DIY 4S Lithium Battery Pack With BMS

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Introduction: DIY 4S Lithium Battery Pack With BMS

About: I love scrapping and dissecting electronics and taking out the guts to see all the goodies inside. I think everyone should take something apart with no idea on how or intention to put it back together.

I have watched and read more than one tutorial or how-to guide on lithium ion batteries and battery packs, but I haven’t really seen one that gives you a lot of details. As a newbie, I had trouble finding good answers, so a lot of this was trial and error (and sparks).

When I decided to build a battery pack out of 18650 lithium ion cells for a project, I took apart my old laptop battery, got the batteries out, soldered them together with metal strips into a battery pack. However, I learned on my first attempt that it wasn’t that easy. Lithium ion batteries are not like nickle metal hydride, lead acid, or nickle cadmium batteries. They are sensitive to over discharging, over charging, and short circuits, and need special care to keep them from overheating, melting, or exploding.

Why use them? They are really great for projects because they have a higher voltage than other chemistries and hold a lot of energy, which means you can use fewer of them than if you were using nickle metal hydride or nickle cadmium cells (only 1.2 volts). Power tool batteries and electric vehicle batteries are made of lithium ion cells for that reason. They come in all shapes and sizes and capacities. High quality cells can withstand high discharge rates of over 20 amps, and work well in multiple cell configurations. You can also get them for cheap or free if you look around because pretty much every laptop has a lithium ion battery in it that people sometimes throw away because it’s “dead,” but may have lots of life left in it.

I am building a 4S2P pack that has 4 cells in series, and 2 in parallel for 8 cells. This will give you a full charge voltage of 16.8 volts, a nominal 14.8 volts, and a discharged rating of 12 volts, and double the capacity of the series cells. It also has a battery management system, that is necessary to protect the cells and keep it working right. I was able to finish this project for around $20 USD. Plus, I made it!

So, let’s get started! Links to the materials I used will be included.

Step 1: Materials, Tools, and Safety

Lithium ion cells are pretty harmless, but you do need take some precautions. Avoid shorting them out, and be careful with the soldering iron and the tools.

For the tools, you need a soldering iron that's at least 30 watts, a digital multimeter, knife or wire strippers, side cutters or flush cutters.

Next, some good quality solder like this: https://www.ebay.com/itm/Kester-44-Rosin-Core-Sold... This is some of the best you can get for electronics.

The other necessary items are, of course, some 18650 lithium ion batteries, either an old laptop pack, or some like these: https://www.ebay.com/itm/1-10x-Lot-Samsung-INR1865...

Pure nickle strips like these: https://www.ebay.com/itm/Kastar-Nickel-Solder-Tab-...

A battery management system/board: https://www.ebay.com/itm/4S-10A-18650-Li-ion-Lithi...

4S balance plug connectors: https://www.ebay.com/itm/10-PCS-4S1P-Balance-Wire-...

Deans T-type connectors (or XT60 connectors): https://www.ebay.com/itm/5-SETS-DEANS-STYLE-T-PLUG...

Balance charger to charge the battery pack: https://www.ebay.com/itm/Imax-B6-LCD-Screen-RC-Lip...,

Other miscellaneous items were 18 gauge (1.02 mm diameter), 26 gauge (.40 mm diameter) to 24 gauge (.51 mm) wire, masking tape, and or electrical tape, or heat shrink film.

Step 2: The Batteries

First, you’ll need some 18650-size lithium ion batteries. Because I’m doing this cheap, I looked for old laptop batteries, and found a 9-cell Dell pack at the thrift depot for less than $3. This pack was made up of some good quality red Sanyo brand cells. I checked the data sheet and they are pretty standard 2200 mAh capacity and rated for 4 amps discharge current. Not bad. Yes, they were pretty much dead (under 2 volts each cell), but I was able to revive them. I am making another Instructable that tells you how to do this. You can buy brand new cells on eBay or Amazon, but they can be expensive for the good brands. Stay away from the ones that advertise 5000 or 9800 mAh capacity. They are probably name brand cells that failed quality control tests in the factory and may have 1000 or even 900 mAh capacity. They are re-branded and re-sold at a discount. If you used an old laptop battery, you need to remove the old connectors from the terminals. Use the side cutters to do this.

Step 3: Connecting the Cells

Next you need a way to stick the cells together. You can use steel solder tabs or nickle strips. I am using pure nickle strips, not nickle plated steel because at high current draws, steel has higher resistance than nickle, which can cause heat buildup.I am soldering them to the cells. This isn’t the recommended way because if you hold the soldering iron on the cell for too long, it will damage the cell and cause it lose capacity. The best way is to use a purpose-made spot welder like this: https://www.ebay.com/itm/2-in-1-1-9kw-Pulse-Spot-W...

However, unless you make lots of battery packs and can justify spending $200 or more for one, soldering is fine. Just be careful.

For the soldering iron, I recommend at least a 30 watt iron and good solder. Good solder is critical. Do not use lead-free solder for this because it has a higher melting temperature. Also, a weaker soldering iron won’t get hot enough to properly bond the cells to the nickle strips.

To build the battery pack, we are taking 4 cells in series and adding a parallel cell, so we have double the voltage and capacity per cell. See the diagram above for how to go about connecting the cells. The only limiting factor is that all of the cells need to be identical. Even with the BMS, unequal capacities would cause one cell to charge and discharge unequally and this could cause that cell and the others to fail more quickly. This is why it's good to use laptop batteries, since they have always been used together.

To solder the cells, rough up the positive and negative terminals of the cells and apply a small amount of solder. Next, arrange the cells into the proper order for the series/parallel connection as shown in the diagrams. I taped the cells together with masking tape for this, but you can also use battery spacers.

Cut the nickle strips to the correct length to connect the cells together. I used some side cutters for this, but tin snips or sheet metal cutters work too. Apply solder to each end of the strip, and solder the strip to the battery terminals. Don't hold the soldering iron on too long, just enough to melt the solder. I taped the cells together before soldering the final connections to keep them aligned correctly.

Step 4: BMS Board and Balance Connections

To get the most out of the battery pack and keep it from failing prematurely, w need to add a way to make sure they are protected and charged properly. Lithium ion or polymer cells need to be protected from under or over discharging, which can be really bad. This is done by a battery management system/board, or BMS. It's a device that combines battery protection for multiple cell batteries like we are building. It’s called a battery management system or BMS for short. It is a device that protects the cells from over and under discharging, current spikes, and short circuits. There are a lot of different type and configuration of BMS boards for different cell arrangements and applications. I am using a 4S BMS board rated for a 10 amp working current, which is fine for my application (100 watt LED flashlight).

Connecting it is easy. Once our battery is soldered together, we need to measure the voltages across the series cells with a multimeter. You should have 14.8 volts for battery positive, 3.7V volts, 7.4V volts, and 11.1 volts. There are 5 connections for a 4S balance plug: one for battery positive or cell #4, one for negative, cell #1, cell #2, and cell #3. Measure these by putting the negative probe on the negative side of the pack, and measuring across the connections. Once they all match, you can solder the balance wires from each connection to the correct pads on the BMS.

I used 26 gauge wire (.40 mm diameter) for the balance connections, and 18 gauge (1.02 mm diameter) for the battery +/- and load outputs since they will be handling almost 10 amps of current. You can use smaller wire for the balance connections since they aren't handling hardly any current, just the respective voltage from the connections. I wouldn't go under 26 gauge though for this. Once you have the pack connected, you can connect the balance plug leads to the proper battery outputs.

Step 5: Balance Charging

Now that we have everything connected, we can connect our pack to the charger and make sure it charges. This is how you will know if your connections are wrong, because your charger will not charge and warn you for incorrect voltage connections.

To start, we need a balance charger for lithium batteries. No other charger will work for this because it needs to have a balance mode! I'm using a Chinese clone of the SkyRC iMax B6. No, it's not the real-deal, but I found the copy to work just fine. Connect the battery positive and negative leads to the charger. My charger has banana plugs with a Deans T-type connector that connects to various connectors. You can use alligator clips or wire in a charger plug like a Deans or XT60. I'm using a Deans connector, and connected it to the outputs on the balance board. Make sure this is where you connect the charger because the BMS needs a 12.6 volt signal to activate itself. If you are intending this to be a removable battery, then wire the output to whatever connector your device will use. I'm wiring mine with spade connectors and a Deans plug because it will be mostly permanently mounted to my project.

