From small consumer devices to electric vehicles, many systems rely on rechargeable battery packs to provide power. Often these are multi-cell Li-ion battery packs that must be repeatedly charged and discharged during operation. In the process of charging and discharging batteries, it is possible that they may not accumulate evenly across the cells in a battery pack. There are several reasons for this that are often very difficult to compensate in terms of physical design, battery manufacturing, and PCB layout.
Instead of trying to eliminate every possible defect that contributes to variations in battery charge/discharge, systems that use multi-cell battery packs can implement a battery balancing and management system. Lower power devices that use a small number of batteries do not normally need to have a battery balancing and management system because the batteries are cheap to replace. But for a larger battery-powered system like an electric vehicle or watercraft, battery balancing is essential for maximizing the operating lifetime of a device and it can be implemented with a small module added to these systems.
How to Design Battery Balancing
The purpose of battery balancing is to distribute charge among cells in a battery pack such that the state of charge (SOC) is very similar across all batteries. Larger systems like electric vehicles and appliances use large arrangements of battery cells to provide the required voltage, discharge current, and total available power. The battery balancing method needs to be implemented based on the arrangement of cells in the battery pack. Battery cells are typically arranged in series and parallel configurations to provide higher voltage and total discharge current respectively.
When a battery pack is placed into operation, different cells in the system can discharge at different rates. When this occurs, the SOC in the various cells will be different. