Building A Battery Pack From 18650 CellsPosted on: Tue Apr 05 2022
Lithium-ion is currently the best battery chemistry humanity has. So, it's no surprise that you will find a lithium ion battery in your laptop, in your drone, in your power tools, and even in your medical devices.
In fact, it's simply a matter of time before all the cars on the road are electric. Isn’t it fascinating to live in these times? All of us alive today will see the world make its major shift away from lumbering, fossil fuel-powered vehicles to a wider array of light electric vehicles that have far simpler and more efficient powertrains.
The most common type of lithium ion battery cell by far is the 18650. Like all cylindrical battery cells, this one gets its name from having an 18mm diameter and 65mm height. So, as you would expect, an 18650 cell looks like a slightly larger AA battery.
For these reasons, there are a lot of people wondering how to build a battery pack from 18650 cells. In this article, we will teach you everything you need to know about how to build a battery pack from 18650 cells.
How Are 18650 Cells Different From Other Batteries?
While a lithium ion cell may be only slightly larger than a AA, an 18650 is vastly more powerful than any AA ever could be. There are several key differences between these types of battery cells.
For starters, 18650 cells have a higher voltage than AA cells. This means that it takes less 18650 cells in series to produce a voltage suitable for consumer electronics. Also, 18650 cells have far lower ISR (internal series resistance) than a AA battery. This makes it so less 18650 cells are required to achieve a given current carrying capacity.
Another great thing about 18650 lithium ion cells is their power density. While a typical AA battery contains only about 3.9 watt-hours of energy, a 18650 lithium ion cell can store 13 watt-hours or more. This is no surprise, as energy density figures for modern lithium ion cells are between 100 and 265 watt-hours per kilogram.
Parts And Tools Required
1. 18650 Battery Cells
2. 18650 Cell Holders
3. Nickel Strips
4. Charge & Discharge Connectors
5. PVC Heat Shrink
Tools Needed For Building A Battery Pack From 18650 Cells
Whether you are building a pack with a soldering iron or a spot welding, you are going to need a soldering iron. While we do cover how to build a battery pack from 18650 cells using both methods, we strongly recommend going the spot-welding route.
This is because when spot-welding a pack, soldering is not used to connect the cells together electrically. Instead, a soldering iron is only needed to make auxiliary connections and to make the connectors.
1. Spot Welder
2. Wire Stripper / Cutter
3. Heat Gun
5. Lithium Ion Charger/Capacity Tester
6. Electrical Gloves
7. Kapton Tape
8. Barley Paper
9. Insulator Rings
10. Soldering Iron
Where To Get 18650 Cells For A Battery Pack
Buying New 18650 Lithium Ion Cells:
There are all kinds of 18650 cells on the market ranging from $2 to $10, but which one should you buy? If you are in the market for new cells, I recommend the following brands 18650 cells from Panasonic, Samsung, Sanyo, LG, and Molicel.
These 18650 brands have good track records and you can trust their datasheets and ratings. Most of the time, 18650 cells from these brands will cost more, but they are a battery value for the money. This is because higher-end cells often have higher capacities and current carrying capacity.
Salvaging Lithium Ion Cells:
Many devices use 18650 cells. You can find them in everything from discarded scooter battery packs to old laptop batteries. You can also find excellent 18650 cells in modem and medical battery packs.
If you don’t feel like salvaging 18650 cells on your own, feel free to check our selection of high-quality salvaged cells. We have a vast library of pre-inspected and pre-sorted 18650 lithium ion cells that would be perfect for building your first battery pack!
How Many 18650 Cells Do I Need For My Battery Pack?
To make the battery pack you need, you must first know what voltage, amp hours, and current carrying capacity the battery needs to have. Connecting cells in series will increase the voltage, while connecting cells in parallel increases their current carrying capability. Any time you add cells, whether it's series or parallel, you gain capacity.
E-bike batteries typically run somewhere between 36 and 72 volts. Home Powerwall systems can be designed to run on 12, 24, and 48 volt systems, and just about anything in between. So, for this example we will build a simple ‘24V’ battery using 2200mAh cells. I put the 24 in quotes because with lithium ion cells, you can’t really build a perfect 24 volt battery.
