Week 8 Multi cell Battery Pack
Multi-Cell Battery Pack
The weakest cell is one with lower capacity or higher impedance which has a higher voltage than other batteries. This cell continuous to weakens by overcharge cell.
This also causes capacity degradation if the maximum recommended charging voltage is exceeding by 10% it causes degradation to increase.
On the discharge side, the weak cells tend to have lower voltage than other cells which is due to either higher internal resistance or faster rate of discharge. i.e. if any
weak cells hit the cell at under-voltage protection limit while the pack voltage is still sufficient to power the system, the full potential of the battery will never be used.
The pack protection will prevent it from over-discharge by stopping the discharge of the whole pack.
Hence the solution for such imbalance in the cell is cell balancing. Cell balancing is the process of equalizing the voltage and state of charge.
No to cells are same there is always some difference in capacity, soc, impedance.
Types of Cell Balancing
Cell balancing technique can be classified into
- Passive Cell Balancing
- Active Cell Balancing
Passive Cell Balancing
Passive Cell Balancing allows the stack to look like every cell has the same capacity as the weakest cell. Using a relatively low current it drains
a small amount of energy form high SOC cells during the charging cycles so that all cells charge to maximum SOC.
This is accomplished by using a switch and a resistor with each battery cells.
The high SOC cells are bled off i.e. is power is dissipated from the resistors. Passive balancing allows all batteries to have the same SOC but it
does not improve the run time of the battery pack. It provides a fairly low-cost method for balancing the cells but also wastes energy in the process
due to discharge resistors.
Active Cell Balancing
Charging
When charging the battery stack without balancing as the weak cell reaches at its full capacity than stronger batteries. The weak cells in this case
are the limiting factor they limit the total charge in the system that it can hold. With active cell balancing charge redistribution during charging the
stack can reach its full capacity.
Discharge
When discharging, the cells starts at full capacity. In the diagram bellow full capacity is to be keep at 90% as the battery to charge till100% can cause
degradation and reduction in life span. And 30% is kept as max cut off voltage to prevent the deep discharge.
Overtime some cells will become weaker than other as shown below
We can see that there is more capacity in the other batteries hence the weak batteries limit the run time of the system. Which could let to the
excessive amount of energy that is left unused.
Hence with the help of active cell balancing charge is reintubated from the storage cell to the weaker cell resulting in full depletion
