A battery pack is a set of any number of identical batteries or individual battery cells. They may be configured in a series,parallel or a mixture of both to deliver the desired voltage, capacity, or power density. 18650 Li-ion cells are a great way to power Projects. Which provides a nominal voltage of 3.7V which is not sufficient for most of the application for let's say you want to power a BLDC motor with an ESC which requires 9V to start. So we need more voltage which can be achieved by connecting them in series. We will show you how you can make a 18650 Li-ion Battery Pack with a BMS circuit and all the things you need to know before you built one!
1. Everything We Need for Building a 18650 Battery Pack
1)18650 Li-ion cells - 18650 batteries are used for powering everything from laptop batteries to electric vehicles. It is a standardized type of lithium-ion battery, cylindrical in shape and measuring 18mm in diameter by 65mm in length (give or take a few 1/10s of a millimeter).
2)BMS - A battery management system (BMS) is an electronic system that manages a rechargeable battery (cell or battery pack),such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondarydata,reporting that data, controlling its environment, authenticating it and / or balancing it.
3)Spot welder - One of the purposes of spot welding is to join 2 or more components together in a mainly permanent fashion. you can buy one or built one.
4)Nickel Ribbon - Nickel ribbon is commonly used in battery building due to its ease of use in spot welding and soldering as well,its high corrosion resistance over time.
2. Check the Battery and Charge Them Up!
First, we need to check the capacity of all the cells & make sure all the cells are at the same voltage. It is mandatory that all the cells are at the same voltage else cells will end up charing each other and it can lead to unexpected results.
3. Calculations for the battery pack voltage and capacity
18650 cell can provide a Nominal voltage of 3.7V, Minimum voltage of 3V and Maximum voltage of 4.2V. So if we consider nominal voltage, connecting 6 cells in series will give us 22.2V which is a 6S1P Configuration. Where 6S means 6 Cells in series and 1P means 1 cell in Parallel. By adding another 6 Cells in parallel we can not only double the capacity but also the amount of current
Pack can deliver.
4. Building the Pack and Spot Welding
For building the battery pack we need tiny black Plastic spacers which snap like Lego and can be very useful as we don't need to use adhesive instead. Now make 2 pairs of 6 plastic spacers, Insert the cells in alternation pattern. Now we measured the spacing between 2 cells and it was about 25mm. So I used Nickel Ribbon marked 25mm on it and used sharp scissors to snap it. Now Place the Nickel pieces on top of cells and make a series connection use a spot welder to fuse nickel ribbon and battery top.
5. Adding BMS
Now we can add and plug and call it for the day but we can also add a BMS which adds over charge, over discharge and some of the also add Balance charging to the battery pack. we just have to solder it to the battery pack as shown on the board. Also, some BMS like mine does not offer Balance charging so you need to add and Balance charging connector and You are done!
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