The battery we build in this example will be used to power a 500W air condition through an inverter.
Determining Series Configuration
We have already determined that we need to run a 24 volt load. When building a 24 volt battery pack, it's best to use 7 cells in series. This is because lithium ion cells have a depleted voltage of about 2.6 volts, a nominal voltage of 3.7 volts, and a fully charged voltage of 4.2 volts. So, that means 7 lithium ion cells in series will have a nominal voltage of 25.9 volts, a fully charged voltage of 29.4 volts, and a dead voltage of around 20 volts.
As the battery dies, its voltage will fall into the low 20s, but this voltage range works fine with 24V inverters and other 24V appliances. So in this example, the battery will have a 7S Configuration.
Determining Parallel Configuration
In this example, the air conditioner is rated for 500W. So, that means it requires 500 watts per hour to run. It’s generally safe to assume that a decent 24V inverter is about 90 percent efficient. That means for the inverter to supply 500W of power, you would need to put at least 556W of power into it.
NOTE: This also means that to be able to power a device that uses 500W on the AC side of the circuit, you would need an inverter that is capable of more than 500W. Remember, the 500W limit is a limit on the overall power going through the inverter, rather than just a limit on the output.
It's important to not max out your equipment. So, if you need to draw 500W continuously, then you should go with an inverter that can provide no less than 750W.
Watts is volts times amps. So, if you have a nominal voltage of 25.9, and you need to supply 556 Watts to a load, the load will draw about 21.5 amps from the battery pack. This total battery current is divided among the number of cells in your P group. So, all you need to do is divide 25.9 by the amount of Amps that an individual cell can provide to find out how many cells you need in your P groups.
NOTE: Each and every cell has its own figures, so for this example we will assume that the 18650 cells can provide 5 amps of current. A quick calculation shows that a 7S battery would need 5.18 cells in its P groups to be able to run a window unit air conditioner. When doing this calculation, it's always best to round up. So, in this case, you would need a 7S6P configuration.
That means that to run a window unit air conditioner from a 24V inverter using 18650 cells that have a 5 amp CDR (Continuous Discharge Rating) and a 2200 mAh capacity, the absolute minimum is 7S6P Configuration.
How To Calculate Battery Running Time
But for how long will the air conditioner run? Remember, after the inverter losses, the air conditioner will require about 556W of power. The cells in this example contain 8.14 Wh of energy (3.7 volts nominal x 2.5Ah). This figure is obtained by multiplying its normal voltage by its nominal amp hours. In a 7S6P battery pack, meaning you have a total of 42 cells.
That gives the battery pack in this example a capacity of just 341.88 Wh. That’s only long enough to run the air conditioner for about 37 minutes. While this does not seem like a lot of power, I would like to give you some context.
340Wh of energy is enough to fully recharge the average cell phone about 17 times. With 340Wh of stored energy, you could use a desktop PC and a monitor for about 7 hours. With that amount of power, you could fully recharge most laptops about 6 times.
If you need to add more capacity, all you have to do is add more cells to your P groups. In the case of this example, adding an additional cell to the P groups means adding 7 more cells. So, for every number of cells you add to the P groups, you get another 57 Wh of energy storage.
You could build one large 24 volt battery, or you could build several small ones. But if you are using 2200mAh cells that have a 5A CDR and you are running an air conditioner through an inverter, the absolute minimum the pack should be is 7S6P.
Online Tools For Building Lithium Ion Batteries
If all of this sounds confusing or maybe just a bit too much to keep up with, we highly recommend you check out the tools section. We have developed several tools that make building lithium ion batteries easier.
Pack Planner tells you how long a given load will run on a particular battery and provides an estimation of the size battery that would be needed for a given application. Pack Builder helps you arrange cells in most effective configuration based on each cell’s individual capacity.
18650 Lithium Ion Cells To Avoid
Steer clear of brands that offer impossible cells. As of this writing, the highest capacity 18650 cell is the Panasonic NCR18650G, which has a capacity of 3600mAh. So, if there is a company out there claiming to make something larger than a 3600ma 18650 battery cell, don't buy it.
In fact, it’s a good practice in general to avoid buying products from brands that are known to use inaccurate labelling.
