This project assumes you already know how to solder and splice wires together. Also, lithium batteries can be dangerous if not handled properly. Go to this page of my other instructable for a detailed warning of everything which could go wrong.
Step 1: PCB
Because Lithium ion batteries are so volatile, a PCB must be used to protect the batteries. Lithium-ion cells can be ruined or even catch fire if they are over charged or if the current limit were exceeded. Their most useful function in my opinion it to prevent the cell voltage from dropping below 2.5 volts. Lithium-ion cells would be ruined if they were allowed to discharge below 2.5 volts. The PCB shuts off the circuit once that lower limit is reached.
If you short the pack the PCB will close the circuit and will not turn back on unless you fix the short and plug the charger into mains power. Also, the first time you put the pack together it may not work until you apply 4.2 volts to the charge/power leads on the PCB. This may be necessary even if the batteries are most of the way charged. Applying 4.2 volts will turn the PCB on.
The PCB protects against shorts coming out of the PCB itself, but it is still possible to cause a short from the wires coming directly out of the cells. The PCB would not save the pack in that instance.
Step 2: Wire Up the Cells
In the picture you see 5 battery holders with 4 18650s each. Later I felt that much juice was overkill so I removed 3 of the packs. That is the beauty of this pack because I can customize it to the capacity I need. Cycling around town I use 8 cells but on a long bike tour I would probably use all 20. All I need to ad or remove cells is a screwdriver.
I separated the battery holders with cardboard and pieces of plastic to make sure they would not short.
Step 3: Charger
12 amp hour pack / 6 amp charger = 2 hour charge time
I cut the alligator clamps off the charger and wired them into the charge leads on the PCB.
Step 4: What Can You Do With This Pack?
Another use is to charge my cell phone. I found a tiny step up switched mode power regulator at Dimension Engeneering. It is perfectly designed to take varying output of a 3.7 volt nominal pack and step it up to the 5 volts necessary to charge a phone. Having a way to charge a phone on long bike tours is very useful. When I am using my phone's gps to navigate the battery is drained in 2.5 hours. With the external pack (with all 20 cells) I can extend that to about 75 hours!
I am going to cycle across the US in the summer and I hope to use the gps logging cellphone app called "My Tracks" to log every mile of my journey. I will charge the pack at restaurants and hostels whenever I get the chance. This is very easy to do because the pack and charger are housed in my removable handlebar bag.