Your charger might be different, but this is how it works for pretty much every clone of the SkyRC iMax B6 charger. Plug the balance lead into the 4S socket on the charger. It only goes in one way, and is marked for the positive and negative sides of the battery. Connect the charger lead, and set the charge mode to "Balance." Make sure the charger is also set to "4S" mode. Because this is a 4400 mAh pack, I like to set the charge current to 1/2 or less of the max current rating, so 2 to 2.2 amps. I'm using 1.5 because this is a test. These batteries are pretty much fully charged, so the voltages are high. When it's running, you should see the 4 series cells charging equally, within 0.1 to 0.2 volts of each other. When the charging finishes, all the cells should be at the same voltage, which is 4.2 volts. The pack should read a full charge voltage of 16.8 volts. When it's at the nominal voltage, it's 14.8 volts (3.7 volts per cell). If you're charging it for the first time, start at a low current setting for the first charge, then ramp it up when you charge it again.

Step 6: Conclusion

That's it! You've made a functional and reliable lithium ion battery similar to a 4S 5000 mAh LiPo pack for a fraction of the cost! Yes, you need a charger, but if you have an old laptop battery lying around, some wire, charging plug, and solder tabs, then all you need is the BMS to get going which costs around $10 USD or less if you buy it from China. This cost me about $24 USD. It would be even cheaper if I got it all from China, but I didn't want to wait a month for the parts to get here! I had the charger, soldering iron, multimeter, solder, tools, and wires already, so all I had to buy was:

Laptop battery

BMS board

Balance plugs

Nickle strips

It was cheaper than buying a LiPo pack and was more practical because I needed something to fit in my project. On top of that, it's fun and I learned a lot doing it!

I hope you like this guide and most of all, I hope you know more than you did before reading it. It's my first attempt, so please comment and let me know how I did, or could be better for the future! Thanks for reading!

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    30 Comments

    1
    johanmoberg
    johanmoberg

    9 months ago on Step 6

    Guys if you're not 100% on your electronics and battery knowledge, I would advise from building your own battery packs. The equipment and time required to construct safe and high quality packs is usually not worth it unless you really want to learn and construct many packs the coming years. Yes, a brand new pack and charger is much more expensive but it will also last much longer and you wont burn your house down (a real lithium fire extinguisher is around €600). You will find many guides showing you that it's easy, fast and cheap but that's mostly because its done in an unsafe way and resulting in poor quality packs. I don't want to bash the author but there are many things in this tutorial you should NOT do.

    1. Using old cells with under 2 V charge is both unsafe and a waste of time. When discharged below 3 V chemicals build up in the battery permanently increasing resistance and the cells can become unstable and prone to short circuit, overheating and fire. DONT USE!! Best case you have a "working" cell with reduced capacity. The increased internal resistance in the battery will lead the voltage to drop heavily when put under load. The energy is instead converted to heat. Yes, it will charge to 4.2V but will go flat very quickly (power loss increase proportional to the square of the current) and become hot during discharge.

    2. Never use a BMS with higher current rating than the batteries(!!!). Here you have two 4A batteries in parallel, max current is 8A (for brand new functioning batteries which this is not). This BMS wont cut the current until 20A (rated for 10A continuous current, 20A cutoff). When you use bad batteries they wont discharge evenly, in this case you can have one cell discharging 15A. Discharging a battery several times its listed discharge current is a great way to overheat your battery pack.

    3. When designing a BMS, the best practice is two fuses, one where the IC will cut the current (well before reaching battery max current) but also a second, conventional fuse in case the IC doesn't work. Cheapest China BMS's skip this to cut cost. You also need to verify that the protection really works as intended before using the pack.

    4. A proper BMS has a temp sensor for the battery, in case the battery goes above say 70C it turns off. A black pack laying out in the sun in the summer can reach 60C, even without being used. Too hot batteries leads to thermal runaway and battery fire occurs.

    5. Packs needs to be spot welded, yes many people online says you can solder them if you're just fast, don't. Both dangerous and the heat deteriorates your cell. You can buy good solderless cell holders if you don't want to buy a spot welder.

    6. Parallel cells needs to be balanced before connected, if one cell is 4v and second one is 3.2 v, you basically charge one battery with an uncontrolled high current.

