Project 1 Mechanical design of battery pack

AIM:- To prepare a detailed battery pack drawing along with its enclosure. State your assumptions.

DATA SHEET INCLUDED.

Links used:- https://a123batteries.com/anr26650m1-b-lithiumwerks-nanophosphate-3-3v-2-5ah-lithium-iron-phosphate-battery/

https://www.instructables.com/DIY-Professional-18650-Battery-Pack/

A123’s 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. Our cells have low capacity loss and impedance growth over time 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.

Specification:-

Nominal Ratings:-

Voltage: 3.3 VoltsCapacity: 2.5 Ah
Energy: 8.25 Wh
Specific Power: 2600 W/kg
Impedance (1KHz AC Typical) 6 mΩ
Cycle Life at 1C/1C, 100% DOD: > 4000 cycles
Cell Type: 26650 Lithium Ion Power Cylindrical Cell

Discharging:-

Max Continuous Discharage: 50 A

Max Pulse Discharge Current > 50% SOC (10s): 120 A

Minimum Voltage: 2 Volts

Temperature: -30 to 55 Celcius

Charging:-

Recomended Standard Charges:2.5 Amps

Max Charge Rate: 10 Amps

Max Pulse Charge Current <50% SOC (10s):25 A

Flat Voltage: 3.45 Volts

Recomended charge V & Cut-off Current 3.6 Volts, taper to 125mA

Temperature ( reduce charging current to 250mA when under 0 degrees centrigrade):0 to 55 degress centrigrade.

Mechanical:-

Diameter: Ø25.96 +/- 0.5 mm (1.0")

Length: 65.15 +/- 0.5 mm (2.6")

Mass: 76 g / cell

FOR A 18kWh Battery Pack the following calculations are made:-

Let us consider

Cell nominal voltage = 3.3V

Cell nominal capacity =2.5 Ah

Let us consider a 48V battery pack.

To make this voltage battery pack we need 'Ns' number of cells in series and 'Np' number of cells in parallel.

Ns=48V/3.3V = 14.545 or 15 number of cells in series connection to generate a total of 48 Volts or pratically 49.5 Volts.

Now we need 18kWh so 18000/48V=375 Ah total capacity, but each cell is 2.5 Ah,

so Np=375 Ah/2.5 Ah = 150 modules in order to produce 375 Ah capacity.

Therefore to produce 18kWh we need 15 cells in series and 150 cells in parallel where each we can get the following:-

3.3Volts x 15 series cells = 49.5 volts

2.5Ah x 150 parallel cells = 375 Ah

total capacity = 49.5 V x 375 Ah = 18562.5 Wh or 18.5625 kWh or 18 Kwh.

The extra power can be accounted in other losses like connection losses or heat, etc.

The total number of cells required for the pack is 15x150=2250 cells.

the weight of each cell is 76 grams . there fore the entire battery pack is 2250 x 76 grams= 171 kilos of cell weight only.

the bms would weigh approx 1 kilo and nickel wire strip could be 2 to 3 kilos. This all excludes any additional cooling system like water jacket of heat sink, etc

Therefore the approx weight of the 18kWh battery pack is around 175 kilos.

THEREFORE OUR PACK CONFIGURATION IS 15S150P

CONSTRUCTION:-

Step 1: Selecting the Right Cells for the Battery Pack

Although there are many manufacturers supplying different types and capacity of cells, it is more appropriate to stick to one particular type and capacity of the cells to prevent any complications. In addition we need to make sure that the cells are equally balanced or this could cause a bottleneck in the perfomance of the entire battery pack. All it takes is one single under/ over charged cell to disrupt the perfomance. Therefore before making a pack, balancing is required.

Step 2: Choosing the Right Battery Strips

To make the battery pack, you have to connect the 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 3: 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.

Step 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 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.

Step 5: Battery Pack Capacity and Voltage

{THE ABOVE FIGURES DO NOT REPRESENT THE ACTUAL 18 kWh battery pack but an approximate idea of what they may look like}

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 ).

Step 6: Assemble the 18650 Cells

Place the first parallel group of cells 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 26650 battery holder. 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

Step 7: Spot Weld the Nickel Strips

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. 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.

Safety: Before starting the spot welding, always wear safety goggles.

Step 8: Adding the BMS