Batteries in series and parallel

                                             2.  BATTERIES IN SERIES AND PARALLEL.

Content:
1.Cell vs Battery.
2.Cells/Batteries in series.
3.Cells/Batteries in parallel.
4.Battery pack design.

For all projects, we can't use a single cell and power up all circuits and components. We might need a voltage that is greater than the maximum voltage of a cell or we might have a current requirement that is greater than the single cell's current discharge capacity or we might need to power the circuitry for a long time.
This blog will be useful for those who are into electric vehicle development or any projects which involve the usage of batteries. It has information about series and parallel connection of batteries.


1.Cell vs Battery.

Li-ion cell



Battery


A Cell is a single unit that converts chemical energy to electrical whereas A battery is a group of cells.

2. Cells/Batteries in series.

3.7v nominal

Vab = 3.7v x 2 = 7.4v , 2600mAh
    
  • Connecting cells/batteries in series increases(adds up) the voltage. In any of the projects if we need to increases the voltage or we have batteries of less voltage then we can connect them in series to increase the voltage.
  • Connecting cells/batteries in series don't affect the mAh rating and C rating. In the above example, 2600mAh remains the same. Means a single cell/battery can discharge continuously 2.6A for an hour and 2s(2cells in series) battery can also deliver 2.6A continuously for an hour but at 7.4V.
  • It increases the capacity,  3.7v x 2.6Ah = 9.62 wh.  This battery can handle 9.62 watts of load for an hour.

3.Cells/batteries in parallel.

             
Vab = 3.7v, 2600 x 3 = 7800mAh.


  • Connecting cells/batteries in parallel increase the mAh value. In any electronics device if we have a more current demand from the components in the circuit then we can go for parallel connection of batteries.
  • Connecting cells/batteries in parallel don't affect the voltage. In the above example, 3.7v(nominal voltage.4.2v peak) remains the same. All the cells discharge at the same rate and maintain the same voltage level.  
  • In the above example, the battery can give 7.8 Amps for an hour at 3.7v (nominal) as the battery discharges the voltage decreases.
  • We should make sure that we connect batteries of the same voltage in parallel.
  • 3.7v x 7.8Ah = 28.86wh. This battery can handle 28.86watts of load for an hour.
  • C- rating of each battery doesn't change it remains the same, in the above example it is 1C. But the overall discharge current increases 7.8A x 1C = 7.8 A maximum current.

What happens if we connect batteries in parallel with different voltage levels?



  • We have 2 batteries connected in parallel but one is having voltage 3v and the other 3.7v. The battery with higher voltage discharges and starts charging the battery with lower voltage. This continues until both the battery reaches the same voltage level around 3.2 to 3.3v each. 
  • In this process, the battery gets heated up a bit and battery life also reduces. The current 'i' mentioned in the above figure might exceed the C rating of the battery as there is no current limiting resistor in between.
  • So it is better to avoid connecting batteries of different voltage levels in parallel. If there is 0.2v or 0.3 v difference then it is ok to connect in parallel. 

4.Batter pack design.

 1.2S2P -  2 cells/batteries in series and 2 in parallel.




Vab = 3.7 x 2 = 7.4v, 2600 x 2 = 5200mAh. This battery pack can discharge 5.2Amps for an hour at 7.4v (nominal).

2. 3s 2p.   


            Vab = 3.7 x 3 = 11.1v, 2600 x 2 = 5200mAh. This battery pack can discharge 5.2Amps for an hour at 11.1v (nominal).  Capacity = 11.1v x 5.2Ah = 57.72Wh.  It can handle the 57.72Watts of load for an hour, 115.44 watts of load for half an hour but the load should draw less than or equal to 5.2Amps of current.     

Let's work out:

You need a battery pack of 48V & 1 KW. Consider cell Nominal Voltage is 3.6 V, Nominal Capacity is 2.6 Ah, Calculate the number of cells required in Series, Cells in parallel & the Total number of cells.

Solution:   V = 48V,  P = 1KW.

                  V = ns x 3.6
                  48 = ns x 3.6
                                                     ns = 13

                                                      I = np x 2.6

                                                     P = V x I
             
                                                    P =1k = ns(3.6) x np (2.6) -----( ns,np-number of cells in series and                                                                                                                                                       parallel)
                                                    1k =48 x np(2.6)
                                                    np = 8 
                                                  
              
                                                  P =1k = ns(3.6) x np (2.6) = 13(3.6) x 8(2.6) = 973.44watts (nearly equal to 1k watts)

                    
* Tesla uses 18650 cells in their model S battery pack. There are 7104 cells.




Next post:  Battery charging, BMS(battery management system).  





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