Project 1 Mechanical design of battery pack

To make the battery pack, you have to connect the 18650 cells together by means of Nickel strips or thick wire. Generally, Nickel strips are widely used for this. In general two types of nickel, strips are available in the market: nickel-plated steel strips and pure nickel strips. I will suggest buying a pure nickel. It is a little bit costlier than nickel-plated steel, but it has much lower resistance. Low resistance means, less heat generation during the charging and discharging, which leads to longer useful battery life.

Nickel strips come with different dimensions and lengths. Choose the strips according to the current rating.

Step 4: Spot Welding Vs Soldering

You have two options two connect the 18650 cells together: 1. Soldering 2. Spot Welding

The best choice is always Spot welding, but Spot Welder is much costlier than a good quality Soldering Iron.

Soldering :

You should know why Spot welding is preferred over soldering, the problem with soldering is that you apply a lot of heat to the cell and it doesn’t dissipate very quickly. This enhances the chemical reaction in the cell which damages the cell's performance. Ultimately you will lose some capacity and life the cells.

But if you are not interested to buy a costly Spot Welder, you can solder the nickel tabs to the cell by following some precaution and tricks :

1. To minimize the contact time of your soldering iron on the cell, make sure the surface is scuffed up sufficiently and you use plenty of flux to allow for fast solder flow.

2. It is better to have a good quality high wattage ( min 80W ) iron with good thermal capacity so it can deliver the heat to the joint quickly so you don't have to hold the iron to the battery for ages and let the heat seep into it, causing damage to the battery.

Spot Welding :

The reason we spot weld because it securely joins the cells together without adding much heat to them. There are two grades of spot welders currently available in the market: hobby grade and professional grade. A decent hobby-grade Spot welder costs around $200 to $300, whereas a good professional grade may cost around ten times more. So I will suggest buying a hobby-grade spot welder from any online store like Banggood, Aliexpress.

Step 5: Check the Cell Voltage

Before connecting the cells in parallel, first, check the individual cell voltages. For paralleling the cells, the voltage of each cell should be near to each other, otherwise, a high amount of current will flow from the cell with a higher voltage to the cell with a lower voltage. This can damage the cells and even result in fire on rare occasions.

If you are using brand new cells, the cell voltage is near 3.5 V to 3.7 V, you can join them together without worrying much. But if you are going to use an old laptop battery, be sure the cell's voltage is nearly the same, otherwise, charge the cells to the same voltage level by using a good Li-Ion Battery Charger. I used my Nitecore SC4 Charger to charge all the 18650 cells before joining them together.

Step 6: Battery Pack Capacity and Voltage

To make the battery pack, you have to first finalize the nominal voltage and capacity of the pack. Either it will be in terms of Volt, mAh/ Ah, or Wh. You have to connect the cells in parallel to reach the desired capacity (mAh ) and connect such parallel group in series to achieve the nominal voltage (Volt ).

For this project let the requirement is: 11.1 V and 17 Ah Battery Pack

Specification of 18650 Cells Used: 3.7V and 3400 mAh

Capacity (mAh):

The desired capacity of the battery pack = 17 AH or 17000 mAh.

The capacity of each cell = 3400 mAh

No of cells required for parallel connection = 17000 / 3400 = 5 nos

Commonly cells in parallel are abbreviated in terms of ‘P’, so this pack will be known as a “5P pack”.When 5 cells are connected in parallel, ultimately you made a single cell with higher capacity ( i.e 4.2V, 17000 mAh )

Voltage(Volt) :

The desired nominal voltage of the battery pack is 11.1V.

The nominal voltage of each cell = 3.7 V

No of cells required for series connection = 11.1 /3.7 = 3 nos

Commonly cells in series are abbreviated in terms of ‘S’, so this pack will be known as a “3S pack”.

So we have to connect the 3 parallel groups (5 cells in each group ) in series to make the battery pack.

The final pack configuration is designated as a “3S5P pack” with a final specification of 11.1V,17AH.

From the previous step, it is clear that our battery pack is made up of 3 parallel groups connected in series ( 3 x 3.7V = 11.1V ), and each parallel group has 5 cells ( 3400 mAh x 5 = 17000 mAh). Now we have to arrange the 15 cells properly for making the electrical connection among them and with the BMS board.

Place the first parallel group of cells (5 nos) positive side up, then place the second parallel group negative side up, and then finally the last parallel group positive side up. For better understanding, you can see the above picture.

