Week 8 Multi cell Battery Pack
WEAKEST/FAULT CELL
Weakest cell limits the usable capacity of the battery pack. Any element which is imposing certain limitation of charging and discharging battery pack, it imposes a limit on overall operation, due to to weakest cell reaching the limit earlier than others, battery pack is going to be under utilised. The cells in series below Fig 1 is showing a faulty cell in red with 3.1V
Fig 1 Faulty Cell
REASONS OF FAULT
Capacity
SOC
Leakage
Internal Resistance
IMBALANCE
If lithium-ion cells in Electric Vehicle 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 a few hundred millivolts
CELL BALANCING
It is another important task carried out by the BMS, in a module, multiple cells are combined in series or parallel to form a battery pack. The cell voltage of all the cells should always be equal, otherwise cell with lower voltage will be over discharged and the cell with higher voltage will be over charged. To prevent this BMS performs something called Cell balancing, it detects the cells with higher voltages and discharges them till they all matches. All the cells are exactly at the same SOC.
in SimuLink below screen-shot. Simulation was carried out of Lithium-ion battery pack showing a fault in one of the cells of 20
Results
The scope above is showing simulation result of 20 cells, the yellow graph is of a fault cell undergoing high temperature causing to overheat, compared to the rest which are at nominal battery temperature
The red graph on SOC above is showing cell 10 fault at 0.5 and the other 19 cells are at the required battery state of charge.
Solutions
Protection of individual cells. With proper battery pack integration and improved thermal management, the battery pack experiences lesser imbalance in a lifetime and the overall requirement of balancing is not very extensive but a must-have feature. The passive balancing with the proper board-level thermal management can handle all balancing requirements with cost-effective solutions.
The fundamental solution of balancing equalizes the voltage and state of charge among the cells when they are at a fully charged state. Cell balancing is typically categorized in two types: a). Passive and b). Active
The passive balancing method is, by discharge the cells through a dissipative bypass route. This bypass can be either integrated or external to the integrated circuit (IC). Such an approach is favorable in low-cost system application. The fact that 100% of the excess energy from a higher energy cell is dissipated as heat makes the passive method less preferable to use during discharge because of the obvious impact on battery run time.
Active Cell Balancing
Active balancing, which utilizes capacitive or inductive charge shuttling to transfer charge between battery cells, is significantly more efficient because energy is transferred to where it is needed instead of being bled off.
Balancing cells is not only important for improving the performance and life cycles of the battery, it adds an element of safety to the battery, With improved battery cells and new cell chemistry, the battery variation is stabilizing. Hence, passive balancing is a widely accepted approach for most of the applications.