Week 8 Multi cell Battery Pack
Aim:
To study the effects of the weak cells present in the battery pack and to find the solution for the low capacity issue caused in the battery pack because of the weak cell.
Battery Pack:
* A battery pack is a set of any number of identical and individual battery cells. They maybe configured in both series and parallel to deliver the desired voltage, capacity and power density.
*Components include the individual batteries or cells and the interconnects, provide the electricity between them.
* Rechargeable battery contains a temperature sensor, which the battery charger uses to detect the end of charging.
* Common configurations for Li-Ion cells in battery packs consistof three or four cells in series, with more cells are in parallel.
* This combination gives both the voltage and power necessary for Portable computer, medical and industrial applications.
* While these configurations are common today, they may not be as efficient as they could be.
* There are two prime mismatches which affects the overall output of the battery pack., which are State of Charge, Capacity/Energy Mismatch.
* These kind of problems limit the pack capacity to the capacity of the weakest cell and it is important to recognize this.
* The chargers detect full charge by checking the voltage of entire string of cells has reached the maximum regulation point.
* However the weak cell tend to exhibit higher voltage than the rest at full charge termination.
* On the discharge side, the weaker cells tend to have a lower voltage than the other cells due to either higher internal resistance, or a faster rate of discharge that results from the lower capacity.
* If the lithium cells are overheated or overcharged they are prone to accelerated cell degradation. They can catch fire or even explode as a thermal runaway condition can occur if a lithium ion cell voltage exceeds 4.2V by even few 100 millivolts.
Solutions for problems with weak cell in a battery pack:
* The solution for solving the problem with weak cell in a battery pack by the method of cell balancing.
* Cell balancing is the process of equalizing the voltages and State of charge among the cells when they are at the full charge.
* Classified into
-Active Balancing
-Passive Balancing.
Passive Cell Balancing:
* It is somewhat simple and straight forward, by discharging the cells through a diissipative bypass route.
* This bypass can be either integrated or external to integrated circuit.
* The fact is 100% Of the excess energy from the higher energy cell is dissipated as heat because of the obvious impact on battery run-time.
* The sample circuit is that 4 cells each of which is connected by two by-pass resistors through MOFSET.
* The controller measures the voltage and turns on the mofset whose voltage is higher than the other cells.
* The cell begins to discharge through the resistors when MOFSET is turned ON.
* This method is not very efficient because electrical energy is dissipated as heat in the resistors and also occurs with switching losses.
ACTIVE BALANCING:
* Active cell balancing utilizes capacitive inductance shuttling to transfer charge between battery cells, that is more efficient because energy is transferred to where it is needed.
* In active balancing, the excess charge is transferred to another cell of low charge to equalize them.,which is achieved by capacitors and inductors.
* In the given circuit, chane can be transferred from cell 1 to cell 2 by switching the MOFSETS sw1 and sw2.
* First the sw1 is closed and will make the charge from cell 1 to flow into th inductor with current 1 charge.
* Once the inductor is fully charged, the switch SW1 is openedand the sw1 is closed for repeating the same.
* It is faster and efficient method of cell-balancing in a battery pack.
* Not only improving the performance and life cycles of the battery, it adds an element of safety to the battery.
* One of the emerging technologies for enhancing battery safety and extending the battery life is advanced cell balancing.
* Since new cell balancing technologies track the amount of balancing of cells, the usable life of the battery packs is increased and overall safety of battery is increased.
Conclusion:
Thus, the study the effects of the weak cells present in the battery pack and to find the solution for the low capacity issue caused in the battery pack because of the weak cell is determined, and the solution of the issues in the battery pack is analysed.