I think the single biggest reason for following this instructable to the T is because by the end you will have a self contained pack and charger which completely fits into this project box. Dimensions are 6x4X2 inches. Here is the name of the project box on ebay
aluminum project electronic enclosure box 6 x 4 x 2
Since I started working with battery packs I have learned that there is a lot of math involved. It's only 4th grade math mind you, but very important. All through the instructable I explain the numbers involved so you will be better equipped to make your own pack.
Step 1: The Math and Other Important Details
"Watt Hours" means how many watts can be drawn from the pack for how many hours. A single 18650 cell is 10 watt hours. You can draw 10 watts from one for 1 hour or 2 watts for two hours. My pack is 140 watt hours so you could draw 20 watts for 7 hours. There is an upper limit for power drain. If I were to draw more than 140 watts / 28 amps from the pack I would be at the maximum current load for the pack. Before I got anywhere near that though the pack would shut off because the PCB only allows no more than 8 amps to be drawn. This pack is designed to power low current devices for a very long time.
Amp hours and watt hours can be converted back and forth using the formula Volts x Amps = Watts. If you have a 10 amp hour battery you can convert it to watt hours by multiplying by volts. 10 amp hours X 3.7 volts = 37 watt hours. If you have a 64 watt hour hour battery you divide by nominal voltage to get amp hours. 64 watt hours / 14.7 volts nominal (a 4S pack) = 4 amp hours
Nominal voltage means average voltage. A 1s battery configuration, as in this instructable, varies from 4.2 volts (charged) to 2.5 volts (discharged). 3.7 is the average voltage. In a 4s configuration the voltage varies from 9.6 to 16.8 volts (14.v volts nominal.
18650s come in different capacities for several reasons.
1. It is cheaper to make cells of lower capacity
2. The older it is, the less advanced at the time was the technology to make them
3. By the 200th charge cycle, an 18650 only hold 80% of its original charge.
New 18650 are 2.6 amp hours / 10 watt hours. I will use those numbers when referring to the capacity of my pack for the rest of this instructable for simplicity's sake. Remember that these numbers are for best case scenario.
Can you mix cells of different capacities? You are not supposed to but it is okay. In a perfect world you would use cells all of the same manufacturer, same age, same capacity and you would get a higher capacity battery. Since this project is about doing it yourself for low cost, then it is okay to mix capacities. 14 new 18650s would cost 140 dollars. I got all mine for free. The average amp hour capacity of the individual cells in my pack is 2 amp hours. That gives a 14x2=28 amp hour theoretical maximum capacity of my pack. i drained the pack using a watt meter to measure the actual capacity which turned out to be 21 amp hours or 75% theoretical capacity which I was satisfied with.
Step 2: Cells and Charger
New 18650s at Batteryspace.com
I am using a 3 amp 4.2 volt charger from batteryspace. The battery pack is 28 amp hours so at 3 amps it takes 9 hours to fully charge. It would be more practical to use a larger charger here. 18650s can be safely charged at up to 1C. C means rate of charge. A single new 18650 is 2.6 amp hours and so could be safely charged at a maximum of 2.6 amps (1C). Since the whole pack is 28 amp hours it could be safely charged at up to 28 amps. That would be a little scary if you ask me. With the charger I have the pack is charging at .1C
There is also a 6 amp charger on batteryspace but it is 3 times the size of the 3 amp charger. It would not have fit in my project box. It will still charge completely overnight.
Step 3: PCB
Here is the PCB I used.
The PCB is simple to wire up. There are only 4 contacts on the board. 2 are for pos and neg from batteries and the other are for pos and neg to the charge/discharge terminals. Charge the battery though the PCB to lower the risk of oversharging. This is unlikely beause the smart charger automatically shuts off at the correct voltage. I soldered the charger to the charge/discharge terminals.
Step 4: What Do Can You Do With This Voltage Range
1. The battery pack is less complex so you can make it smaller, and cheaper
2. You don't need a balance charger
3. New advances in DC boost converters allow you to step up the voltage to more usable voltages.
4. At a range of 2.5 to 4.2 volts it is easier to charge with low voltages produced by solar, wind, USB, or bike dynamo voltages
Step 5: Light
Normally this light is powered by 1 18650 and lasts an hour. With 14 cells in parallel it lasts 14 hours at full brightness.
Step 6: Charge Cell Phones and Other USB Devices
LM2577 Boost DC-DC Voltage Step-up Power Converter 3-34V to 4-35V
DC-DC Boost Buck Converter 3.5-28V to 1.25-26V Step Down Step Up Voltage Module
The first one is 2 amps and the second is 1 amp. Because the capacity of this pack is so large you should get the highest current DC boost module you can find. As long it can boost voltages at least as low as 3.5 volts (the lower limit of lithium ion) it will work. Most of the time DC boost converters are 80 percent efficient at converting 3 volts to 5 volts. That is a little inefficient but because of the size of the battery and the small current draw of USB devices it doesn't really matter.
Consider a cell phone for example. My cell phone is 5.5 watt hours / 1200 milliamp hours. I have used watt meters to determine that my cell phone draws 2 watts while charging. Because the DC boost converter is 80 percent efficient it draws 20% more than 2 watts which is 2.4 watts. The battery pack expends 20 percent more energy than my cell phone's 5.5 watt hours to charge which is 6.6 watt hours. The battery pack is 140 watt hours divided by 6.6 watt hours is 21 times I could charge my phone.
However, because my batteries are old and because actual battery capacity is always less then theoretical capacity I should really expect 75 percent of theoretical which is 15 times I could charge my phone.
I epoxied a 4 port USB hub to the outside of my project box. I am using a 2 amp boost module. Standard USB power is 500 milliamps so this boost module could power 4 USB devices simultaneously. This way I could charge my cell phone, gps device, camera, external speakers, all at once.
Most DC boost modules have a lower limit of 3.5 volts and lithium ion batteries can be discharged down to 2.5 volts. That looks like it would be a problem but it is not. 95 percent of the capacity has been used by by the time the voltage dips below 3.5 volts.
Well there you go. Feel free to ask if you have any questions.