Week 8 Multi cell Battery Pack

AIM:

To define the weakest cell limits the usable capacity of the battery pack and to suggest a solution

Procedure:

# Weakest cell limits are to be explained with how it works with the usable capacity of the battery pack.

# To give a valid solution for the usable capacity of the battery pack within the weakest cell limits.

Weakest cell limits:

# The weakest cell limits set a barrier for the entire battery pack to work with limitations. The weak cell may not fail immediately but will get exhausted more quickly than the strong ones when on a load.

# On charge, the low cell fills up before the strong ones because there is less to fill and it remains in over-charge longer than the others. On discharge, the weak cell empties first and gets hammered by the stronger brothers. Cells in multi-packs must be matched, especially when used under heavy loads.

# Most battery chemistries lend themselves to series and parallel connections. It is important to use the same battery type with equal voltage and capacity (Ah) and never to mix different makes and sizes. A weaker cell would cause an imbalance.

# This is especially critical in a series configuration because a battery is only as strong as the weakest link in the chain. 

Cell balancing:

# 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.

# Every pack that is design and manufacture has an overvoltage protection circuit (sometimes even a backup) to go along with standard cell balancing that will prevent such an event from ever occurring. In a multi-cell battery pack, which is commonly used in laptop computers and medical equipment, placing cells in series opens up the possibility of cell imbalance, a slower but persistent degradation of the battery.

Eg:

Consider a lower voltage cell connected in a series configuration

A battery pack in which “cell 3” produces only 2.8V instead of the full nominal 3.6V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack. The voltage collapses and the device turns off with a “Low Battery” message.

Consider a lower capacity cell connected in a parallel configuration

A cell that develops high resistance or opens is less critical in a parallel circuit than in a series configuration, but a failing cell will reduce the total load capacity. It’s like an engine only firing on three cylinders instead of on all four.

A weak cell will not affect the voltage but provide a low runtime due to reduced capacity. A shorted cell could cause excessive heat and become a fire hazard. On larger packs, a fuse prevents high current by isolating the cell.

What is Need for Cell Balancing?

# When you need several cells grouped together to power a device, you need to do some sought of balancing. The reason is that battery cells are fragile things that die or get damaged if they are charged or discharged too much. For your cells that have different SoC and you start using them, their voltage starts dropping until the cell with the least amount of energy stored in it reaches the discharge cut-off voltage of the cell.

# At that point, if the energy keeps owing through the cell, it gets damaged beyond repair. Now, if you attempt to charge this group of cells to the correct combined voltage, the healthy cells get overcharged and thus get damaged as they will take the energy that the already dead cell is no longer able to store. Imbalanced lithium-ion cells die the first time you try to use them. This is why balancing is absolutely required.

Other reasons for cell balancing include:

Thermal Runaway

# Battery cells, especially lithium cells are very sensitive to overcharging and over-discharging. This leads to thermal runaway when the rate of internal heat generation exceeds the rate at which the heat can be released.

# By the use of cell balancing, every non-defective cell in the battery pack should be balanced to the same relative capacity as the other non-defective cells. Other than using cell balancing, you can keep the pack cool since heat is one of the primary factors that lead to thermal runaway. This minimizes the retention of heat in the pack. You should maintain the battery environment at room temperature.

Cell Degradation

* When a lithium cell is overcharged even slightly above its recommended value the energy capacity, efficiency, and life cycle of the cell reduces. Cell degradation is mainly caused by:

*  Mechanical degradation of electrodes or loss of stack pressure in pouch-type cells.

*  Growth of solid electrolyte interface (SEI) on the anode. SEI is seen as a cause for capacity loss in most, if not all, graphite-based Li-ion when keeping the charge voltage below 3.92v/cell.

* Formation of electrolyte oxidation (EO) at the cathode that may lead to sudden capacity loss.

* Lithium-plating on the surface of the anode generated by high charging rates. Cell degradation is a serious economic problem that varies according to how the battery is being used.

Incomplete Charging of a Cell Pack

# Batteries are charged at a constant current of between 0.5 and 1.0 rate. The battery voltage rises as the charging progresses to peak when fully charged then subsequently falls.

# Consider three cells with 77 Ah, 77 Ah, and 76 Ah respectively and 100 percent SoC, and all cells are then discharged and their SoC goes down. You can figure out quickly that cell 3 becomes the first to run out of energy since it has the lowest capacity.

# When power is put on the cell packs and the same current is owing through the cells, once again, cell 3 lags behind during charging and may be considered fully charged as the other two cells are fully charged.

