Week 8 Multi cell Battery 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.