Project 1 Mechanical design of battery pack

Given cell Datasheet :

Constant power discharge charecteristics :

Constant current discharge charecteristic:

Cell perameter according to cell datasheet :

• Cell dimension = 26 x 65 mm
• call weight = 76g
• cell capacity = 2.5 Ah
• Nominal voltage = 3.3v 
• internal resistance = .006 ohms 
• power = 2600 W/kg

Calculation :

• Battery capacity = 18 Kwh   given
• BAttery voltage  = 330v    assuming

series connection :

Each cell voltage is 3.3 v and battery pack voltage requirement is 330 v ,

Nuber of cells required to be connected in series are as follow :

Ns = battery pack voltage / individual cell voltage 

    = 330/33

  = 100 

So ,here we required 100 number of cells in series connections.

Parallel connection :

Required capacity is 18000 Wh ,

voltage is assued to be 330 v ,

hence current is given by ,

I = capacity /voltage 

 = 1800/330

 = 54.54 Ah

Single cell has capacity of 2.56 Ah 

So , number of cells required to be connected in parallel are as follow :

Np = Battery capacity / individual cell capacity

     = 54.54 / 2.56

     = 22 aprox.

so ,here we required 22 number of cell in parallel connection .

Total Number of cells are -

 = Number of cells in series x number of cells in parallel

 = 100 *22

  = 2200

So we required 2200 cells for battery pack

Cell Architecture :

There are 100 cells in series and 22 cells in parallel.

  100s22p

Design of battery pack :

• Cell diameter = 26 mm
• cell height  = 65 mm
• cell weight = 76 g

Length of casing (L) = number of cells at series  x  (cell diameter  +  margin at the both side of cell ) + Clearance at side 

                              = 100 x ( 26 +4)  + 60

                               =3060 mm

Breadth of casing (B) = Number of cells at parallel x (cell diameter  +  margin at the both side of cell ) + Clearance at side 

                                = 22 x (26 + 4) + 60

                                = 720

Height of casing (H) =  height of cell + margin of wiring + cell holder and metal strip 

                               = 100 mm

Weight of battery pack :

cell weight = 76 g 

number of cells = 2200 

Battery pack weight (W) = cell weight x number of cells + weight of accessories 

                                    = 76 x 2200 + 4.5

                                    = 167.2  kg

Connection of cells :

• Cells are connected with the help of Nickel strips .
• Nickel is used because it has high heat capability .

Welding :

• To connect cells the nickel strip are used  and are welded by following methods :
• Spot weiding 
• Laser welding 
• Wire welding .

Cell insulation :

• The outside case of battery is electrically connected to the positive end of the cell .
• This surface is electrically isolated from an electrical bus bar or mechanical support that may be of different voltage potential .
• Insulation ,such as tapes , shrink wraps , or sleeves , must have at least a 150 degree C melting point .
• This helps to ensure that the cells do not short circuited to each other at a high temperature.

Cell protection :

• Cell is protected from exposure to corrosive substance and oxidizing catalysts ,such as dust and moisture .
• The necessary level of protection for cells in a battery pack varies depending on the intended application .
• The battery pack design for use in an HEV must have an enclosure that isolated cells from shock and vibration , protect them from dirt and debries , as well as shield them from other environmental dangers ,such as salt spray.
• Even a sealed enclousre is subjected to pressure differentials between its inside and the ambient ,causing minute amounys of air exchange .
• Over a time , some moisture may accumulate and condense on inside srface .
• Enclosure protecting cells must work with the thermal management system to achieve optimum durability and safety of the battery pack. 

Cell Testing :

• Various call testing is done to check the battery performance and mechanical strength.
• Battery pack are subjected to comprehensive testing to ensure that the electronic are performing correctly .
• The protection circuit is check by applying a short circuit across the terminals for  1 to 2 second and checking that the current path is cut with in prescribe period and the battery recovers afterward.

Aging test :

• Various test is taken to check the self depliting charecteristics of battery .
• Life cycle test 
• capacity fade test 
• calender test 

Performance test :

Following performance test are performed :

• Dynamic load test
• HPPC test

Safety test :

• Cell short circuit test : This test is carried out to check the short circuited conditions .
• cell overcharging test : This test is carried out to check the what will happed if battery is overcharged.
• Cell nail penetration test : in this test the cell is given indentation to see that how much toughness it has .
• Fire resisitance test :  To check the fire resistivity of the cell .

Mechanical test :

• Vibration test : in this test the battery pack is applied to external load to check the vibration capacity of battery pack during various condition .
• Shock test
• water proof test .

Thermal management :

• Cell operates throughout a wide temperature range , they are most effective between 25 degree C to 35 degree C .
• However it can be operated between 15 degree C to 45 degree C.
• Carefull attaintion to the thermal management is necessary to keep the cells operating at peak efficiency and avoid fault conditions .
• In HEV vehicles operating in cold climates - where a heater is beneficial to keeping the cell operating in their optimal range .
• There are various method to avoid thermal rumaway occuring in the battery .
• Passive cooling and active cooling .

Battery management system :

• It is electronic device which combination of hardware and software .
• The BMS and cells together form the energy management system .
• BMS isused to control various parameter like current ,vlotage and temperature of the battery .
• To protect the cell from being damage .
• To inprove the life cycle of the battery pack.
• To maintain same SOC level of battery .
• To report to user or other devices.

Recycling :

• Recycling of battery is very important process .
• After some time the capacity of battery is decrease .
• So that battery is not usable for EV.
• The process of recycling ,refabricating and reusing only battery are called secondary life of battery.
• Second way is to extract the material from the battery .