Week 8 Multi cell Battery Pack

1.)The weakest cell

The weakest cell is an element which rules the battery pack.

• The weakest cell is an element which is imposing certain limitations on charging and discharging of complete battery pack
• Beause it is very important to protect an indiviual cell,over voltage/under voltage limits aplpy to all series connected cells.

e.g A circuit in series consisting two cells ,1 and 2 .

assuming cell 1 having 0% and cell 2 100% charge.

In this condition charging of cell is impossible since we can not overcharge cell 2 if current flows towards cell 2.

if we consider the opposite direction(towards cell1) ,& discharging condition

its is not possible to discharge cell 1 ,hence cell 2 will be un affected as well.

In the above example the bottom three cells have 3.0v and the upper most cell has 2.7v making it the weakest cell, now assuming the under voltage limit be 2.6 when the the weakest cell reaches that limit the BMS(Battery management system) turns off the load.

The remaining cells (3.0v) haven't reached their limit yet they have little Ah wh left,hence they are under utilized. therefore the weakest cell rules the battery pack.

similarly assume one of the cell have 4.2v(over voltage limit) and rest three 3.6v . now if a charger is connected to the BMS due to over voltage in cell with 4.2v the charging will stop by BMS leaving the other three without reaching the optimum voltage.

The above model consist of 20 cells with cells 1-9 and 11 -20 with capacity_LUT, open circuit volta=EM_LUT, terminal resistance =Ro_LUT , cell resistance = R1_LUT , capacitance = c1_LUT

And the faulty cell 10 has capacity reduced by 5%--- capacity_LUT *0.95, open circuit volta=EM_LUT* 0.9, terminal resistance =Ro_LUT*5 , cell resistance = R1_LUT*5 , capacitance = c1_LUT*0.95.

the differnce between the faulty and normal cells can be seen below.

after simulation

Battery cell tempreture

The cell temp in(K) is increasing in case of cell 10(faulty) having higher internal resistance , 5 times more than other cells. The tempreture is gradually rising and comparetively higher than cells 1-9 and 11-20 and the cell -10 tempreture reaches upto 330k while the rest of the cells reach 320k.This will affect the way the battery package operate together.

The battery pack might have limit of operation e.g 325k which is not reached by cells 1-9 and cells 11-20 but reached by cells 10 , which means that the tempreture protection will be operated & it will shut down the battery pack.

SOC

we can observe that SOC of cell 10 is less than other cells. The peak of cell 10 -0.50 and Peak of rest of the cells-0.52 ,such that at certain stage cell 10 will have 50% and rest have 52% charge

Due to weakest cell reaching the limit earlier than other, battery pack may be under utilized.

Solution

Cell balancing -

A balanced battery is one in which , at some point in its cycle, all the cells are exactly at the same SOC

there are two type of cell balancing

• Passive balancing
• Active balancing

Passive balancing :- passive balancing works by dissipative ,drawing from cells having extra charge. asuuming two cells in series with voltage 3.3 v and 3.4 v , according to passive balancing the cell with 3.4v will dissipate to 3.3v by using a ciruit with a resister I=(Vc/R) which will drop the voltage from 3.4v to 3.3v , zener diode or a resister with a switch. and we need to keep checking the voltage of the cell while it is begin balanced,it is checked with the mean value & if it reaches to other cell, balancing will stop.

Active balancing :-In active balancing ,connect overcharge cell to discharge cell.we do not waste extra charge of cell into the heat.

asumming two cells of 3v and 3.6v , the 3.6v is going to charge the 3v cell. This is done by various circuit based on inductor and capacitor, these circuit comparises of L & C are used to transfer the charge from higher voltage cell to lower voltage cell.