    7. Don't work on fully charged batteries, that one mistake will do much more damage.

    8. Placing a pack you're working on piece of wire, screw, wedding ring etc etc will effectively make your pack into a welder.

    9. Cheap china BMS is you get what you pay for. You are going to store this high energy battery in your home with the lowest quality safety system.

    10. How is the pack exterior designed? Can you drop the pack on a rock without it denting a cell? Can in vent gas if the cells overheat? Does it protect against a spilled cup of coffee or condensation?

    11. Packs should to connect to BMS with a connector. First fix connector, then solder wires to the pack. You don't want to solder wires straight on the BMS which are live, if you drop one cable and it accidentally touches another nearby wire you have a short circuit.

    As I said earlier, if you want to learn how to build packs, go ahead its a lot of fun, but only do once you really understand how everything works. There are two kinds of people who thinks battery packs are easy, those who are experts and those who are amateurs. People will say I'm overzealous and that they have built many batteries the "bad way" and they work just fine. I have also driven my car for 20+ years without an accident yet I never drive without my seatbelt on.

    0
    alfredbriffa50
    alfredbriffa50

    Reply 1 day ago

    Very interesting comments....while reading your comments I realised how many mistakes I made on my first and only pack.
    I was going to ask the author of the article/tutorial about the thermo coupler or temperature sensor...which I observed on every charging pcb boards of renown appliances such as cordless vacuum cleaners, etc.
    I wonder whether you could tell me what is the use of a small pcb board on which there is a microswitch on situation on the side of the pack plastic cover on a PARKSIDE 20v 4amp battery pack, p;ease?!

    0
    NickB6
    NickB6

    Reply 7 hours ago

    Hi. Those switches can be for anything and it's hard to say what specifically unless I see a picture of it. They can be for a battery 'fuel gauge' to show how much charge is in the. battery.

    The thermocouple sensor is for a temperature limiting circuit to break the current flow to keep the battery from overheating (the device auto powers off or charging is terminated due to overtemperature). Some BMS have them, some don't. I've found the higher end ones do.

    Hope that helps

    0
    AshinP2
    AshinP2

    Question 1 year ago on Introduction

    Heyy! Can you please answer how much is the output amps??? Is it 14.8v with 2amps or above???

    0
    scandihandy
    scandihandy

    Question 1 year ago on Step 5

    Hi,

    Great article, thank you!
    I’ve been trying to figure out how to safely charge my battery pack once it’s built. It’s going into a portable speaker. I know you need a balance charger initially, but can you use a regular 16V charger once that’s done? I would really appreciate your help. I’m using this BMS board. Protection Board 4S 30A 14.8V PCB BMS 18650 Li-ion Lithium Battery Protection Board with Balance https://www.amazon.co.uk/dp/B07T1BJKJ3/ref=cm_sw_r_cp_api_glt_fabc_YJTQ1RD81CT5DWTKEEW7?_encoding=UTF8&psc=1

    0
    peaps5
    peaps5

    Answer 1 year ago

    If you ensure that the input to the pack is exactly 16.8 volts, the BMS will ensure balancing of the cells during charging. I'm using this same method at home with a 4S12P pack and its working fine. I have not tried increasing the voltage above 16.8v, however in theory this should also work too because the BMS will prevent each cell bank from rising above 4.2V by cutting it off. The advantage with a higher than 16.8v input is obviously faster charging times.

    0
    anngower54
    anngower54

    Question 2 years ago

    Can I have a diagram for a 4 s by 4 ? I want to make sure Its right

    0
    NickB6
    NickB6

    Answer 2 years ago

    I don't have a diagram of a 4s4p, but the same principle applies. You cabn search Google and probably find one.
    Best

    0
    islo83
    islo83

    2 years ago

    Unfortunelly do you forget to talk about the parallel charge issues.
    Example if you use a 4S BMS with 4-5-6P large capacity batteries then the BMS after reach the 4.2V turn into CV mode and start to decrease the charging current AS SET BY developer (it's about 1P charging current). In this case the rest of charging time will be increase 4x-5x-6x. Normally the CV charging time less than an hour....

