Project 1 Mechanical design of battery pack

AIM

The objective of the project is to create a 18 kWh battery using ANR26650M1-B cells & prepare a detailed battery pack drawing along with its enclosure.

Battery pack

It 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. Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging. Interconnects are also found in batteries as they are the part which connects each cell, though batteries are most often only arranged in series strings.

When a pack contains groups of cells in parallel there are differing wiring configurations which take into consideration the electrical balance of the circuit. Battery regulators are sometimes used to keep the voltage of each individual cell below its maximum value during charging so as to allow the weaker batteries to become fully charged, bringing the whole pack back into balance. Active balancing can also be performed by battery balance devices which can shuttle energy from strong cells to weaker ones in real time for better balance. A well-balanced pack lasts longer and delivers better performance.

Case 1: Ladder, linear, F type, or radial

Note that the straps will both come off the top when there are an even number of cells, and one off the top, the other off the bottom when there is an odd number of cells. With a connector and heat shrink wrap they look like this:


The size of a ladder pack is D x nD x H where D is the diameter of the cell, n is the number of cells, and H is the height of the cells.

Case 2: Multi-row cells

There are two ways to start packing them. One could be called the cubic, and the other face centered cubic, or nested.

Cubic packing is in neat rows. The size of such a pack is nD x mD x H, where n is the number of cells in a row, m is the number of rows, D is the cell diameter, and H is the cell height.

 

Case 3: Face centered "cubic"

Face centered cubic packing is nested to take up less room. Calculating the size takes a little geometry, which follows:

The size is L x W x H where

L = (n +½)D
W = [0.866(p-1)+1] D
H=H
p is the number of rows wide.

Case 4: FCC with alternating long and short rows

If there are alternating long and short rows, such as the 3,4,3 ten cell pack, the formulas are

L = mD
W = [0.866(p-1)+1] D
H = H

Where m is the number of cells in the longest layer, and p is the number of layers. With heat shrink it looks like this:

Case 5: Fixed channel width optimization

What if you have a fixed width channel and want to stagger the cells to maximize the number of cells per unit width? Pull out your high school geometry book, this is trickier!

The distance P between the cells of Diameter D in a channel with height T is P = sqrt (2TD-T^2)
So if you have 18mm diameter batteries fitting into a 25.4mm channel, the distance between the cells is 16.8mm. In this case you can fit in an extra cell for every 15 cells, or save 1.2 mm of width per cell. 

ANR26650M1-B Specifications
 Nominal Ratings
  Voltage: 3.3 Volts
  Capacity: 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: -30ºC to 55ºC

 Charging
  Recommended Standard Charge: 2.5 Amps
  Max Charge Rate: 10 Amps
  Max Pulse Charge Current <50% SOC (10s): 25 A
  Float Voltage: 3.45 Volts
  Recommended charge V & Cut-off Current: 3.6 Volts, taper to 125mA
  Temperature
    (reduce charging current to 250mA when under 0ºC): 0ºC to 55ºC

 Mechanical
  Diameter: Ø25.96 +/- 0.5 mm (1.0")
  Length: 65.15 +/- 0.5 mm (2.6")
  Mass: 76 g.

Assumptions 

The Chevrolet Volt car is battery pack is used

Nominal voltage - 340V.

Current capacity - 52 Ah.

Battery holding techniques

Heat Shrink Tubing

The most common way to hold the pack together is to use heat-shrink tubing. This has sufficient strength for small packs, but as the weight increases more structural strength is necessary. This is done by adding a sheet of structural material, usually plastic or fish paper, to the top and the bottom of the pack. If the battery is to be put into another structure, either a plastic case, or the system box, it is still important to tie it together with heat shrink or tape for ease of handling.

When building and using battery packs be careful not to inadvertently short the cells. A pack of cells wired in series will become shorted if the cases of adjacent batteries touch, since the outer case is a terminal. This can happen if the cells are shrink wrapped, film wrapped or painted and the batteries rub against each other. Brittle shrink wrap is known to shred under stress, leaving the bare cell walls to touch. If there is danger of this happening cardboard sleeves are used instead of just the heat-shrink tubing.
Another source of strength is the use of glue where the cells touch. Cyanoacrylates are sometimes used to tack things together, but can weaken the vinyl heat-shrink if not reinforced. A hot-melt glue is more forgiving.

Battery Holders

When using or designing battery holders make sure there is adequate provision for short cells, long cells, or wide cells. Keep sharp clip edges from touching the cell where they could cut the film or paint, causing a short between cells held by the same clip.

Calculation

The nominal voltage, current rating & Energy capacity is 3.3V, 2.5Ah & 8.25 Wh.  

Inorder to achieve the 18kWh battery requirement number of cells required =  18000/8.25

                                                                                                             = 2122 cells

The 2205 cells can be divided into 2205 cells in series & 1 cells in parallel which makes 3.3 * 2205 = 7276.5 V & the current capacity = 2.5Ah but the 7000V system is too high for the voltage running the system & the current capacity is too low for the system. 

Calculation 

The battery pack is 105S 21P & the nominal volatge of the pack is 346.5 V, Current capacity of the pack is 52.5Ah. 

Energy Capacity - 18.2 kWh.

Max pulse current - 525 A.

Max charge current - 210 A.

Mass - 165 kg.

Single cell calculation

= pi*r^2*h

= pi*(13)^2*65

= 34510.4 mm3.

Cell peak current (A)

Charging

= Max pulse charge current * single cell capacity

= 25*2.6

= 65 A

Discharging

= Max pulse discharge current * cell capacity

= 120*2.6

= 312 A.

Continous current

Charging

= Max continous current * cell capacity

= 10* 2.6

= 26 A.

Discharging

 

= Max continous current * cell capacity

= 50 * 2.6

= 130A.

Battery pack peak current

= Cell peak current * No of cells in parallel.

= 130 * 21

= 2730 A.

Battery pack peak power

= peak current * total voltage 

= 2730*346.5

=  945945 W.

The total battery pack is divided into many modules (i.e) 2205 cells can be divided into 7 modules each module have 15S3P.

Module voltage - 15*3.3 = 49.5 V.

Module current = 3 * 2.5 = 7.5 A.

Module peak current = 7.5 *130 = 975 A.

Module peak power = 48262.5 W.

The BMS system is used in the each module where the voltage is sensed across the series & current is sensed across each parallel cells. The temperature is measured across each cell & the module gets disconnected when the cell exceeds the operating range.