Introduction: 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.

Step 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.

Step 2: Cells

The words 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.

Step 3: Battery Holders

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 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 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.

Step 5: Wiring the PCB and Balance Leads

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.

Step 6: Balance Charging

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.

Step 7: Activate the PCB

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).

Step 8: Charger and C Rating

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.

Step 9: Balance the Pack

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.

Step 10: Safety

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.


ajayt7 (author)2017-07-26

If all batteries are not at the same voltage there might be internal current circulating among the batteries itself

ajayt7 (author)2017-07-26

Good, practical project. Power supply is a must for pursuing other electronics projects

swaldo (author)2017-05-31

do you have the link of where the wire diagram you used at the beginning of your build?

bpbansal (author)2017-01-29

Whenever I try to put 3rd battery in slot, it starts generating heat getting red immediately forcing me to withdraw 3rd battery. What should I do?

untg (author)bpbansal2017-04-22

It's most likely that you haven't charged all the batteries to the same voltage. What is probably happening is that the battery you added is either a lot higher or lower voltage than your other and they are trying to dump charge (or remove charge) from your battery. To fix this, make sure ALL your batteries start at the same voltage.

pmap93 (author)2017-03-10

Hi! I'd like to ask if you can use the battery pack while charging it if I made it this way?

I plan to power a portable device with a similar method to your battery pack (with balance connector), I want to be able to use the device while charging it, like a Laptop, is it possible If i follow that method?

Amazing tutorial btw!

MecreeL (author)2016-06-26

I'd like to know if it's possible to connect two 6s4p battery packs which i bought from in series to double the voltage?

can anyone help me

millingby (author)MecreeL2017-02-11

Of course you can. the capacity will remain the same, but the voltage will double. Hope this helps.

curtis.newton.104203 (author)2017-02-09

if the charger has to know the voltage of each battery independently to smart/balance charge them, the how is it that the wires to the charger are connected to 3 batteries at a time ?

AntonisK12 (author)2016-11-12

Thank you very much for all your effort. Very detailed and very well explained. Many thanks.


canklot (author)2016-10-06

can you advise a board for 4s4p on aliexpress ? for example,searchweb201602_3_10056_10065_10055_10068_10054_10069_10059_10073_10017_10070_10060_10061_10052_10062_10053_10050_10051,searchweb201603_3&btsid=2f1f8179-8987-4db3-8b58-7928095e4f89

DavidC199 (author)2016-10-03

DavidC199 (author)2016-08-18

I have bought new laptop batteries that are inexpensive because of being a high demand battery. Tgen i take them apart to get the 18650 batteries. Benifits are i get 6 or more same batteries with solder tabs and it costs about 10 to 12 bucks delevered. And they are good ones also. Just thought i would share.

TaylorM82 (author)DavidC1992016-10-03

what model laptop do you find the cheapest battery for?

DavidC199 (author)TaylorM822016-10-03

i just look for laptop batt then narrow down to but it now, then organize lowest price. here is a new 6 cell 5200mah for dell e6500 for 13.00.
this should help find deals.

NeroZ2 (author)2016-08-13

It IS Great.I think i can use the method to make a lithium polymer battery pack. like this

pmap93 (author)2016-08-11

Hello! I'd like to follow your amazing instructable!

I have a question tho.. the balance port is the only port required for charging?

I remember using a LiPo Balance Charger before, i used to connect the Balance port and the +/- of the Output

CarmeloO (author)2016-06-16

Hey I'm new to this hopefully you can help... Want to make a 18v battery pack for powertool i want to work with 2p5s configuration with 18650 cells ... Now live seen couple things that might help but I'm not shut soooo. Here's a list of the things

18650 cell of course, a pcb with balancing

A power supply

Or a adjustable power supply Would this even work i mean I've never done this before but indeed to do this for my personal tools but my problem is in in Puerto Rico and I don't have a place to go to and ask my questions so I decided to ask you guy's. A lot of questions but for now with these items mentioned will they work for what I want? Or if possible link me what would help. Please thanks for all your time..... Let me know....

