went for Black and Red for the different polarities - I know, pretty cool.
A soldering iron was used to melt a suitable sized hole in the lids. I used some M4 machine bolts to create the connections.
Step 4: Getting Sticky
I put a layer of SuperGlue over the end of the tube and slid the bottle top on. I twisted it around to get a strong bond. You want to make sure that the bottle overhangs the end of the tube by about the depth of a magnet - check the photo. I then put a magnet on the screw head of the machine bolt.
I repeated this at both ends.
To make sure I was getting a secure connection I put the batteries in, screwed the caps on and pushed the whole lot together whilst taking a reading with a multimeter. I even went crazy and wrote on the tube with a Sharpie so that I'd know what I'd created. Once the glue was cured I had myself a 12v battery that, when needed, I could replace the internal cells.
Step 5: Final Thoughts and Disclaimers
I'm assuming that because you're reading an Instructable that you are a tinkerer and that you can appreciate the rough and ready quality of my build. I'm also assuming that you can see potential uses for multi-cell 18650 batteries that are easily swapped out.
The Tube Batteries are easily connected together if you need additional capacity or voltage and I'm sure you can think of many ways of using them.
Obviously 18650s have to treated with respect as they have been known to burst into flames - I have actually had one do that when I badly shorted it so it isn't just anecdotal. That being said, I learned my lesson and since then have used many harvested batteries without incident.
One last point, don't try and go through any security checks or board airlines with them as they do look suspiciously like pipe bombs.