Multi cell Management of Electric Vehicle Battery Pack

How weakest cell limits the usable capacity of the battery pack? What is the solution?

Ans:-

The battery pack consist of different cells connected in series and parallel as the requirement. But although the all cell are identical, made from same manufacturer with same specification there is always some variation in SOC of state due to some factors.

The different SOC of cells may limit the usable capacity of battery. Like if we have different SOC’s cells and we start to use them, first of all 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 flowing through the cell, it gets damaged beyond repair. Now, if we 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 we try to use them.

The way to avoid this limitation we can use the cell balancing to improve the usable capacity & safety of battery.

Cell Balancing:-

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 a functionality in a battery management system that performs cell balancing often found in lithium-ion battery packs electric vehicles and ESS applications.

Typically, individual cells of a battery pack have different capacities and are at different SOC levels. Without redistribution, discharging must stop when the cell with the lowest capacity is empty, even though the other cells are still not empty. This limits the energy delivering capability of the battery pack.

During balancing, higher capacity cells undergo a full charge/ discharge cycle. Without cell balancing, the cell of the slowest capacity is a weak point. Cell balancing is one of the core functions of a BMS, along with temperature monitoring, charging, and other features that help maximize the life of a battery pack.

Above image shows the comparison of cell balancing with unbalance cell SOC.

Types of Cell balancing:-

• Active cell balancing
• Passive Cell balancing

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 we have a pack of cells with lower capacity, we can extend the life or the SoC that we 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.

Advantages of Active Cell Balancing:

• It improves capacity usage. It performs great when we have different cell capacities in a series.
• It increases energy efficiency. It saves energy instead of burning the excess energy in a cell by transferring the excess energy to a lower energy cell.
• Lifetime extension. It improves the life expectancy of a cell.
• Fast balancing

Disadvantages of Active Cell Balancing:

• When we transfer energy from one cell to another, approximately 10-20% of the energy is lost.
• The charge could be transferred only from higher cell to lower cell.
• Although an active cell balancer has high energy efficiency, its control algorithm may be complex and its production cost is expensive because each cell should be connected with an additional power electronics interface.

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 we have cells packed in series and we notice that some of the cells have higher energy than the other lower energy cells, we can balance the cells in burning energy of the top cells simply by attaching a resistor to the cells which releases the energy into heat thereby equalizing the cell energy of the battery pack.

Initially, we burn off the excess energy until we 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.

Advantages of Passive Cell Balancing:

• We should never have to balance a pack that is working perfectly.
• A cell cannot waste energy that it does not have. As soon as the energy bank is full, that is only when the cell has enough energy to balance.
• It allows all cells to have the same SoC.
• It provides a fairly low-cost method for balancing the cells.
• It can correct for long-term mismatch in self-discharge current from cell to cell.

Disadvantages of Passive Cell Balancing:

• Poor thermal management.
• They do not balance during the full SoC. They only balance through the top of each cell at around 95%. This is because if we have different cell capacities, we are forced to burn off the excess energy.
• Its energy transmission efficiency is usually low. Electrical energy is dissipated as heat in the resistors and the circuit also accounts for switching losses. In other words, it results in a high amount of energy loss.
• It does not improve the run-time of a battery-powered system.