Week 8 Multi cell Battery Pack

Cells which are manufactured by the same manufacturer may be slightly different, no 2 cells are identical when it comes to usage, Manufacturers are often unable to explain why some cells develop high electrical leakage or an electrical short while still relatively new. The suspected culprit is foreign particles that contaminate the cells during manufacturing. Another possible cause is rough spots on the plates that damage the separator. Better manufacturing processes have reduced the 'infant mortality' rate significantly.

In a battery pack which consists of x number of cells in series and y number of cells in parallel, where the current in series cells are assumed to be constant and the voltage across parallel cells are assumed to be constant, but in reality, some cells may not be the same age as other cells and discharge faster than other cells, the weakest cell in a battery pack can cause big problems in the battery pack as its voltage and current properties will also affect the total voltage and current of the pack.

in case of parallel cells when 1 cell reaches the required peak voltage quicker than the other cells, then the pack will cut off the voltage as it assumes the required voltage has been achieved, whereas only one cell has reached optimal voltage and the other y cells are not which will cause is lesser outputs from the battery pack.

The weakest cell in the battery pack will define the shortcomings of the entire battery pack as it can lead to under usage of the pack which leads to less performance like low rage, a higher rate of discharge at cruising speed, thermal runaway due to over-charge and over-discharge and reduction in top speeds. The higher voltage of the weaker cells at charge completion causes accelerated capacity degradation. If the maximal recommended charging voltage is exceeded even by as little as 10 percent, it will cause the degradation rate to increase by 30 percent

Solutions to the weakest cell limit problem:

There needs to be some way to detect the weakest cell which would help the user to know when a problem arises, this can be done by 2 methods

1) Active cell balancing

2) passive cell balancing

ACTIVE CELL BALANCING

Active cell balancing redistributes charge during the charging and discharging cycle, active cell balancing increases system run-time and can increase the charging efficiency. Active balancing requires a more complex, larger footprint solution; passive balancing is more cost-effective.

When charging the battery stack without balancing, the weak cells reach full capacity prior to the stronger batteries. Again it is the weak cells that are the limiting factor; in this case they limit how much total charge our system can hold. The diagram below illustrates charging with this limitation

With active balancing charge redistribution during the charging cycle, the stack can reach its full capacity, the charge can be redistributed from one cell to the top of the battery stack or to another battery cell or combination of cells within the stack.

PASSIVE CELL BALANCING

Passive cell balancing is cheaper compared to active cell balancing as it can be used in places where the budget is a big constrain. There are two types of passive cell balancing

Charge Shunting

In this method, a dummy load like a resistor is used to discharge the excess voltage and equalize it with other cells. These resistors are called bypass resistors or bleeding resistors. Each cell connected in series in a pack will have its own bypass resistor connected through a switch

the drawback is that the entire discharge current flows through the MOSFET which is mostly built into the controller IC and hence the discharge current has to be limited to low values which increases the discharging time. One way to overcome the drawback is to use an external switch to increase the discharge current

Charge Limiting

The charge Limiting method is the most inefficient method of all. Here only the safety and lifetime of the battery are considered while giving up on the efficiency. In this method, the individual cell voltages are monitored continuously, During the charging process even if one cell reaches the full charge voltage the charging is stopped leaving the other cells half the way. Similarly during discharging even if one cell reaches the minimum cut-off voltage the battery pack is disconnected from the load until the pack the charged again.