Lithium-ion batteries are the most popular batteries out there. You can find it anywhere, from your laptop to power tools.
The most common type of lithium battery cell is the 18650 model. It looks like a large AA battery and is used widely. Even, you can make car batteries from it.
But, how to make a car battery from 18650 cells?
You can do this by arranging the cells in series or parallel. But the parallel configuration is better for a constant flow of electricity. The cells need to be spot welded together and a BMS will help with the voltage distribution. Finally, a well-enclosed casing will ensure safety.
That’s for the short summary. I have prepared a step-by-step guide just for your ease. I promise it won’t take more than five minutes.
Can You Make a Car Battery From 18650 Cells?
Yes, it is possible to make a car battery from 18650 cells. It is theoretically and practically possible to construct such a car battery.
Lithium-ion cells can be paired together to provide the necessary starting current for starting a car. Even fully-built lithium-ion cell batteries are sometimes better than regular AGM batteries.
However, there are some disadvantages to this method. The battery can heat up and have malfunctioning issues. Hence, making a car battery from 18650 cells is possible but not recommended.
How To Choose The Right Nickel Strip To Build a Car Battery?
One of the most vital components of building a lithium-cell car battery is stripping the 18650 cells together.
The 18650 cells must be linked together. The linkage should be through Nickel strips or heavy wire to complete the battery pack.
In most cases, nickel strips will be enough for this purpose. The market typically offers two distinct varieties of nickel, stripping, including nickel-plated steel and pure nickel. If you want to invest in something, I recommend pure nickel strips.
Because it is slightly more expensive than nickel-plated steel. But it offers far less resistance and therefore is more practical. Batteries with low resistance generate less heat during the charging and discharging processes. This extends their lifespan.
The width and length of nickel strips might vary. Make your selection of strips based on the star rating as of right now.
Building A Car Battery Pack from 18650 Cells: 8 Step-guideline
Building a car battery pack from 18650 requires delicate work and time. However, it is not hard at all. You can DIY the whole project at home using regular tools.
To make things easier, I have discussed the whole process and necessary tools in 8 simple steps.
Required Tools & Equipment
You will need two types of tools and parts for the whole build. I have listed them separately for your ease:
- 18650 Battery Cells
- 18650 Cell Holders
- Nickel Strips
- Charge & Discharge Connectors
- PVC Heat Shrink tube
- Spot Welding Machine
- Wire Strippers
- Heat Gun
- Lithium-ion charger
- Safety Gloves & Goggles
- Kapton Tape
- Barley Paper
- Insulator Rings
- Soldering Iron
A soldering iron is essential for any soldering job, including spot welding and pack construction. While I explain both techniques for assembling a 18650-cell battery pack, spot welding is highly recommended.
Because electrical connections between cells are not soldered in the spot-welding process. An alternative is the use of a soldering iron. This is only required for making the connectors and any auxiliary connections.
Step 1: Finding 18650 Cells For A Battery Pack
There are two ways of finding 18650 cells for the build. Here I have described them for your ease-
Buying New 18650 Lithium Ion Cells
What 18650 cells should you buy?
You may find so many to choose from, with prices varying from $2 to $10. If you need new batteries, I suggest the 18650 models made by Panasonic, Samsung, Sanyo, LG, and Molicel.
For example, you can use the Duracell rechargeable AA batteries or EBL 3.7V Li-ion Rechargeable Batteries 3000mAh for the purpose.
It is safe to rely on the specifications and ratings provided by these 18650 manufacturers. 18650 cells from these manufacturers tend to be more expensive overall. But they provide excellent performance for the price. This is because the capacity and current carrying capacity of high-end cells tend to be greater.
Salvaging Lithium Ion Cells
The 18650 battery is widely used in day-to-day electronic devices. You can discover them in old laptop batteries as well as those from discarded scooters. The battery packs used in medical equipment and modems typically contain high-quality 18650 cells.
You can recycle your old laptops and scooters and use the good 18650 battery cells. This will save you a lot of time and money.
Step 2: Determine How Many 18650 Battery Packs For A Car Battery
The battery specifications you need are its voltage, amp hours, and current carrying capacity. You must know these before you can begin making the battery pack.
When cells are connected in parallel, the current can flow more freely than when they are connected in series. The capacity of a system increases when more cells are added, whether in series or in parallel.
Most car batteries are between 12.6 and 14.5 volts. Hence, we will take 12 volts as the reference voltage to build the whole battery pack. A typical car battery requires around 70 ampere-hours at a 3.5A rating.
Determining Series Configuration
For reasons we’ve already discussed, we’ve settled on a 12.6-volt load. Using 4 cells in series allows for a 12-volt battery pack to be constructed. Because lithium-ion cells have a depletion voltage of around 2.6 volts. Then a nominal voltage of 3.7 volts, and a fully charged voltage of 4.2 volts.
The nominal voltage of 4 lithium-ion cells connected in series is 14.8 V. The maximum voltage is 16.8 V. Lastly, the minimum voltage is roughly 10.4 V.
Determining Parallel Configuration
The desired capacity of the battery pack= 70 AH or 70,000 mAh
The capacity of each 18650 battery cell= 3400 mAh
No. of cells required for parallel connection= 70,000/3400 = 20 nos
This pack of cells will be referred to as a “20P pack”. But that’s after the common abbreviation for parallel cells. The parallel connection of 20 cells results in a single cell with a greater capacity.
Step 3: Calculating Battery Run Time for Car Battery
Determining the car battery runtime is very crucial for the build. Most automobile batteries operate at 12.6 volts and 105 amps.
If you convert this to watt-hours, you’ll get 1,323W. To start an engine at least 1000 watts are needed. Our build can handle up to 1300 watts which qualifies it for the car start-up.