AlexZ45 made it! (author)2016-05-21

balancing current from the separate battery balancer would reach 6A at max.

much better performance on balancing battery cells

02.jpg01.jpgEVL Balancer.jpg
AlexZ45 (author)2016-05-21

its better to use separate battery balancer rather than a balance charger or charger with balancing function.

rnijland (author)2016-05-16

What should I use for a motorcycle battery, specially because it has to work with the original alternator....I dont think the usual pcb's works right with it, so maybe something like a led driver before? (alternator from 0 - 14,4 volt)

JadinA2 (author)2016-01-11

Is it possible to connect these PCB's in series or parallel? I'd like to know if it's possible to connect two 6s4p battery packs in series to double the voltage? I'd prefer a 12s controller but can't seem to find any online.

foszz (author)JadinA22016-01-12

I have a 13s I am working on bulding currently.

JadinA2 (author)foszz2016-01-12

I think you meant a 12s, but thanks that is a great find! Could you perhaps answer my question about connecting them in series/parallel though? For example say I have two 4s4p battery packs running at 14.8v each and delivering a maximum of 15 amps each. Could I connect them in series to get one 29.6v pack, or parallel and get one 14.8v pack but capable of delivering 30 amps? I understand that charging them in series might be an issue?

Basically I would like to build a modular system that can be easily expanded by adding more modules?

foszz (author)JadinA22016-01-13

Nope, 13s. It's a pretty standard replacement size for Sealed Lead Acid (SLA) x4. SLA are called commonly called 12 volt batteries when actuality they are 13.2 volts fully charged. When placed in series by 4 (48v applications) the actual charged value is 52.8 Volts. This configuration is used in wheelchairs, lawn mowers, golf carts, ebikes, etc. A Li Ion max charge is 4.2 Volt each. 12x4.2=50.4, not high enough for the max voltage. I choose to run a 13s @ 4.2v each for 54.6 V.

As for your groupings, why would you do it that way? Is it not modular enough at 4p8s or 8p4s? I would think the 8s4p would be a pretty good way to go with a 2A charger and a BCM. This would charge them at 500ma each.
You could create a "battery bank", and have it be removable. But don't run a charged battery and a discharged battery together in any circuit ever, unless you isolate them from one another. Bad things could happen.

bobcool09 (author)2012-11-30

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!

Kolby12309 (author)bobcool092015-11-20

use a 3.7 to 5v boost board

DennisJ4 (author)2015-04-22

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.

Kolby12309 (author)DennisJ42015-11-20

just look on ebay

NickF15 (author)2015-09-01

Please correct me if i'm wrong here but the charge profile of a li-ion is constant current then constant voltage with capacity measured by current dropping off. if you are using a balance charger across 4 cells in parallel the current will be 4x higher than its expecting thus providing incorrect feedback to the balance charger.? you could over charge your batteries

Kolby12309 (author)NickF152015-11-20

it just acts like a bigger battery

blakehx (author)2015-11-06

So where is the best place to get cheap or free batteries / cells?

GetyormaG (author)blakehx2015-11-18

Get used laptop cells from your local recycling site, usually there only 2 or 1 damaged cells inside, that make the pack fail, so the consumer throws it away, although the other cells are in a good used condition.

dasimpson1981 (author)2011-10-22

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

pmhyden (author)dasimpson19812015-11-15

You would want a buck converter which can handle up to 2A draw. Its about 90% efficient, whereas the lm7805 dissipates the excess voltage as heat.

boost converters step up voltage and drop max current, which is not possible in dc-dc applications using a transformer.

Kolby12309 (author)dasimpson19812015-10-01

use a car adapter to usb, they can take 5-24 volts in and 5 volts out


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

rainman002 (author)dasimpson19812012-02-22

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.

dasimpson1981 (author)rainman0022012-02-22

i was compareing ah dc to dc boost converter like they minty boost to a 7805

vov35 (author)dasimpson19812012-01-07

the 7805 is self adjusting resistive voltage splitter, really. you're not gainging any amp-hours by using it, but you are reducing the voltage.

EVMadrid (author)2015-10-13

Very nice!

dasimpson1981 (author)2011-10-22

i think i just found 18650 at 5000mah (5amph) will know more when money comes to get them

Kolby12309 (author)dasimpson19812015-10-01

those dont exist.

ironsmiter (author)dasimpson19812011-10-22

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.

dasimpson1981 (author)ironsmiter2011-10-22

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


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

dimvasilk (author)2015-04-04

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!

Kolby12309 (author)dimvasilk2015-10-01

dont even buy those. they are so fake. buy a 12 cell laptop battery from amazon.

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