There is nothing wrong with getting cheap cells or used cells. What you want to avoid, however, is buying cells that are marketed as new when they are used. Another thing you want to avoid is buying low-capacity cells that are marketed as having a much higher capacity.
So, it’s important to make sure the cells you are buying have been tested. The most reputable battery resellers have a policy of testing a selection of cells in each batch and usually post screenshots of the results.
Check The Cell Voltage
It is of critical importance to check the voltage of your cells before connecting them in parallel. Remember, if there is any voltage difference between the cells that you are connecting in parallel, energy will transfer from the higher voltage cell to the lower voltage cell as fast as it can. The difference in voltage is what determines the speed. So, the larger the voltage difference, the more the risk of a fire when you connect them.
While it's not dangerous to connect out-of-balance cells in series, it certainly can be inconvenient. This is because it takes quite a bit of time before most consumer BMS systems are able to bring cells within balance, and until it does, you won’t be getting anywhere near the full performance of your battery pack.
While it's still important to check your new cells, this is generally more of a problem when using used cells. As a general rule of thumb, you don't want to connect two cells together in parallel if they are any more than .1 volts off.
Different Types of Cell Balancing
There are various methods employed to keep cells balanced in lithium ion battery packs. The most common way for most BMS systems to handle this is to essentially burn off excess energy in any cells that may have a slightly higher voltage than the others.
Depending on your application, you may want your battery back to either top-balance or bottom-balance your cells. In top-balancing, the cells are brought into balance as the battery pack is being charged. This ensures that all cell groups have an equal voltage when the battery pack is fully charged.
In contrast, a bottom-balancing system will balance the cells as they discharge. This method ensures that all of the cell groups have equal voltages when the pack is dead.
How To Choose The Right Nickel Strip To Build a Battery
To make the battery pack, you will have to electrically connect the 18650 cells together. This is most commonly done using a spot welder and nickel strip. Avoid nickel-plated strips, and instead opt for pure nickel strips. They cost a little more, but pure nickel strip has far lower electrical resistance.
When the electrical conductor used to join 18650 cells has a lower resistance, that means the battery pack will have less voltage drop under load. This leads to less heat generation during charging and discharging, which extends the battery’s life-span while also ensuring that the battery maintains optimal performance throughout this extended life span.
Nickel strips are made in many different shapes and sizes for different cells and ways of building a battery pack. So, make sure to choose the right type of nickel strip based on your needs.
High-quality pure nickel strip with a thickness of 0.015 inches is usually the standard for building a battery pack with 18650 cells. At this thickness, each millimeter of width can carry about 1 amp of current.
If you can’t afford an expensive spot welder, you can always pick up a lower-cost model. You would be surprised how effective entry-level spot welders are these days. If neither are an option for you, you can make a lithium ion battery pack using a soldering iron and wire.
Spot Welding Vs Soldering Lithium Ion Battery
When it comes to how to connect 18650 cells together, there are many different methods. In high-end industrial settings, it's not uncommon to find batteries that are laser welded, ultrasonically welded, or even robotically welded.
When it comes to building a lithium ion battery pack in the consumer space, however, there are two methods: Soldering or Spot Welding.
Generally speaking, spot-welding will always be the best choice compared to soldering. There are, however, some exceptions to this rule that we will explore a bit later. The problem with spot welding a battery pack is that a good spot welder typically costs substantially more than a soldering iron of equivalent quality.
Spot welding is the most common method used to build an 18650 battery pack. This is because spot welding enables the cells to be joined together without adding a lot of heat. Also, each spot weld takes far less time than an equivalent soldered connection would.
A high-end spot welder used to make professional battery packs typically costs somewhere between $200 and $300. There are, however, lower-cost spot welders that may not have all the bells and whistles that the K-weld has, but they will certainly get the job done.
If you have no other way to put a battery together, then soldering a lithium ion battery would be a good choice. The main problem with soldering is that you must apply a large amount of heat to the cell to form a bond. That heat does not dissipate quickly.
Sure, with the right soldering iron this issue can be minimized, but it's difficult to completely eliminate. If you don’t do things just right, that amount of heat directly on both sides of a cell will certainly affect its long-term performance.
