Week 8 Multi cell Battery Pack
HOW WEAKEST CELL LIMITS THE USABLE CAPACITY OF THE BATTERY PACK, AND ITS SOLUTION
To get high performance which means better Voltage and current characteristics we use combination cells or so called batteries instead of cells itself. Combination of cells helps to achieve this kind of properties that is either parallel or series. In Series combination Voltage gets added same time in parallel we get high capacity by adding Ampere Hour. Practically battery pack contain of both series as well parallel combination. It is important to keep in mind that we use only same kind of cells otherwise it cause imbalance in the combination.
MISMATCH IN SERIES COMBINATION
Cells are connected in series if the requirement is higher voltage in portable equipments.
The above shown figure shows 4S configuration otherwise called 4 series configuration. In this figure 4 , 3.6 Volt batteries are connected in series. As per the law Voltage gets added in series where as current remains the same for all cells. High Voltage batteries keep the conductor size small. These batteries need careful cell matching specifically speaking operating in high voltage or at cold temperatures. If any one of the cell fails the whole battery pack is also useless. To prevent this from happening a solid state switch is used which bypasses the cell in large battery packs.
Cell matching is a very difficult task in ageing packs. Because the added cell will always have more Voltage than the older one. Hence it is advised to change the pack as a whole.
Above figure shows a pack with a faulty cell which cannot be replaced hence as a whole is recommended.
PARALLEL CONNECTION
Three cases demand parallel connection. 1) If higher currents are needed, 2) Large cells are not available, and 3) Big one does not fit. Parallel connections posses some side effects also.
Figure shows 3.6 Volt cells connected in P4 configuration to obtain a net voltage of 3.6 and a capacity of 13600mAh. Comparing series configuration with Parallel, later posses less problems in net resistance and cell failure but it may cause less load capability. Dendrite growth and reverse polarization are the main disadvantage caused by parallel circuit. Shorted cell will provide less working time and will get excessively heated and may cause fire so fuses are used to prevent this.
SERIES AND PARALLEL CONNECTIONS.
With a standared size and desired properties,series and parallel connections are used.