Now, a typical car stereo system uses 12 watts per hour to run. We will take 20 watts for a safe margin as the number may vary. Hence, the total runtime for the battery will be;
(10 x battery capacity in amp hours) / (appliance load in watts)= (10 x 70) / 20 = 35 hours.
This is a theoretical nominal runtime. The figures may vary depending on car, weather, and situation.
Step 4: Assembling the 18650 Cells
In a stack of 20 cells, arrange them. This way the tops will be facing up in the first parallel group. Then the bottoms will be in the second, the tops in the third, and so on. The above image can help you visualize the concept being discussed.
Use a plastic 18650 battery holder and hot glue to assemble the individual cells into a functional pack. To put the cells together, I use plastic 18650 cell holders/spacers. To summarize the key benefits of these cell holders are-
- A custom pack can be made in any size to suit your needs. Much like putting together a jigsaw, this is a challenging yet rewarding activity.
- Second, it separates the cells so that air may flow freely between them and cool the battery.
- In addition to these benefits, your battery pack will be more durable and dependable after doing this.
- Four, it protects your battery pack from vibrations.
Step 5: Spot Welding the Nickel Strips
You can spot-weld the nickel strips in three ways. But I will suggest the fixed welding head with foot switch method. This is the best option for welding nickel strips.
A nickel strip should be laid on top of the cells so that it covers all cell terminals. Ten millimeters of the extra strip should be left so that it may be connected to the BMS.
Miniature nickel strips, as shown in the diagram, should be cut for a series connection. Parallel connections use four long strips, while series connections use ten short strips.
Join the negative end of the first parallel group to the positive end of the second. Then the negative end of the second to the positive end of the third.
Nickel and nickel-plated steel strips can both be welded with this spot welder. The nickel strip thickness will dictate how much you turn the welder’s pulse and current knobs.
Set the current dial to 4-5 amps and the pulse dial to 4P for nickel strips of 0.15 mm in width. For nickel strips with a thickness of 0.2 millimeters, set the current control to 7-8 amperes and the pulse knob to 4P, 6P.
Put some pressure on the nickel strip and the battery terminal of the welding pen. Then hit the switch. Two dots and a tiny spark appear on the strip.
If you pull on the nickel strip, you’ll get a good idea of how well the weld holds together. Welds are considered strong if they can’t be broken by hand or need considerable effort to do so. If it comes off easily, it might cause a bad battery and the engine might misfire.
Step 6: Adding the BMS
Following the wiring schematic, attach the BMS. A diagram is given in the box of the BMS. Usually, B-, B1, B2, and B+ are the four soldering pads on the BMS.
To do this, connect the first parallel group’s negative terminal bus to the B- and the positive terminal bus to the B1. The third parallel group also has a negative terminal bus going to the B2 and a positive terminal bus going to the B+.
Nickel strips can be soldered to a PCB pad or spot welded to a BMS. For a more permanent connection, I opted to solder the nickel strips to the PCB.
Solder flux should be applied to the nickel strip ends and the PCB pads first. The next step is to solder all the pads together after first tinning them.
Step 7: Wiring and Assembly
A typical car battery has two terminals to charge and discharge. You can add a charge indicator if you want to know how much charge is left in the battery. You can use a 5mm DC jack ( 12V /3A ) for input/output and a 3S battery level indicator. You can also use a rocker switch to ON/OFF the battery level indicator.
Connect the BMS’s P+ terminal to the positive (red) wire coming from the DC jack and Rocker switch. Connect the BMS’s P- terminal to the negative (black) wires coming from the DC jack and the battery level indicator.
When the adhesive has cooled, fasten the battery pack to the bottom of the battery compartment. To ensure a secure fit and avoid disconnected wires.
Step 8: Charging and Testing the Battery Pack
You will need a 12V DC Charging adapter for initially charging the newly made battery. Connect the terminals with the charger using alligator clips and plug in the charger. After the charging, connect the battery to the car and start the ignition.
If everything is okay, your car will start like any normal car. If it doesn’t check the battery connections and wiring. If your battery voltage is low, it may crank but won’t start.
5 Tips & Precautions For Making the Battery
Here are some necessary tips and precautions which you can use during the battery build-
- Before starting spot welding, always wear safety gloves and goggles.
- Don’t solder the wires ( P+ and P- ) to the BMS before fitting in the battery to the case.
- Do not work with leaked or puffed battery cells, always avoid this. Using such cells may result in fire and explosion.
- Use the heat gun to place the heat shrink tubes in place. But do not overheat them or they will burn.
- Place the cells carefully in the casing, and make sure the battery cells don’t get damaged.
Follow these tips and precautions to perform a safe and hassle battery build.
- Can Lithium-Ion Batteries Be Used In Cars?
Yes, they can be and they are used in cars. Most EVs and Plug-in hybrids use lithium-ion batteries in their car. It is very common now in the industry. Also, it is more reliable and efficient than conventional technology.
- Will My Car Alternator Charge A Lithium-Ion Battery?
Yes, your car alternator will charge a lithium-ion battery. And it will work normally like any other battery charging. However, you will need some additional accessories. This includes Battery Isolation Manager (BIM).
- Can I Overcharge My Lithium Car Battery?
You should not, by any means overcharge your lithium car battery. Because the internal pressure of the battery rises due to overcharging. So, the battery may overheat and undergo thermal runaway. When this happens, the protective circuit turns off the flow of ions to keep the pressure from becoming too high.
That will be all from our side on how to make a car battery from 18650 cells. I hope that you can do it by yourself after reading this article.
Always check the charge level of your battery regularly. Don’t deplete it fully. Also, check the wirings and connections sporadically.
See you soon.