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.
I use this one on my bike for a headlight, tail light, radio, and cell phone charger.
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Signing UpStep 1: The PCB
If the batteries are the heart of the Li-ion 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...
Electric performance:
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.
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...
Electric performance:
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.
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In short, if I want to build a battery that can power a 72 volt motor, what gauge wire should I use when wiring up my parallel/series pack?
i had 2s4p and found that they was one bettery not charging right when i stripped the pack the others were higher then they should of been the dead one wasent charging to 4.2 but maxed at 4v the other were at 4.4 after the unit was broken down
it's bigger but it seems worthwhile if you're going to be charging a laptop, which can draw more than 5 amps.
how do you set up the charger to use one lead per cell? I only see one lead for four cells?
I HAVE found some 4200mAh rated 18650 cells, but they were Ni-MH(like here http://www.all-battery.com/browseproducts/One--4-3-AF-%2818650-Size%29-4200-mAh-high-capacity-NiMH-battery.html).
Those are completely different animals from Li-Ion 18650s.
You COULD use them for the same style project, but you'd need a different charger/charging setup.
Not to mention, they are still 1.2 volt.
So to get equal capacity...
for every two 18650 Li-Ion cells(we'll use an "average" 2600mAh@3.7v) you would need 3 18650 Ni-MH cells.
2x2600mAh@3.7v = 5200mAh@3.7v
3x4200mAh@1.2v = 4200mAh@3.6v
Trade offs are, safer to use/charge, but more weight, more space, less capacity, and good luke finding cheap/free Ni-MH.
were the ones i was looking at to me it says 10x 18650 at 5000mah but it could also be 10 18650 5000mah total.
just depends id wording is right
With all the components for the summer project, my setup totals $200 plus unknown shipping for some things.
If the batteries are found in laptop batteries, why not buy some of the mass-produced laptop batteries, take them apart, and save a few bucks over buying new individual cells?
for example, buying 3 dell 11.1 volt, 4400 mah laptop batteries would cost $52 and gives 2 extra cells in case of a dead cell.
link: http://www.amazon.com/Dell-Inspiron-1545-Laptop-Battery/dp/B002MG6OO6/ref=sr_1_4?s=pc&ie=UTF8&qid=1366750261&sr=1-4&keywords=dell+laptop+battery
Individually, the best price/quality I could find was trustfire @ $5/cell with unknown shipping cost, which adds up to $80 plus shipping.
Thank you!
Looking forward to trying this out and using it to power a Raspberry Pi! (You should look them up, it's a nice little computer the size of an Altoids Tin)
However considering you are using used cells, I guess you could overlook this detail for the profit of simplicity.
I am using lifepo4 7 cells in series 2 in parallel,i dont know what kind of PCM I shoud use,my machine need 100A to boost first,then after started,the current only need around 30-50A.
Please help to choose one good PCM from BesTech Power.
I found that RCA plugs are a bit dangerous as the two poles are exposed as the batteries have a lot of punch if short circuited!
They were connected in 3S2P, which i want to refix as 2 X 3S1P connections, as it provides a good 12v supply 4 my circuits.
It Would Be if You could please tell the necessary mods in your circuit for that....
Thnx A Lot
=V=
what i use with my diy mobile charger 2 18650 in series for 7.2 volt then reduced to 5 volt by the 7805 ok losses of heat but if you look at that compared to a dc -dc converter that can use 1.5amp the little loss in heat is worth it
these would make a hell of a mp3 player battery lol
hence the code PCB-S4A5-GS 4 series amps 5
Yes, it's rated at 4 series at upto 5 amp.
no mention of how many parallel. ;-)
As long as it stays under 5 amps max rating...
Here's MY take on it ;-)
Yes, doing a 4s4p is beyond what it was intended for.
However!
It is a protection circuit, NOT a charging circuit.
As long as your charging and discharging values are at or below the circuits rating, it shouldn't trip the protection, and you should be fine.
Because it has 4 times the batteries intended, charging should/will take 4-5 times longer, possibly more. But this isn't an issue, since the author is charging cells individually! (Noblenutria (author), chime in here, if I'm wrong)
Discharging will also be limited to the rating of the circuit.
Think of that protection circuit as a bottle neck.
As long as there is no dead shorts, the circuit will safely limit the outgoing power to IT'S maximum.