Picture of Make an inexpensive Lithium-Ion Battery Pack
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.
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Step 1: The PCB

Picture of 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.
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darter229 days ago

I have a friend that works for a cable company. They use back up battery packs in telephone modems and he had a bunch that were taken out of service and he gave them to me. They all had 2 or 4 18650 batteries in them. I ended up with 24 of them for free. Some were light blue and some of the better ones were purple.

CosminA11 month ago
Thanks, these are the most detailed instructions ever about li-ion battery packs.
DennisJ42 months ago

I love this project. I am a total novice when it comes to electronics but it is very interesting. I am trying to build a light weight battery to run my telescope. It will run on the 11.1 volt configuration. I have 21 good cells, so I was going to place them in a S3P7 configuration. My question to you is, can you recommend a PCB for that configuration? I the one you have is only for a P4? I do not understand that part of the build. Thank you so much for posting such a cool idea.

dimvasilk2 months ago

Very informational instructable! I'm planning on using it to make a battery pack for a quadcopter. Originally, I was planning to use Panasonic NCR18650B's for it, but then I found these on eBay. What do you think - can I use these? I'm going for ~ 2 C discharge rate. Thanks!

MarcC83 months ago

is it really usefull to have a pcb + hobby charger?

DJEnD5 months ago

Hi there, stumbled upon your tutorial whilst searching for how to DIY a battery pack.

Here's my issue, I don't come from an electronics background so pardon me if this sounds a bit simple.

From what I gathered on your 2nd page and what I have in mind, I'm actually planning to make a single pack of 6 18650s, 7.4v, into a camera battery casing which is meant exactly for that, but I'm using higher capacity cells because the originals are quite low cap.

Anyway, from your explanation, it seems that I'm looking at a 2S3P configuration am I right? If I am, the problem now is the soldering. I need help wiring them to each other. Could you hit me up at Appreciate it tons!

QuyenC5 months ago

Dear experts,

Look like you can solve the problem that I am facing now.

I have about 20 x 5mw lasers and I need to design a system to drive it. At the moment I have 8 x 3.7V batteries (18650) connected in series and I found that even if I don't turn them on for a few days all the batteries became flat. Maybe because some of them are faulty and used up all the power?

The system that I am going to design include a Microchip PIC16F876A which needs 5V to drive it. So should I connect 2S4P (2 in series & 4 in parallel) and use a voltage regulator to regulate it to 5V? I want to monitor it and charge them when required. Is this a good way to connect them? My concern is if one of them faulty then the whole thing will upset? How to avoid this? What is the best practice? I heard you guys mentioned about charging/discharging protection device - will this help? What sort of charger should I use because I am going to use this one from ebay to charge it:

If you have any suggestion please let me know.

Many thanks in advance

blorbyblorb6 months ago

Just so you're aware, for less than $30 you can head on down to any battery shop and pick up an 8AH SLA that'll run all your equipment just as long, and doesn't require expensive or complicated equipment.

fromtherootsup12 months ago

First and foremost thanks for the article. I was wondering if it is necessary to use a PCM/PCB if you are buying tabbed batteries (built in protection circuits). Also, I'm thinking of charging straight from my PCM with a voltage regulated line from solar - thoughts? Again thank you so much!!!

caimartin1 year ago

hello i need help i'm starting out making portable chargers and speaker systems powered by recycled li-ion batteries and this helped alot but couldn't i charge 2 li-ion batteries in parallel if it is being powered by a 5v charger

assaad1 year ago

hello .. amazing project , well built

but i have some question here

the pcb has an over charge protection beside other functions . yet you used a charger to charge the batteries with ballance . im a little confused here !!

can the pcb be used to charge the batteries instead the charger ?

because when reading the specs of the pcb it says ( charge and discharge terminal on p+/p-)

plz do correct if i missed any thing .

(sry . old post but i need some answers .. thanks any way)

You got it right. The PCB is a protection board which does all of the balancing on its own. Connect any power supply under 20 volts and it should work just fine. Buying a balanced charger defeats the whole purpose of trying to save money.

john52471 year ago

Any 18650 above 2600mA hour is probably unprotected. Some of the space is used for the protection circuit in the cell and prevents fires from faulty chargers. The Chinese batteries with no built in protection use the space for more chemistry, but I would be suspicious of anything more than 4000mA Hour.

The protection board in this 'ible provides full protection for single cells connected correctly to it's pads. You can then use a laptop power supply to drive the board. If you only need 15 volts at 3Amp / hour just use 4 Chinese cells.

If you need to parallel the cells they should have individual protection (and cost twice as much and have a maximum 2.6A/h capacity.) To get your 15 volts at 3 A/h you will need 8 cells wired in parallel pairs to the protection board in this 'ible.

