Is it possible to build your own battery pack, and if so, is it even cost-effective? Earlier this year, I built a DIY one-wheel, and one of the things that surprised me the most was the cost of the battery pack. Granted, the battery pack I bought is a 20 amp-hour battery at 48 volts—that’s almost one kilowatt-hour of capacity. It also came with the charger, but still, this thing cost about $340, which is about half the price of the overall project.
How to Custom Battery Pack
1. Choosing the Battery Cells
For building a battery pack, you’re obviously going to need some battery cells. For this build, I’m going to be using a very common lithium battery cell called the 18650. It’s called the 18650 because of the dimensions of the battery: 18 millimeters in diameter and 65 millimeters long. These batteries are found everywhere—from laptop batteries to drill batteries and even in electric vehicles.
Each battery cell has a voltage of 3.7 volts nominal and a capacity of 2500 milliamp-hours. I want to make my battery pack 48 volts, so how do I get from 3.7 up to 48? I need to connect 13 of these batteries in series. Just like any other AA or AAA battery, these have a negative terminal and a positive terminal. They’re a lot harder to see because these don’t have the button on top like a AA or AAA, but they are marked on the outside. It’s really important to keep those straight as you’re connecting them because if you do this wrong, it will result in a short, an explosion, and a fire—and we want to avoid that at all costs.
2. Assembling the Battery Pack
The first problem we need to overcome is how to hold all of these cells and orient them the right way. Luckily, to help with that, they make little battery clips designed to hold 18650 battery cells, and they’re actually modular. You can snap more of them together and custom-build the size and shape of your battery. I’m going to start by making a grid of 13 by 4.
Because I have these cells alternating, I can connect them in series pretty simply by just bridging the gaps. Then it’ll go down, bridge the gap underneath, come up, bridge the gap on top, and so on. That will allow me to put all 13 of these in series.
To make a good solid electrical connection, I’m going to be spot welding some nickel strips across these battery cells. You might be asking yourself why I made a grid of 4 by 13 if I’m only going to be using 13 cells. If I just use the 13 cells, that would give me 48 volts, but my battery capacity would only be 2.5 amp-hours. I want to make a battery pack with more capacity than that, so I’m going to make three more sets of 13 cells and connect all of those in parallel to give me a total capacity of 10 amp-hours. That is about half the capacity of the battery that’s in my one-wheel.
You can order this nickel strip material in individual strips, but because I’m going to be doing a lot of these cells in series, I bought nickel strip which makes that job a lot easier. I’m going to make a whole bunch of pieces out of this material and cut them to the right length. Once I’m done with that, it’ll be time to start spot welding these pieces onto the battery pack. For that, I’m going to use a battery spot welder that I found online.
3. Spot Welding the Cells
Now that I’ve got the batteries assembled in the plastic clips, it’s time to start spot welding those nickel strips in place. Like I said earlier, there are 13 cells in series and then four of those in parallel, so this is a 13S4P battery.
I’m going to leave the last little tab unwelded for now and come back later to finish it off. That’s because I’m going to be using a battery management system (BMS), and I need to solder balance leads to each one of these terminals. I don’t want a lot of heat from the soldering iron to go into these lithium-ion batteries because that would be really bad. So I’m going to leave this last one loose, come in and solder that balance lead on there, and then tack it into place. That way, I avoid getting too much heat on those batteries.
4. Installing the Battery Management System
At this point, I have all of the nickel strips spot welded to the battery pack, and I’ve checked the voltages at each node and get the voltages that I’m expecting. Right now, I could connect wires to the positive and negative ends of the battery and use it, but in order to charge this effectively, I want to add a battery management system.
A battery management system like this will add a lot of safety features like over-discharge and over-voltage protection, as well as making sure that when you charge the batteries, all of the cells get balanced properly.
Can I be honest with you? It’s getting to the point where my heartbeat is starting to pick up a little bit. I’m hyper-cautious about where everything is, where all of the terminals are, and making sure that nothing shorts out. If there’s a piece of wire or something on my workbench and it shorts it out, that’s not going to be good—that’s going to be pretty scary. So forgive me if I start to slow down and take my time with this because I really don’t want a fire in here.
Now that I’ve got the balance leads soldered onto the battery pack, it’s time to install the BMS board itself. But before I do that, I need to solder on a couple of wires to this board. First, I’m going to solder a big 10-gauge wire to the discharge port on this BMS system. This is the connector that I’m going to use to actually power the load in whatever project I use this in, so I want it to be a big gauge wire.
The second wire that I need to solder onto this BMS board is the charging cable. The charging wire doesn’t need to be as big of a gauge as the discharge one because the current is much lower when you charge the battery. Finally, I’m going to solder the negative terminal of the battery to this board, completing everything. Once I do that, I’m going to add connectors for the discharge and charging cables.
5. Insulating the Battery Pack
For this battery, I want to be able to put some sort of insulating layer around each of the lithium cells. I went to the store and found a piece of craft foam, but this was kind of expensive and not very thick. So I went to the hardware store and started looking around the aisles and came across a thicker rubber material used to line the inside of cabinets. It was a large roll and much more cost-effective. This is what I’m going to use to wrap around the battery to keep it protected and also keep an insulating layer around it.
6. Final Assembly and Testing
At this point, the battery is pretty much done and ready to go. I like how it turned out—I love the blue shrink wrap on the outside; it makes it look very professional. The last thing to do is to test this thing out on my one-wheel to see if it works.
I’m going to go ahead and swap out the old battery and put in this new one to see if it works. I’m going to turn on the power button here. I’m looking at my voltage meter, and it’s reading at 35 percent, which means that these cells are not fully charged—which makes sense because it wouldn’t be safe to sell them and ship them in a fully charged state. So I’m not really worried about that.
This is ready to go. I think if I press down on the footpad and then lean forward, it should start going—yep, there it goes. Cool, it works!
Knowing that this battery works is awesome because it means I can use this in future projects. It also gives me the confidence to build more of these as I need them in the future.
Was It Cost-Effective?
So, was it cheaper to build my own? Obviously, I built a battery that’s only half the size of the one that I bought for the one-wheel, but I ran the numbers and created a bill of materials. If I were to build one with the same specs as the one that I bought, it would cost about $270 as opposed to $340. That’s about a 20 percent savings.
If you’re only ever going to need one of these batteries, it’s probably better just to buy one. But if you’re going to be building projects that require lots of different batteries, it’s probably wise to invest in the tools and the materials to build your own.
FAQs
Q1: Is building your own lithium battery pack safe?
A1: Building your own lithium battery pack can be safe if you have a solid understanding of electronics and follow proper safety precautions. However, lithium batteries can be dangerous if mishandled, leading to short circuits, fires, or explosions. Always ensure you’re knowledgeable about the process and take all necessary safety measures.
Q2: Why use a Battery Management System (BMS)?
A2: A Battery Management System is crucial because it adds safety features like over-discharge and over-voltage protection. It also ensures that all the cells are balanced properly during charging, which extends the life of the battery pack and maintains optimal performance.
Q3: Is it cost-effective to build your own battery pack with 18650 cells?
A3: Building your own battery pack can be cost-effective, especially if you plan to build multiple packs or need custom specifications. In the example above, building a battery pack saved about 20 percent compared to buying one. However, if you only need one battery pack, purchasing a pre-made one might be more practical when considering the time and tools required.
Disclaimer: Building your own lithium battery pack can be very dangerous if you’re not sure what you’re doing. Do not attempt this project unless you have a solid understanding of electronics and feel comfortable taking on such a project.