NOTE: There are some use-cases in which using soldering is preferable to welding. A large-scale Powerwall battery is an example of such an application. This is because in this type of Powerwall battery, each cell is typically individually fused.
Put The 18650 Cells Together
Place the first parallel group of cells positive side up. After that, place the second parallel group negative side up, and so on. The cell holders will have notches on the side that allow them to be interlocked.
Connect your cell holders together to form the needed arrangement of cells for your battery pack. While it is possible to simply hot glue the cells together, using cell holders gives your battery pack better cooling characteristics among other benefits.
Here are some of the reasons why we recommend building battery packs using cell holders that are specifically engineered for 18650 cells:
Why Use 18650 Cell Holders?
Along with giving you an easy framework to build complete battery packs, using 18650 cell holders provide several advantages:
1. Cell holders provide critical spacing between the cells. This allows for much better cooling characteristics as compared to cells that are in direct contact with each other.
2. Using cell holders makes your battery pack more solid.
3. Cell holders give your battery pack an element of vibration resistance.
For these reasons, we recommend always using 18650 cell holders when building a battery pack.
Option A: Spot Weld The Cells Together
Now it's time to connect the cells. Before welding, be sure to prepare the nickel strips.
The easiest way to do this is to lay your nickel strip on top of your parallel cells. Make sure that it covers all cell terminals. It's always good to leave around 10mm of strip for connecting the BMS sense wires. Remember, solid nickel strip with a thickness of 0.015 inches can carry about 1 amp of current per millimeter of width.
For series connections, simply cut small nickel strips to fit between your P groups.
Start by connecting the negative terminal on the first parallel group to the positive terminal of the second group, and so on, all the way down.
Make sure there is no air-gap between the welding electrodes and the nickel strip. Also, make sure there’s no air-gap between the nickel strip and the battery terminal. When welding, you will see a tiny pair of spark that will generate two small weld marks on the nickel strip.
As you are becoming acclimated to the welding process, it's good to make sure you have produced a quality weld by trying to pull off the nickel strip. So, it's good to practice welding nickel onto a few cells before you do the build.
If the nickel strips are difficult to remove, then chances are it's a good weld. If you can easily peel the nickel strip off, then you know you have a poor weld. If this happens, you may have to increase your welding current.
Option B: Solder The Cells Together
Make sure to thoroughly clean the surface of your cells. After that, add flux to remove any oxidation that would prevent a quick, clean bond.
If you try to solder directly to a cell without using flux, while you may be successful, you certainly will have to press the soldering iron harder and hold it on the cells longer.
Use at least an 80W soldering iron. I know it may seem counterintuitive to recommend using a hotter soldering iron on battery cells. It’s important to consider, however, that the hotter the soldering iron, the less time you will have to spend with the soldering iron on the cell.
NOTE: If you hold your soldering iron on a cell for more than a short moment, you run the risk of damage to the battery.
Make sure to use a heavy gauge wire that can handle the amount of current your battery is going to need to provide. Starting with the negative terminal, carefully connect each terminal. Be careful to not leave that hot soldering tip on that cell for very long.
Using this method, you can substitute nickel strips and a spot welder for wire and a soldering iron. This is not, however, the only way to build a battery pack from 18650s using solder.
Cell Level Fusing
If you are building a very large battery pack that could have hundreds, even thousands of cells. It’s a good practice to incorporate cell-level fusing into the pack. In battery packs like this, each and every cell has its own fuse.
In applications where safety is critical, cell-level fusing is the way to go. In these setups, there is typically a large, copper bus-bar that each cell is connected to with fuse wire. Fuse wire is specifically sized wire that is made to withstand a certain current threshold.
When this threshold is crossed, the fuse wire will burn up in a controlled manner. When this happens, the path of current is broken, which effectively removes the cell from the circuit.
The Importance Of A BMS
Once you have all of your parallel groups connected in series, you have a battery pack. The cell on one end of the chain will have a negative cell with nothing attached to it. That is your main battery negative. On the other end of the series chain, you will find a cell that has a positive end that nothing is connected to. That is your main battery positive.
While your battery is now technically ready to use, I wouldn’t recommend it. Remember, if you don’t have any form of over-current or over-charge protection, a battery can be dangerous. If something goes wrong with your battery, load, charger, or even a wire, you could be in for some serious trouble.
