I use this one on my bike for a headlight, tail light, radio, and cell phone charger.
Step 1: The PCB
PCBs come indifferent varieties depending on number of cells, voltage, and capacity. Here is a list of all the PCBs you could use with 18650s on batteryspace.
Here are the specs for the one I used and I will explain what everything means...
Overcharge protection voltage for single cell: 4.35V
Over discharge protection voltage for single cell: 2.40V
Over current detection protection: 4-6A
Supply current: Max 30uA
Short circuit protection
Protection circuitry resistance: <=50mohms
The PCB prevents overcharging because the delicate lithium ion chemistry of the battery can be damaged if charged with too high a voltage and the PCB will cut power to the cells if you did so. This should not be a problem if you charge with a smart balance charger. If you charge a cell with 4.2 volts, then the cell voltage will never rise above 4.2 volts, even if you charged the cell for weeks. You still don't want to charge a cell beyond the point at which it is charged. A smart charger will turn off once it has finished charging.
Many batteries can be discharged all the way to zero volts, this is not one of them. If the voltage of a lithium ion battery dropped to zero, or even below 2 volts, it would be damaged, and would never charge back up. Cell phones have this same protection. If you measured the voltage of a "dead" cell phone battery it would probably read 2.5 volts.
Over drain protection is necessary because this is a small PCB with tiny components and can only handle so much current. It shuts down to save itself when drawing between 4 and 6 amps.
Supply current is the current draw from the electronics on the PCB. It is practically nothing and will not drain your battery.
Short circuit protection means the PCB will turn off if it detected a short; if a wire became disconnected or if the wires crossed.
Protection circuitry resistance is the resistance caused by the PCB. All circuitry produces a little resistance. Again the drain is so little you will not notice it.
Step 2: Cells
The cells are a common format, and therefore widely available and cheap, called 18650s because they are 18mm by 65mm. They use 6800 18650s in the Tesla Roadster! My brother works at a university where he has access to an e-waste dumpster. Old laptops are often powered by these cells. Often you can look up the date of manufacture and capacity by looking up ID numbers on the cells. There is no way to tell how many cycles they have been through but the savings are so great over new ones that it is easy to ignore most lack-of-charge problems.
There are 18650s all over the net which claim capacities all the way up to 4.2 amp hour and they are half the price of the ones on batteryspace. The highest capacity 18650s on batteryspace is 2.6 amp hour. I called a tech guy and asked him about these 4.2 amp hour Ultrafire 18650s. He said he never heard of Ultrafire and that 2.6 amp hour cells have been the industry standard for years. I haven't tried the Ultrafire batteries and I am not sure if they would work for this project. You can read more about them on Candlepower Forums .
New cells are 7.25 dollars each new on batteryspace here so I saved 116 dollars by finding used ones. If you can't find old batteries for free you can sometimes find used 18650s on ebay.
Step 3: Battery Holders
Battery holders are a much better option because you can remove cells easily if they die. This is very likely since we are using used batteries. You can get battery holders here . You will need 4 of them. Play around with the shipping options. There are around 10 different ways to ship and I don't remember which is the cheapest.
Solder the contacts in the back together so all the positives are connected to the other positives and do the same with the negatives. In the end you will have two wires coming out, one pos and one neg. There are 4 cells in this holder but it will act as one cell once it is connected to the PCB. This batteries in this holder will be in a 1S4P configuration.
You could use any number of cells provided you have a number divisible by 4. If you had 8 four-cell battery holders and 32 cells you could make a 4S8P pack. If you only had 4 cells you could make a 4S1P pack.
Step 4: Project Box
I used some nuts and bolts to connect the terminal blocks to the project box. Use any size that will fit through the terminal blocks.
Put thin plastic or some sort of insulation between the layers of batteries. The battery holders are covered with pointy metal objects which could cause a short.
Step 5: Wiring the PCB and Balance Leads
You cannot electrocute yourself with 16.8 volts but you could get a burn. You could damage your cells or PCB as well. Take necessary precautions.
This is technically a 14.4 volt battery. It ranges in voltage between 9.6 and 16.8 volts depending on its state of charge. 14.4 volts is in the middle, its nominal voltage. Follow this guide and everything should turn out fine. Once you are done there will still be no power coming out of P+ and P-. We will fix that in step 7.
To be more clear I don't mean individual 18650 cells here. I mean 4 18650s in a battery holder. That is a single cell as far as the PCB is concerned.
