This instructable will cover how to build a multiple cell battery from rechargeable 18650 cells. These kinds of cells can be found inside laptop batteries, in particular the ones marked as Lithium Ion (or Li-Ion). I won't cover how to get at the cells, since not all batteries are the same, and there is the chance of something bad happening (shorting out a cell or puncturing a cell are the main concerns) if due care is not taken. But, assuming you've managed to get a hold of some, here's how you can make up your own multi-cell battery pack. I'm building a 2 cell pack, but this method will work for larger packs, you just have to use a larger balance cable.

Supplies

Tools required:

Soldering Iron
Solder
Wire cutters and wire strippers
Hot glue gun
Helping hand/third hand (to hold stuff steady while soldering)

Supplies required:

18650 rechargeable lithium ion batteries
Appropriate balance cable (this cost me $4)
Battery connector (I didn't have to buy this, but is only a couple of dollars if you need one)
Heat-shrink tubing
Insulation tape

Step 1: A Bit of Theory First...

In order to make a 2 (or more) cell battery pack from 18650 batteries it is necessary to connect them in series with each other, so that their voltages add up. Wires will be added at each end, with an appropriate battery connector attached to them to allow the new pack to be used (please ignore my kludged together battery connectors in this instructable and use appropriate connectors). That gives us a nice battery pack, but if we try and charge it, we will almost certainly end up damaging the batteries. This is because the batteries are unlikely to be the same voltage, so we risk over charging one battery to get the other fully charged.

To fix this problem we need to attach what is called a balance cable to the battery pack. A balance cable simply has a connection running to the positive end, the negative end, and each join between cells in the pack. This allows each cell in the battery to be charged independently, so all can be fully charged without risking overcharging any cells.

Step 2: Prepare the Batteries

So, now we know how a battery pack is built up, let's get cracking and prepare our batteries. First of all you need to decide if the cells will be placed end to end (great if you have a long, narrow space they will be going in) or side by side. I chose to have my cells side by side, because they fit the available space I had the best that way. If you want to have the pack constructed end to end, simply follow these directions, but don't hot glue the cells together, and they can be straightened out to be inline.

The batteries need to be lined up beside each other, so that the positive end of one battery is next to the negative end of the next (see the first picture for this step). To stop them rolling everywhere, and to give the completed pack a bit more strength I used a small dab of hot glue to stick the batteries together. The ends of each battery now needs to be tinned* with solder to allow wires to be soldered to them. Strictly speaking this is not a great thing to be doing, because the heat can damage the batteries, but they didn't cost us any thing, so it's not a huge deal, right? I found that the solder did not want to stick to the battery terminals at all. The best way I found is just to keep adding solder until it sticks.

* Tinning is the process of coating a wire/connection with solder prior to joining to make the joining process easier. Two tinned wires can simply be held together, and the solder heated with the soldering iron until it melts and the wires join together.

Step 3: Start Attaching Wires

To attach the wires they also need to be tinned. The first wire I attached was the centre wire of the balance cable, which attaches to the join between the batteries. After thinking about how best to join the batteries and add the wire from the balance cable, I hit upon the idea of simply attaching a couple of short wires to the balance wire, which I then soldered to the batteries. I put a length of heat-shrink tubing over the join to prevent short circuits. Check out the photos for more detail.

Having attached the middle wire, I ran it up the gap between the batteries to the other end, where the other balance wires will connect along with the battery connections. I cut the other balance wires to an appropriate length, and then soldered them to the battery wires. In my case the red balance wire gets soldered to the red battery connector wire, and then soldered to the positive end of the battery pack. The black wires get soldered together, and then soldered to the negative on the end of the battery pack.

Step 4: The Final Step

At this point the battery pack is almost finished, it just needs some insulation to prevent short circuits. I used two narrow strips of gaffa tape to hold the batteries together. I then wrapped a length of insulation tape over then exposed ends of the batteries to stop short circuits from happening. Finally I wrapped a layer of insulation tape around the sides of the battery pack, covering everything up. The battery pack is now finished, and can be charged and used.

I hope you found this instructable useful, and can use it to save a few dollars. The only parts I needed to buy for this project was the balance cable (cost me a whopping A$4). Since a LiPo battery with a similar capacity would cost around A$20 I've made quite a saving.