This is why a BMS is essential. A BMS, or Battery Management System, is a small circuit board that converts several dangerous, fairly useless lithium ion cells into one safe, useful, battery pack. A BMS has several roles, and some more expensive BMS boards do more than others. Here are some of the most common functions that a BMS performs:
A BMS Monitors the voltage of all cell groups. When any one of those cell groups reaches 4.2V, charging is stopped. This prevents your cells from being damaged from being overcharged, and also helps prevent fires.
If one or more of your cell groups are at a higher voltage than the others, a BMS uses a circuit and small balance resistors to lower the voltage of any high cells.
A BMS disables the battery pack when any cell's voltage falls below a certain threshold. This threshold is usually 2.5 to 3.0 volts. Remember, if a cell goes much lower than that, it will get damaged.
Another critical job of a BMS is to make sure the battery pack is not put under too much stress. So, every BMS has a maximum current that, if achieved, will turn the battery pack off. Over-current protection applies to both charging and discharging the battery pack.
Short Circuit Protection
A short circuit is equivalent to a 0 Ohm load. Ohm’s law states that the amount of current that will flow through a given circuit is a function of, among other things, the circuit’s resistance and voltage. When a short circuit condition is created, the only resistance to the flow of current will be the very low resistance of the conductor (wire or nickel) and cell itself.
So, because a short-circuit condition creates an over-current condition, any BMS that has over-current protection also has short-circuit protection.
Making The Connections To The BMS
BMS systems are usually composed of N-channel components. What that means is that the board regulates the battery pack’s output on the negative end. So, after connecting all your cells together, you can route the main battery positive directly to the DC connector, but the main battery negative needs to go into the BMS.
The main battery negative is usually marked as B- on most BMS boards.
The next step is to attach the sense wires. In the case of a 7S BMS, there is usually a break-out connector and cable that comes with the BMS. Where exactly each pin goes is often not clearly marked on BMS boards, but there is usually one wire that is a different color than the other.
Usually, a BMS cable will have 1 black wire on the end. This goes to the main battery negative and is known as the B0 connection. Then, the very next wire in the cable will go to where the first cell’s positive end is connected to the second cell’s negative connect. This is known as the B1 connection, and so on.
Simply solder each sense wire where it goes on the battery pack and the BMS will be able to read, change, or completely disable the battery pack power in the event of an emergency condition.
The very last step is getting the power out of the BMS. Remember, we ran main battery positive directly to the battery connector and main battery negative into the BMS using the B- connection. So, to get the power out of the BMS, we are going to need to locate the P- connection. Luckily, that is usually clearly marked on most BMS boards.
Solder a wire from the P- to the negative side of your DC connector and you should be good to go. Keep in mind that some BMS boards do not send any voltage out until they see a charge on the input. So, if you built your very first battery pack and you are seeing 0V on the output, don't panic! Just put it on a charger for a moment to activate the BMS.
How Many 18650 Batteries Make A 100Ah Battery?
That depends on which cells you are using and what voltage you are trying to achieve. For example, if you are wanting to build a 12V battery and were using 2500mAh cells, you would do the math like this:
To build a battery using lithium ion cells that is close to 12V without going too much over is going to be a 3S configuration. This is because lithium ion cells have a nominal voltage of 3.7V.
So, 3 cells in series would give you a voltage of 11.1V. Remember, connecting cells in series adds their voltage but does not change their mAh. This 3S cell group on its own would have a voltage of 11.1V and a capacity of 2500mAh.
So, to find out how many 3S groups need to be put in parallel to get 100Ah, simply divide 100 by 2.5.
100 / 2.5 = 40P
So, to make a 100Ah battery in this case, you would need to put your cells in a 3S40P configuration.
3 x 40 = 120
You would need 120 2500mAh lithium ion cells to make a 100Ah battery.
As you can see, there is quite a bit to consider when building a lithium ion battery pack from 18650 cells. It can be quite difficult for a busy person to take the time to learn all of these terms when they really just want a battery.
We hope this article helped you build your very first lithium ion battery pack from 18650 cells. Thanks for reading!