After experimenting with many different cell configurations, I found a 3.7 volt pack (one without serial cell configurations) to be exceedingly simple and inexpensive. It's more simple because you don't have to worry about different cells draining at different rates. In a 4S pack for example, one cell might drain long before the others, leaving a lot of unused energy in the other cells. You don't have to balance charge with 1S packs. It is cheaper because 1S PCBs are much cheaper. The only drawback to a 1S pack is that the voltage range (2.5 to 4.2 volts) isn't as useful as higher voltages. In spite of this, I have found this pack to be very useful and in this instructable I will show you how to make a pack and what to do with it.
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
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This is the PCB I used from batteryspace. Even the one rated for the highest current is only $2.50. Compare that to the cheapest 5S PCB at $21. Wiring it up is very simple. Leads for the batteries are on one side and leads for charging and for drawing power (sharing the same contact point) are on the other side. I used European style terminal blocks to ensure I would only have to solder onto the PCB once.
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
Even though it is possible to charge many batteries in parallel by charging "through" the first battery, I do not recommend this. It is not dangerous to do so, but what happens is the cells closer to the charger are charged first and then the charger shuts off once it has detected the first cell has reached 4.2 volts. The other cells down the line absorb charge so slowly that they have not charged fully by the time the first cell is charged. Make sure there is a direct electrical path from the charger to each cell.
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
Get a smart charger. This is the one used. It is designed to be used used on packs at least 6 amp hours. My pack is about 12 amp hours. It should take 2 hours to charge.
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?
I use this pack on my bicycle. It powers my bike light and cell phone. The light is a UltraFire 1300 Lumens CREE XM-L T6 C8. It is powered by a single 18650 Lithium ion cell. I realized that I could solder a wire to the positive and negative contacts on the light and run it from an external battery pack. My external pack has 8 18650s in parallel so the light now lasts 8 hours instead of 1 hour on full power.
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.