You can assemble the cells to make the pack by using hot glue or by using a plastic 18650 battery holder. I used plastic 18650 cell holders/spacers to assemble the 15 cells. The main advantages of using these cell holders are

1. You can make a custom pack of any size according to your requirement.It's like solving a puzzle.

2. It provides space between the cells, which allow fresh air to pass and the battery gets cooled easily.

3. It makes your battery pack solid and reliable.

4. It provides safety anti-vibration to your battery pack

Now it is time to know the procedure for using the Spot Welder ( I am talking about the Spot welder that I have used in this project). The Spot welder has three welding choices: fixed welding head, fixed welding head with foot switch, movable spot welding pen with the footswitch. I prefer to use the second option. Before welding, you have to prepare the nickel strips and welder.

Cut the nickel strips :

Lay your nickel strip on top of the 5 cells ( parallel ), ensuring that it covers all cells terminals, leave 10mm excess strips for connecting it to the BMS, and then cut it. For series connection cut small nickel strips as shown in the figure. You will need four long strips for parallel connection and 10 small strips for series connections.

Connect the first parallel group negative terminal to the positive terminal of the second group and then the negative terminal of the second group to the positive terminal of the third group.

Weld the Battery Strips :

This spot welder can be used to weld the pure nickel as well as nickel-plated steel strips. You have to adjust the welder pulse and current knob according to the thickness of the nickel strips.

For 0.15 mm nickel strips, press the pulse knob 4P and current knob to 4-5. Similarly for 0.2 mm nickel strip, press the pulse knob 4P,6P, and current knob to 7-8. Make sure the welding pen is compressed with the nickel strip and battery terminal, then press the footswitch. You will notice a small spark and two-dot mark on the strip.

Successful Welding :

You can check the weld quality by pulling on the nickel strip. If it doesn’t come off with hand pressure or requires a lot of strength, then it’s a good weld. If you can easily peel it off, then you have to increase the current.

Step 9: Adding the BMS

5 More Images

A battery management system (BMS) is an electronic system that manages a lithium battery pack and the main functionalities are

1. Monitors all of the parallel groups in the battery pack and disconnect it from the input power source when fully charged ( near 4.2V )

2. Balance all the cells voltage equally

3. Doesn't allow the pack from over-discharged.

The two important parameters required to buy a BMS are: i) Number of cells in series - like 2S / 3S / 4S

ii). Maximum discharge Current - like 10A/ 20A /25A /30A

For this project, I have used a 3S and 25A BMS board. These are the specifications of that BMS :

Overvoltage range: 4.25~4.35V ± 0.05V

Over-discharge voltage range: 2.3~3.0V ± 0.05V

Maximum operating current: 0~25A

Working temperature: -40℃ ~ +50℃

Step 10: 3D Printed Enclosure

The battery pack has all-around exposed nickel strips, to avoid any accidental shorting, I designed an enclosure for it. I used Autodesk Fusion 360 to design the enclosure for my battery pack. The enclosure has two parts: The main Body and top lid. You can download the.STL files from Thingiverse.

I used my Creality CR-10S 3D printer and 1.75 mm green PLA filament to print the parts. It took me about 6.5 hours to print the main body and around 1.2 hours to print the top lid.

My settings are:

Print Speed: 70 mm/s Layer

Height : 0.3

Fill Density: 100%

Extruder Temperature: 205 deg C

Bed Temp: 65 deg C

Normally a standard battery has only two terminal for connecting the load and to charge the battery. Apart from this, I have added a battery level indicator, to see the battery level whenever required. I have used a 5mm DC jack ( 12V /3A ) for input/output, 3S battery level indicator module to see the battery status, and a rocker switch to ON/OFF the battery level indicator.

Now let's move on to the wiring of the components. I've prepared this simple wiring diagram for all the components. It's pretty simple! To insulate the conductive parts, I used heat shrink tubing.

Solder the positive (red wire ) from the DC jack and Rocker switch to the P+ of the BMS, negative wires from the DC jack, and Battery level indicator to the P- of BMS.

Then apply hot glue at the base of the battery compartment, then secure the battery pack. So that it will seats firmly and prevent any loss of wire connections.

Finally, screw the top lids in place! I used 3M x 10 screws for securing the lid. Now the battery pack is ready to use.