    0
    NickB6
    NickB6

    Reply 2 years ago

    Hi. You're right and thanks for the input. Increasing the capacity increases charge time, but we're talking about series cells, so the end charge voltage will be whatever the BMS amd charger allows (within tolerance of course). This is why we use a proper balance charger to prevent overcharging and keep track of cell voltages. The BMS is good, but not a reliable way to charge cells properly.

    0
    TonyA3
    TonyA3

    Question 3 years ago

    Hi Nick,
    Great article, and since you have studied the subject a bit, I wonder if you can help. I bought a couple of 4S BMS boards from China and would like to use one with just 2 cells. The BMS has a lot of small transistors in addition to power devices and doesn't use a special chip. I have attached an image. Have you seen any circuits for these boards which might give a clue as to whether it is possible to bypass the unwanted stages?
    Tony

    20191201_101253.jpg
    0
    NickB6
    NickB6

    Answer 3 years ago

    Thanks Tony. I actually accidentally bypassed the balance circuit on cell 3 before by connecting the cells wrong and when I try to charge it my charger throws an error for cell voltage not matching. You could try omitting the 3rd and 4th cell and just connecting your balance charger with a 2S lead and that might work. Give it a short and report back. Thanks

    0
    TonyA3
    TonyA3

    Reply 3 years ago

    Hi Nick, Sorry about the formatting - I copied and pasted my conversation with the eBay supplier! Thanks for replying. I connected the output + pad to the 16.8, 12.6, and 8.4 pads and the top of the two cells. Without the intermediate connections the other pads were too sensitive to touch - they could turn off the mosfets. Both overcharge and low voltage working as per spec.

    Tony

    0
    NickB6
    NickB6

    Reply 3 years ago

    So you got it to work?

    0
    TonyA3
    TonyA3

    Reply 3 years ago

    Yes - the charge cut-off worked at around 8.5V and the discharge at around 5V - as per the specification of the DW01B protection chips on the board. I believe the charge levelling circuits should work OK as well, although I didn't try it.These protection boards are very good value from China and now I have studied the board's circuit with the powerful MOSFETs on board would be good for power switches of up to 40A. I might buy a few more to play with.

    0
    AndreaV106
    AndreaV106

    Question 3 years ago

    Hi Nick. I need to charge a 2s composed by 2x 18650 based Li-ion (batteries are independent and not soldered in series, I would use a battery holder which puts them in series). I would like to use an Imax B6 charger. I was wondering if I could just connect +&- for charging and balancing connector directly to the cells. Something like in the image. Do you think it is enough only to charge them? Or does the BSM board is mandatory? Consider that I will use the batteries in a RC transmitter for drones, like inserting two AA standards in a RC toy. Thanks.

    BATTERIES.png
    0
    NickB6
    NickB6

    Answer 3 years ago

    At the very least you will need some kind of protection circuit. Your charger can balance charge them, but you run the risk of overcharging since the charger can't individually monitor the batteries. The bms ensures the batteries receive equal amounts of charge, so one doesn't finish charging before the other and get overcharged which can be really bad (overheating, venting, fire or at the leas your battery dies from overtemp). It also monitors individual battery voltage and cuts the power if one cell falls below the cutoff voltage of 2.5 to 2.9v instead of letting them continue discharging when one cell is already over- discharged which can damage that cell. If you're using a holder, you can charge them in parallel, but you still need a protection circuit.

    0
    ChadF41
    ChadF41

    3 years ago

    Hi Nick. This is a very nice tutorial. I found it while trying to find info regarding the BMS I have for a 24v pack. The issue is that it does not have a P- connection point, only B- and C-. Any help or guidance you can give me is appreciated. Thanks, Chad.

    0
    NickB6
    NickB6

    Reply 3 years ago

    Hi Chad. So you're a 6S battery. On the balance board especially for 6S boards coming from China, which aren't well marked. The P- might not be marked clearly but It is usually parallel to the P+ solder pad . It might just have minus mark next to it. Kind of hard to see, but check there. Hope that helps!

    0
    ChadF41
    ChadF41

    Reply 3 years ago

    Thank you so much for your fast reply. In the meantime I found confirmation matching your response and my suspicion. P and C are paralleled if present, but my cheap (but recommended) board has only battery negative and "whatever else" negative and balance leads. I'm building my pack with one connector that will be used to power my bike or charge the pack. This is apparently common practice but my boss has no documentation and is not marked in the typical way. Thank you again for your response!