Cell 1 + goes to B+
Cell 1 - goes to B1-
Cell 2 + goes to B1-
Cell 2 - goes to B2-
Cell 3 + goes to B2-
Cell 3 - goes to B3-
Cell 4 + goes to B3-
Cell 4 - goes to B-
wires to main power are connected to P+ and P-
To charge the battery you find some 4S JXT type plugs. You can get them here . Put the red wire in at the same place where Cell 1 pos and B pos from the PCB are wired into the terminal block. Wire the rest of the black wires in order to the rest of the cells.
This site is where I got the wiring diagram and shows different ways of wiring of batteries with balance leads.
Step 6: Balance Charging
Let me explain how packs can become unbalanced and why that is really bad. Batteries will inevitably wear our at slightly different rates. Say the voltage of each cell in your pack is 4.2 volts after charging when new.
Cell 1 is 4.2
Cell 2 is 4.2
Cell 3 is 4.2
Cell 4 is 4.2
Whole battery is 16.8 volts
Now say one of the batteries has started to wear out. They manifest their wear by sometimes refusing to charge all the way to 4.2 volts. Now cell 4 only charges to 3.8 volts. The charger doesn't know this but still charges to 16.8 volts and the other cells are charged higher to compensate.
Cell 1 is 4.3
Cell 2 is 4.3
Cell 3 is 4.3
Cell 4 is 3.8
Whole battery is 16.8 volts
You can see how one bad cell could destroy the good ones because the others are overcharged. Balance chargers never charge cells above 4.2 volts and they can tell you if one cell refuses to charge completely. My charger has four lights and shows blue for fully charged and red for not there yet. They make a volt meter here that is perfect for monitoring the health of cells.
Balance chargers are more common in hobby batteries. People use lithium polymer (LIPO for short) batteries to power model helicopters, airplanes, boats, etc. LIPOs are cheaper and tougher, better for surviving crashes. They require more balancing then lithium ion. Some have C rates of 50! That means a 5 amp hour battery could supply 250 amps and completely discharge in only a few minutes! The chemistries are almost the same so the chargers are interchangeable. I suggest looking through Hobbyking so you know what all is out there. They have an especially dizzying array of chargers.
Step 7: Activate the PCB
I discovered I had to do the same thing if a wire came loose and the PCB lost connectivity to one of the cells. This is a fail safe in the PCB. It turns off so there are no shorts. The PCB assumes you must have reconnected the wire if you are attempting to charge the battery. Lastly, you have to jump the PCB if the battery goes dead from running out of juice. If you never let the battery run completely dry and the batteries do not become disconnected then you don't have to jump it at all (aside from the first time).
Step 8: Charger and C Rating
Battery capacity is also measured in watt hours. To get watt hours you use a formula (Watts= Volts x Amps) to multiply capacity (8 amp hours) with voltage (14.4) to get 115 watt hours. It will run something using 1 watt for 115 hours or any other equal ratio. Of course that is its theoretical run time. Actual time will be less. Calculating remaining battery capacity based on current drain is complex and inexact. If you want to learn more read through Battery University .
You will need a way to plug it into something eventually. I used RCA plugs from radio shack. The gauge of the wire and quality of the contacts are more than enough for the 1.5 amps max I draw from my pack. You may want something more substantial for higher draw.
You generally don't want to charge above 1C. If you did the battery could wear prematurely or catch on fire. Also, the maximum continuous current draw possible from this battery is 12 amps. I calculate that from the ratings of a single 18650 which is 1.5C. 1.5 multiplied by 8 amp hours is 12 amps. Of course the batteries could handle this but not the wires. I used small wires in mine because I don't draw very much current from it. For higher drain use bigger wires. Look up a table of wire gauges and current capacity to pick the right wires. Use the smallest wires possible which can still handle the current. Also use threaded wire because it's easier to work with.
This charger is "smart" because it monitors the voltage of the battery as it charges. It would not charge if it detected dead cells (below 2 volts) or if the voltage was already above 4.2 volts.
Step 9: Balance the Pack
Even better would be to replace the lowest voltage batteries with different ones if available.
Step 10: Safety
However, worst case scenario, it could catch on fire when you are not at home and burn your house down. That doesn't mean this is a terribly dangerous project, you just have to be careful. Many projects on this site could end in fires. Only take on this project if you have a good idea of what you are doing and understand the risks.
A simple precaution is to break the battery in while it is in a bucket of sand or on a bit of concrete at least 8 feet from anything combustible. By breaking in I mean charge it and discharge it a few times to make sure it works with no problems.
So there you have it. Once you make one battery you will have the confidence to make packs in any configuration you need.