# This means that cells 3 have a low Coulometric Efficiency (CE) due to the cell’s self-heating that results in cell imbalance.

Incomplete Use of Cell Pack Energy

# Drawing more current than the battery was designed for or short-circuiting the battery is most likely to cause premature failure of the battery. When discharging the battery pack, the weaker cells discharge faster than the healthy cells whereas they reach the lowest voltage more quickly than other cells.

# Providing regular rest periods during the operation of the battery allows the chemical transformations in the battery to keep track of the demand for current.

Here is a typical example of a weak faulty cell in a battery pack & its simulation in MATLAB:

Lithium-Ion Battery Pack With Fault

# This example shows how to simulate a battery pack consisting of multiple series-connected cells in an efficient manner. It also shows how a fault can be introduced into one of the cells to see the impact on battery performance and cell temperatures. For efficiency, identical series-connected cells are not just simply modeled by connecting cell models in series.

# Instead, a single cell is used, and the terminal voltage scaled up by the number of cells. The fault is represented by changing the parameters for the Cell 10 Fault subsystem, reducing both capacity and open-circuit voltage, and increasing the resistance values.

Model

To overcome the problem of reduced voltage and capacity we need to monitor the voltage and soc of each and every cell in a battery pack. This way technique of maintaining the overall balance in the battery pack is called cell balancing. For example, cells are connected in series the voltage should be equal, and cells connected in parallel the capacity or the soc must be equal to derive the battery pack with maximum efficiency. The method of maintaining all the cells is called cell balancing. 

Result:

Cell 10 graph line shows the faulty battery behavior in the graph line

Cell Balancing to Improve Battery Performance

Cell balancing is a technique that improves battery life by maximizing the capacity of a battery pack with multiple cells in series, ensuring that all of its energy is available for use. A cell balancer or regulator is functionality in a battery management system that performs cell balancing often found in lithium-ion battery packs electric vehicles and ESS applications. The fundamental solution of cell balancing equalizes the voltage and SOC among the cells when they are at full charge. Cell balancing is usually categorized into two types—passive and active. The passive cell-balancing method, also known as “resistor bleeding balancing,” is simple and straightforward: Discharge the cells that need balancing through a dissipative bypass route. This bypass can be either integrated or external to the IC. Such an approach is favorable in low-cost system applications.

Comparison of cells without balancing, Active and Passive balancing

Comparison of Active and Passive Cell Balancing

Passive cell balancing

# A passive system potentially burns off excess energy from the high cells through a resistive element until the charge matches the lower energy cells in the pack. If you have cells packed in series and you notice that some of the cells have higher energy than the other lower energy cells, you can balance the cells in burning energy of the top cells simply by attaching a resistor to the cells which release the energy into heat thereby equalizing the cell energy of the battery pack.

# Initially, you burn off the excess energy until you have balanced cells. Passive cell balancing allows all cells to appear to have the same capacity.

# There are two different categories of passive cell balancing method: fixed shunting resistor and switching shunting resistor. A fixed shunting resistor circuit is usually connected to the fixed shunting to prevent it from being overcharged. With the help of the resistors, the passive balancing circuit can control the limit value of each cell voltage without damaging the cells.

# Energy consumed by these resistors for balancing a battery may result in thermal losses in the BMS. This, therefore, proves the fixed shunting resistor method to be an inefficient cell equalizing circuit.

# The switch shunting resistor cell balancing circuit is currently the most common method in cell equalizing. This method has a continuous mode and a sensing mode, where the continuous mode all switches are controlled to be turned on or off at the same time, and in the sensing mode, a real-time voltage sensor is required for each cell. This cell balancing circuit consumes high energy through a balancing resistor. This cell balancing circuit is suitable for a battery system that requires a low current when it is charged or discharged.

Active cell balancing

# An active cell balancer generally transfers energy from one cell to another. That is from high voltage/ high SoC to a cell with a lower SoC. The purpose of an active balancer is that if you have a pack of cells with lower capacity, you can extend the life or the SoC that you have on the pack by moving energy from one cell in the pack with more energy than the other cell.

# Instead of wasting all that energy as heat, an active cell balancer efficiently balances cells with tiny converter circuits that pass energy from the highest voltage cells to the lowest voltage cells. There are two different categories of active cell balancing methods: charge shuttling and energy converters.

# Charge shuttling is used to actively transport charges from one cell to another to achieve equal cell voltage. Energy converters use transformers and inductors to move energy among the cells of a battery pack.

# Other active cell balancing circuits are typically based on capacitors, inductors or transformers, and power electronics interface.