Mechanical design of battery pack

- Aim: To prepare a battery pack combination for 18KWh capacity using ANR26650M1-B  cell.

- Objectives:

- To make a design model of a battery pack.

- To calculate the number of series and number of parallel cell in order to form a module.

- To calculate the number of modules required to form the battery pack.

- Method:

- Battery Theory:

- A battery is a device consisting of one or more electrochemical cells with external connections for powering electrical devices such as flashlight, mobile phones and electric cars. When a battery is supplying electric power, its positive terminal is the cathode and its negative terminal is the anode. The terminal marked negative is the source of electrons that will flow through an external electric circuit to the positive terminal. When a battery is connected to an external electric load, a redox reaction converts high-energy reactants to lower-energy products, and the free energy difference is delivered to the external circuit as electrical energy.

- Primary(single-use)batteries are used once and discarded, as the electrode materials are irreversibly changed during discharge a common example is an alkaline battery used for flashlights and a multitude of portable electronic devices. Secondary (rechargeable) batteries can be discharged and recharged multiple times using an applied electric current the original composition of the electrodes can be restored by reverse current. 

- A battery consists of some number of voltaic cells. Each cell consists of two half-cells connected in series by a conductive electrolyte containing metal cations. One half-cell includes electrolyte and the negative electrode, the electrode to which anions (negatively charged ions) migrate the other half-cell includes electrolyte and the positive electrode, to which cations (positively charged ions) migrate. Cations are reduced (electrons are added) at the cathode, while metal atoms are oxidized (electrons are removed) at the anode. Some cells use different electrolytes for each half-cell then a separator is used to prevent mixing of the electrolytes while allowing ions to flow between half-cells to complete the electrical circuit.

- Battery Capacity:

- A battery's capacity is the amount of electric charge it can deliver at the rated voltage. The more electrode material contained in the cell the greater it's capacity. A small cell has less capacity than a larger cell with the same chemistry, although they develop the same open-circuit voltage. Capacity is measured in units such as amp-hour (A·h). The rated capacity of a battery is usually expressed as the product of 20 hours multiplied by the current that a new battery can consistently supply for 20 hours at 68 °F (20 °C) while remaining above a specified terminal voltage per cell.

- Anode:

Electrons flow out from the anode in a device connected to a circuit. This means that conventional "current" flows into an anode. On batteries, the anode is marked as the negative (-) terminal. In a battery, the chemical reaction between the anode and electrolyte causes a build-up of electrons in the anode. These electrons want to move to the cathode but cannot pass through the electrolyte or separator.

- Cathode:

Electrons flow into the cathode in a device connected to a circuit. This means that the conventional "current" flows out from a cathode. On batteries, the cathode is marked as the positive (+) terminal. In batteries, the chemical reaction in or around the cathode uses the electrons produced in the anode. The only way for the electrons to get to the cathode is through a circuit, external to the battery.

- Electolyte: 

The electrolyte is the substance, often a liquid or gel, that is capable of transporting ions between the chemical reactions that happen at the anode and cathode. The electrolyte also inhibits the flow of electrons between the anode and cathode so that the electrons more easily flow through the external circuit rather than through the electrolyte. The electrolyte is crucial in the operation of a battery. Because electrons cannot pass through it, they are forced to travel through electrical conductors in the form of a circuit that connects the anode to the cathode.

- Separator:

- Separators are porous materials that prevent the anode and cathode from touching, which would cause a short circuit in the battery. Separators can be made from a variety of materials, including cotton, nylon, polyester, cardboard, and synthetic polymer films. Separators do not chemically react with either the anode, cathode, or electrolyte.

- C rate:

- The C-rate is a measure of the rate at which a battery is being charged or discharged. It is defined as the current through the battery divided by the theoretical current draw under which the battery would deliver its nominal rated capacity in one hour.

- Operation:

- Batteries generally require several chemical reactions in order to operate. At least one reaction occurs in or around the anode and one or more reactions occur in or around the cathode. In all cases, the reaction at the anode produces extra electrons in a process called oxidation, and the reaction at the cathode uses the extra electrons during a process known as reduction.

- When the switch is closed, the circuit is complete, and electrons can flow from the anode to the cathode. These electrons enable the chemical reactions at the anode and cathode.

- In essence, we are separating a certain kind of chemical reaction, a reduction-oxidation reaction or redox reaction, into two separate parts. Redox reactions occur when electrons are transferred between chemicals. We can harness the movement of electrons in this reaction to flow outside the battery to power our circuit.

- Anode Oxidation:

- This first part of the redox reaction, oxidation, occurs between the anode and electrolyte, and it produces electrons (marked as e-). Some oxidation reactions produce ions, such as in a lithium-ion battery. In other chemistries, the reaction consumes ions, like in the common alkaline battery. In either case, ions are able to flow freely through the electrolyte where electrons cannot.

- Cathode Reduction:

- The other half of the redox reaction, reduction, occurs in or near the cathode. Electrons produced by the oxidation reaction are consumed during reduction. In some cases, like lithium-ion batteries, positively charged lithium ions produced during the oxidation reaction are consumed during reduction. In other cases, like alkaline batteries, negatively charged ions are produced during reduction.

- Calculations:

- The specifications for the given cell are mentioned below.

Cell nominal voltage = 3.3v

Cell nominal capacity = 2.5Ah

The cell energy capacity is the product of the above two parameters = 3.3v * 2.5Ah = 8.25 Watt-hr.

- As per the requirement the energy capacity is 18 Kw-hr.

- Lets us take an assumption of 2S & 10P.

- The voltage of each module = 2S * 3.3v = 6.6v

- The capacity of each module = 10P * 2.5Ah = 25 Ah

- Let us assume a 50v battery pack, to make this voltage battery pack we will need a number of series modules = 50/6.6 = 7 modules in series.

- To get a pack energy capacity of the battery = 18000WAh -> 18000/50 = 360 Ah

- To make this pack energy capacity battery we need a number of modules in parallel = 360 Ah / 25 = 14

- Total capacity we get as = 8SM * 14 PM * (2S*3.3v) * (10P*2.5Ah) = 18480 Wh = 18.48Kwh

- Mechanical Design of a battery pack in Catia V5:

- In order to form the battery pack a design is made in design software catia V5.

- All the dimensions are taken as per the cell parameters. The dimension for the cell cover is assumed and the same for all the cells.

Dimension for the holder.
	1 single cell
1 module as per requirement 2Parallel and 10 series combination.

- Learning outcome:

- To perform the calculations for battery pack design to calculate the number of series and number of parallel cells in a module.

- To understand the design procedure for the module.

- To use the Catia V5 software to build the design.

- Reference:

- https://datasheetspdf.com/pdf/1424845/A123Systems/ANR26650M1-B/1

- https://en.wikipedia.org/wiki/Electric_battery