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: 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:

Pure nickle strips like these:

A battery management system/board:

4S balance plug connectors:

Deans T-type connectors (or XT60 connectors):

Balance charger to charge the battery pack:,

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:

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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    3 Discussions


    Question 8 months ago on Step 6

    I thougt that the BMS would manage the balancing so that you could use an ac-dc brick for charging. Or is it the balancing cable breakout only for balancing the cells every once in a while?

    1 answer

    Answer 8 months ago

    Hi! You could use an external power supply to charge the battery through the BMS, but that's not ideal if you want your batteries to live a long life. The BMS is limited by the tolerances built into the protection IC, and it doesn't do a great job monitoring individual cells, which may result in overcharging the series cells before it reaches the cutoff voltage (16.8 volts for 4S). Some cells charge to 4.2, and others to 4.25 to even 4.3 volts before the cutoff is reached. That's not good and reduces lifespan and capacity. This is what the balance plug is for. The balancr charger limits and monitors current and voltage and is calibrated to closer tolerances than the actual BMS. I tested my BMS during charging and after with my multimeter and under load, and it indeed does pull the plug when anything is exceeded, but you need a charger to get the most out of it. Thanks hope that helps!

    DIY Hacks and How Tos

    11 months ago

    Great first Instructable. You should enter this into the First Time Authors contest.