Mechanical design of battery pack
Aim: To calculate detailed battery pack parameters and dimensions using the cell model ANR26650M1-B Nanophosphate high power Lithium-ion cell.
Procedure:
The high-performance Nanophosphate lithium iron phosphate (LiFePO4) battery technology delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package. These cells have a low capacity loss and impedance growth overtime as well as high usable energy over a wide state of charge (SOC) range, allowing our systems to meet end-of-life power and energy requirements with minimal pack oversizing.
The figure below shows a typical ANR26650M1-B Nanophosphate high power Lithium-ion cell
The technical data of the above cell is as given below
Now, let's begin with the calculations.
Cell calculations:
Cell volume = Volume of a cylinder
= `pir^2h`
= `3.14*0.013^2*0.065`
= `0.0000344929m^3`
Cell energy = Nominal Voltage * Nominal Cell Capacity
= `3.3*2.5`
= 8.25 Wh
Cell energy density = Cell energy/Cell volume
= `8.25/0.0000344929`
= `239.18(Kwh)/m^3`
Battery pack calculations:
Assumptions: Taking 50 cells in series and 44 cells in parallel for the battery pack.
Series Strings
Cells combined in series strings can achieve higher operating voltages by connecting the positive terminal of one cell to the negative terminal of the next cell. Connect strings of series cells using their current collection tabs in a manner similar to that illustrated in the figure below.
Parallel Strings
Cells combined in parallel strings can achieve higher operating capacities by connecting like-polarity terminals of adjacent cells to each other. Connect strings of parallel cells using their current collection tabs in a manner similar to that illustrated in the figure below.
Total no. of cells = `50*44`
= 2200 cells
Total Weight of battery pack = Weight of one cell * Total no. of cells
= `0.076*2200`
= 167.2 Kg
Total Volume = Cell Volume * Total No. of cells
= `0.0000344929*2200`
= `0.07588438m^3`
Total Voltage = Nominal Voltage of cell * Number of cells in series
= `3.3*50`
= 165 Volts
Total Capacity = Nominal cell capacity * Number of cells in parallel
= `2.5*44`
= 110 Ah
Battery pack capacity = Total Voltage * Total Capacity
= `165*110`
= 18150 Wh
= 18.15 KWh
This is the required battery pack capacity.
Battery pack specifications:
- Total Nominal Voltage = 165 Volts
- Total Nominal Capacity = 110 Ah
- Battery pack capacity = 18.15 KWh
- Total Energy Density = `239.18(KWh)/m^3`
- Total Weight of battery pack = 167.2 Kg
Power
These cells are designed to deliver high power in pulse and continuous applications. The figure below displays typical discharge curves showing that the voltage remains virtually flat during the discharges and the delivered AH capacity doesn’t change significantly, no matter what the rate of discharge.
Cell resistance changes with cell temperature. The warmer the ambient and/or cell temperature, the lower the resistance. In the figure below, the cells hold up voltages at cold temperatures less effectively than at ambient room temperatures. Voltages reach parity when the cells heat up to or beyond ambient room temperatures.
Life
These cells offer long cycle and calendar life, with minimal impedance growth over the life of the cells. The ambient temperature cycle life graphs in the figure demonstrate that these cells also offer extended calendar life, even at elevated temperatures.
Manufacturing of a battery pack
1. Select the ANR26650M1-B Nanophosphate high power Lithium-ion cell
Nanophosphate is an engineered nanoscale material with specific structural and chemical properties designed to maximize the performance of lithium-ion batteries.
2. Choosing the Right Battery Strips
To make the battery pack, you have to connect these 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. It will be wise to buy a pure nickel cell. 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.
3. Spot Welding Vs Soldering
There two options to connect these 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 :
Why Spot welding is preferred over soldering? The problem with soldering is that there is a need to 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 the cell will lose some capacity and life of the cells.
Instead of buying a costly Spot Welder, soldering the nickel tabs to the cell by following some precaution and tricks will be useful:
1. To minimize the contact time of the 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 to avoid holding 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.
4. 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 using brand new cells, the cell voltage is near 3.5 V to 3.7 V, joining them together is much easier. But if we 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.
5. Battery Pack Capacity and Voltage
The final pack configuration is designated as a “50S44P pack” as per the calculations done earlier with a final specification of 165V, 110AH.
6. Assemble the ANR26650M1-B Cells
From the previous step, it is clear that our battery pack is made up of 50 parallel groups connected in series ( 50 x 3.3V = 165V), and each parallel group has 44 cells (2.5 Ah x 44 = 110 Ah). Now we have to arrange the 2200 cells properly for making the electrical connection among them and with the BMS board.
Place the first parallel group of cells (44 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, refer to the above picture.
Now assemble the cells to make the pack by using hot glue or by using a plastic ANR26650M1-B battery holder. The main advantages of using these cell holders are
1. We 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 allows fresh air to pass and the battery gets cooled easily.
3. It makes our battery pack solid and reliable.
4. It provides safety and anti-vibration to our battery pack.
7. Spot Weld the Nickel Strips
Now it is time to know the procedure for using the Spot Welder. The Spot welder has three welding choices: fixed welding head, fixed welding head with foot switch, movable spot welding pen with the footswitch. Here, we will use the second option. Before welding, we have to prepare the nickel strips and welder.
Cut the nickel strips :
Lay the nickel strip on top of the 44 cells (parallel), ensuring that it covers all the cell 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. We will need 40 long strips for parallel connections and 150 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. We 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 the 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. We will notice a small spark and two-dot mark on the strip.
Successful Welding :
We 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 we can easily peel it off, then we have to increase the current.
Safety: Before starting the spot welding, always wear safety goggles.
8. Adding the BMS
A battery management system (BMS) is an electronic system that manages a lithium battery pack.
Connect the BMS as shown in the figure. The BMS has four soldering pads: B-, B1, B2, and B+. We have to connect the first parallel group negative terminal bus to the B- and positive terminal bus to the B1. Similarly the third parallel group negative terminal bus to the B2 and positive terminal bus to the B+.
We can spot weld the nickel strips to the BMS or solder them to the PCB pad. I preferred to solder the nickel strips to the PCB for a sturdy connection. First, apply soldering flux to the PCB pads and end of the nickel strips. After that tin all the pads by applying a little amount of solder and then solder them together.
9. 3D Printed Enclosure
The battery pack has all-around exposed nickel strips, to avoid any accidental shorting, so designing an enclosure for it. Using Autodesk Fusion 360 to design the enclosure for the battery pack. The enclosure has two parts: The Main Body and the top lid.
Using Creality CR-10S 3D printer and 1.75 mm green PLA filament to print the parts. It will take some time to print this enclosure.
Printing settings are:
Print Speed: 70 mm/s Layer
Height : 0.3
Fill Density: 100%
Extruder Temperature: 205 deg C
Bed Temp: 65 deg C
10. Wiring the Components
Normally a standard battery has only two terminals for connecting the load and to charge the battery. Apart from this, adding a battery level indicator, to see the battery level whenever required is also possible. Using a 5mm DC jack (12V /3A ) for input/output, a 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. Preparing a simple wiring diagram for all the components. To insulate the conductive parts, using heat shrink tubing is preferable.
Note: Don't solder the wires ( P+ and P- ) to the BMS before installing the components into the enclosure.
11. Final Assembling
First, install the components into the respective slots in the 3D-printed enclosure as seen in the above figures.
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! Used 3M screws for securing the lid. Now the battery pack is ready to use.
Reference: