Week 8 Multi cell Battery Pack
Multi Cell Battery Pack
Aim:
* To study on the effects of an Individual weak cell present in a battery pack.
* To find a solution to solve the Low usable capacity problem caused in a battery pack because of the a weak cell.
Battery Pack:
* A battery pack is a set of any number of identical and individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density.
* Components of battery packs include the individual batteries or cells, and the Interconnects, which provide electrical conductivity between them.
* Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging. Interconnects are also found in batteries as they are the part which connects each cell.
Problems with a weak cell in a Battery Pack:
* Common multiple cell configurations for Li-Ion cells in battery packs consist of three or four cells in series, with one or more cells in parallel.
* This combination gives both the voltage and power necessary for Portable Computer, medical, test and industrial applications.
* While these configurations are common today, they may not be as efficient as they could be. The reason for this is that any capacity mismatch between cells in a series connection of cells results in a reduction of overall pack capacity.
* Mainly there are two prime mismatches which affects the overall output of the battery pack and they are,
1. State of charge (SOC) Mismatch
2. capacity/energy (C/E) Mismatch
* Each problem limits the pack capacity (mAh) to the capacity of the weakest cell and It is important to recognize that.
* These types of cell to cell variation more likely occur in Li-Ion prismatic cells, due to more extreme mechanical stresses.
* Most typical battery chargers detect full charge by checking whether the voltage of the entire string of cells has reached the voltage regulation point.
* However, weak cells (ones with lower capacity / higher internal impedance) tend to exhibit higher voltage than the rest of the series cells at full charge termination. These cells are then weakened further by continuous overcharge cycles.
* 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.
* On the discharge side, the weak cells tend to have lower voltage than the other cells, due to either higher internal resistance, or a faster rate of discharge that results from the lower capacity.
* This means that if any of the weak cells hits the cell under voltage protection limit while the pack voltage is still sufficient to power the system, the full capacity of the battery will never be used as the pack protector will prevent over discharge (which would damage the cell) by stopping the discharge of the whole pack when one cell voltage goes below the cell under voltage threshold (usually around 2.7 V).
* If 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 4.2 V by even a few hundred millivolts.
Solution to solve the Problems with a weak cell in a Battery Pack:
* The solution to solve the Problems with a 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 a full charge.
* The fundamental solution of cell 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 type
1. Active Balancing
2. Passive Balancing
Passive Cell Balancing:
* The passive cell balancing method is somewhat simple and straight forward. 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.
* The sample circuit above shows four cells each of which is connected to two bypass resistors through a switch like MOSFET.
* The Controllers measures the voltage of all the four cells and turns on the mosfet for the cell whose voltage is higher than the other cells. When mosfet is turned on that particular cell begins to discharge through the resistors.
* Since we know the value of resistors we can predict how much charge is being dissipated by the cell. The capacitor connected in parallel with the cell is used to filter voltage spikes during switching.
* This method is not very efficient because electrical energy is dissipated as heat in the resistors and the circuit also accounts of switching losses.
Active Cell Balancing:
* Active cell 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.
* Whereas in active balancing the excess charge form one cell is transferred to another cell of low charge to equalize them. This is achieved by utilizing charge storing elements like Capacitors and Inductors. There are many methods to perform Active cell balancing lets discuss the commonly used ones
* Another method of Active cell balancing is by using inductors and switching circuits. In this method the switching circuit consists of a buck boost converter. The charge from the high voltage cell is pumped in the inductor and then discharged into the low voltage cell by using the buck boost converter. The below figure represents an Inductive converter with only two cells and single buck boost converter.
* In the above circuit charge can be transferred from cell 1 to cell 2 by switching the MOSFETS sw1 and sw2 in the following manner. First the switch SW1 is closed this will make the charge from cell 1 to flow into the inductor with current I-charge. Once the inductor is fully charged the switch SW1 is opened and the switch sw2 is closed.
* Now, the inductor which is fully charged will reverse its polarity and begin to discharge. This time the charge form the inductor flows into the cell2 with current I-discharge. Once the inductor is fully discharged the switch sw2 is opened and the switch sw1 is closed to repeat the process.
* It is faster and efficient method of cell balancing in a battery pack.
* Thus the method of Cell balancing is not only important for 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 battery life is advanced cell balancing.
* Since new cell balancing technologies track the amount of balancing needed by individual cells, the usable life of battery packs is increased, and overall battery safety is enhanced.