I started this project out of a desire to keep my phone working on long bike tours. I needed a lightweight, inexpensive battery to put on my touring bike. Unfortunately, the lithium battery I needed costs 200 dollars new. Add a charger and powersupply and that's another 100 dollars. Batteryspace is my favorite place to get anything battery related online. You can see a comparable battery here . Thanks to some good luck, I was able to cobble together an 8 amp hour battery for about 100 dollars. This project takes a lot of soldering. You don't have to be super skilled; just tin a bunch of wires, and soldering the PCB is pretty easy.
I use this one on my bike for a headlight, tail light, radio, and cell phone charger.
If the batteries are the heart of the Li-ion Rockwell WA3525 battery, then the PCB is the brain. This is the one I used. It was 6.50 at Batteryspace. It was easy to solder wires on the PCB. It is designed to stop solder from spilling onto the rest of the board.
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.
The words Rockwell RW9161 batteries and cells are used interchangeably but the difference is that a cell is the most indivisible part of the battery which stores power and a battery can be made of many cells. This battery is made of 16 cells. The configuration is called 4S4P. That means 4 series and 4 parallel. 1S4P means the capacity is multiplied by four but the voltage remains the same. 4S1P means capacity remains the same but voltage is multiplied by four. 4S4P mean this is a battery 4 times the voltage and 4 times the capacity of a single cell. The newest 18650s are 2.6 amp hour. 4S1P is 16.8 volts and 2.6 amp hour. 4S2P is 16.8 volts and 5.2 amp hour. 4S4P is 16.8 volts 10.4 amp hour. Given that these are used batteries I will give it a more modest 8 amp hour rating. 18650s fall to 80% capacity after 200 cycles. You could test the exact capacity on yours if you had a watt meter.
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.
If you bought a pack off of batteryspace it would be wrapped in shrink wrap and the cells would be spot welded together. This makes them a little smaller and lighter than my battery but they are unserviceable. Professional pack builders use spot welding machines. Here is video of one in action. Here is home made spot welder. You could also try tab welding. Here is a video. This is easier and cheaper than spot welding but not as easy or as safe as Rockwell WA3520 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.
I happened to have a little file bin just the right size for the guts of the battery. You can use anything. I recommend something to keep the batteries still so they don't shake apart. There are 32 places where the batteries are held there by nothing but springs. The wires could shake out of the terminal blocks as well. Screw them in tightly. This is just a small price to pay for serviceability.
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.
The best way I have found for wiring everything together is to use European Style terminal Blocks. As I put the pack together for the first time I wired it together in all kids of upside down and backwards ways before getting it right. Having screw type terminals greatly facilitated my trial and error assembly.
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.
There are two ways to charge a battery like this. You can either apply 4.2 volts to individual cells or you can apply 16.8 volts to the whole battery. While the latter is more simple, in this case the former is better because we are using used cells. When new packs are made, they all use new batteries of the same amp hour capacity. This way they can be charged hundreds of times without going out of balance. However, balancing will extend the service life of any pack. With our pack, the batteries are in various states of wear and amp hour capacity, therefore balancing is a must. A balanced battery is one in which all the cells remain the same voltage.
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.
When I first wired everything together I was worried to find that even though I had everything wired correctly and the batteries were charged, the PCB was not working. I got it to work finally when I applied 16.8 volts to the P+ and P- terminals on the PCB. That's because this PCB was designed to be charged serially, with 16.8 volts. I didn't want to do that for reasons I've already described. You have to trick the PCB into thinking it is being charged through P+ and P-. I used a variable power supply to jump the PCB. I touched the wires to the PCB for just a second and this activated 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).
You hear a lot about C ratings when you start building battery packs. C means capacity and it represents whatever your battery capacity happens to be. If I have an 8 amp hour battery and I charge it at 8 amps, then it is charging at 1C and would finish charging in one hour. If I charged it at 2 amps, the rate of my Hextronics charger, then it would be charging at one quarter C, and would charge in 4 hours. If I drain the battery at 8 amps it will last for an hour; that is a drain of 1C.
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.
To be clear I am not talking about balance charging. This is something else. You will inevitable have one cell which has higher capacity than all the other cells. They will drop in voltage at different rates. One cell will drop below 2.4 volts before the others, then the whole battery will shut down. Once dead, if there is a large difference in voltage between lowest voltage and highest voltage cell, you should take two cells from the highest voltage cell and swap them with two cells of the lowest voltage cell. That way the weakest link will not be so weak and your battery will last longer.
Even better would be to replace the lowest voltage batteries with different ones if available.
If your battery shorts, it will most likely heat up and smell like fried electronics. That's what happened to mine a few days ago. It was an easy fix actually. I needed to more insulation between the stacks of batteries
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.