Week 8 Multi cell Battery Pack

Nowadays, Electric vehicles (EV) are the wide application of large battery packs especially lithium-ion type. Single battery cells are serially and parallelly connected to make a battery stack to achieve higher voltage and capacity. However, the charging and discharging process need to stop as soon as any cell reaches its maximum limit or working threshold (below the absolute threshold). Due to this, the capacity of the battery pack is limited by the imbalance in the cells of the pack. This reduces the energy usage efficiencies and shortens the lifetime of the battery pack.

Therefore, the battery cell balancing is a basic, but essential function of a Battery Management System (BMS) and is necessary for battery packs. The following image provides a basic idea about cell imbalance for a series-connected 10S pack.

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.

Below is the figure of the Active Cell Balancer reference Circuit Board which is used to balance 16 lithium-ion cells.

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

Based on capacitors

• Single capacitors – this method is simple because it uses a single capacitor regardless of the number of cells connected in the battery. However, this method requires a large number of switches and intelligent control of the switches.
• Multiple capacitors – this method with multiple capacitors connected to each battery transfers unequal cell energy by multiple capacitors. It does not require a voltage sensor or closed-loop control.

Based on inductors or transformers

• Single/ multiple inductors – a cell balancing circuit with a single inductor has a small volume and low cost while multiple inductors have fast balancing speed and decent cell balancing efficiency.
• Single transformer – this method has a fast balancing speed with low magnetic losses.
• Multiple transformers – this cell balancer has a fast equalizing speed. However, it requires an expensive and complex circuit that prevents the transformer from being flooded.

Based on Power Electronics Interface

• Flyback/ forward converter – the energy of a high voltage cell is stored in the transformer. This cell balancer has high reliability.
• Full-bridge converter – this cell balancer has fast equalization speed and high efficiency.

Active balancers are capable of pushing a lot of current from one cell to another.

Advantages of Active Cell Balancing:

• It improves capacity usage. It performs great when you 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 you 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.

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 releases 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.

Advantages of Passive Cell Balancing:

• You 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 you have different cell capacities, you 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.

Comparing SOC(%) of Active & Passive Balancing at same time:-

CONCLUSION:-

Active cell balancing is an amazing way to handle the cell balancing problem, but, in terms of system integration and optimization, this approach takes a hit on many fronts like cost and complexity. 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, while having some exceptions.