If you charge and discharge your battery pack through the circuit board you can not go above the current ratings for the board without damaging it.

tldr? 4 cheap hi capacity chinese cells on the board in the 'ible will not catch fire

if using 8 or 12 cells they should be protected cells. The board can't protect them all, because it can't monitor separate cells - only paralleled sets of 2/3/4 cells

Protected cells don't really need this board, but would still benefit from a smart (expensive) charger. The more you pay the less chance of Li-ion fires.

Cheap Chinese high A/h cells or reclaimed / secondhand cells need a protection board or smart balancing chargers to avoid fires.

monideth1 year ago

Sorry, to dig up an old thread, but I was interested in this project and was also questioning/converned whether the PCB can be connected to more the battery cells in parallel and series.

I found this PCB on eBay:

which has been designed for 4S battery packs. However, it does state:

"Could also work for 4S2P (8 cells) , 4S3P (12 cells), 4S4P (16 cells) etc."

I enquired about this to the seller and they responded back that it can be done and helpfully provided this diagram of how it can be wired (for 4S3P):

However, I am still concerned by the fact the each individual in each 3P pack cannot be protected.

I found this very useful page about parallel charging battery packs:

You can also use do  parallel balance charging (with appropriate parallel balance adapters). So I'm wondering whether this may be a better (but more expensive) method/option - i.e. you have separate 3S (or more) battery packs, each one having its own PCB. You would then parallel balance charge all the indivial 3S packs.
Li-Ion PCB - 4S3P.png
lawsonrw2 years ago
If I want to make my own battery pack what consideration do I need to make for the wires? As the voltage and amperage increase do I need to use ever larger wires? If I just use the same large wire t/o the build won't the increased resistance keep any of the cells from contributing power?

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?
as i said befor here you need 4 pcb's to keep all batterys balanced
Noblenutria (author)  dasimpson19813 years ago
You only need one PCB. And you balance the pack with the balance leads and charger.
one battery holder with 4 in parallel one battery could be dead and other will charge higher like they would in series i have tried it and this happens

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
same for "protected" batteries?
as far as I know yes cos the power applied keeps the battery reset I could be wrong
this is what i am trying to explain this only happens if it is 4s1p
agoldsmith12 years ago
This PCB has a balance function and is rated at a higher max amperage:

it's bigger but it seems worthwhile if you're going to be charging a laptop, which can draw more than 5 amps.
agoldsmith12 years ago
based on the PCB, wouldn't the amperage be cut above 5 amps? Or is the pcb amp rating for charging the battery?

how do you set up the charger to use one lead per cell? I only see one lead for four cells?
i think i just found 18650 at 5000mah (5amph) will know more when money comes to get them
Be careful here.

I HAVE found some 4200mAh rated 18650 cells, but they were Ni-MH(like here

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
Noblenutria (author)  dasimpson19812 years ago
I don't trust several brands of 18650s like trustfire and Ultrafire. They are poor quality batteries and they overestimate their capacity. There is no way a single 18650 could be 5 amp hours. A good quality cell could be no more than 3 amp hours. The cells on batteryspace are expensive but I am sure they are legit.
you are very right they turned out to be 1100mah
agoldsmith12 years ago
I found some trustfire batteries for $10 per 2 pack.

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.

Individually, the best price/quality I could find was trustfire @ $5/cell with unknown shipping cost, which adds up to $80 plus shipping.
bobcool092 years ago
To activate the PCB you can also put together 11 AA batteries in series, and if they are new the voltage will be higher than the expected 16.5v, mine was around 17.6, but it still works. If you are worried about that just use the batteries a little bit (or short them, if you wanted). This method doesn't require you to have a variable DC power supply, which is nice
bobcool092 years ago
Would the board you use still work if I only connected 2 sets of four? Because I only need 5v and (please correct me if I'm wrong) if I have 4 of them then it would give out about 16.8v, and if I only had 2 of them it would only give 8.4v. Also, hoe would I connect a RCA cord to a USB/Micro USB?

Thank you!
bobcool092 years ago That is a link to the battery holder he is using, the link that he uses doesn't exist anymore.
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)
matroska2 years ago
I might be wrong, but I believe that when arranging li-ion cells it is preferable to have them balanced when charging, as usually cells do not have exactly the same capacity.

However considering you are using used cells, I guess you could overlook this detail for the profit of simplicity.
seems everyone here is an expert,could you please help me?
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.
owendaniel2 years ago
Top work!

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!
Hey i got total six 18650s from an old laptop battery...
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
why 16.8volt you could just make a 7.2 volt battery pack use an 7805 to bring to 5 volts and give your self even more amp hours
Noblenutria (author)  dasimpson19813 years ago
What is a 7805?
l7805cv is a 5v regulator brings the 7.2-12v down to 5 volts but needs a heatsing for voltages higher then 7.2 or if it going to be under heavy load.
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
7805 are extremely inefficient power-wise (hence the heat sinking). They burn of excess voltage with current as heat. Since the real rating of a battery pack is watt-hours, 1. rearranging the batteries for a different voltage wont change the energy capacity (watt-hours) and 2. a 7805 will drastically reduce the effective watt-hours.
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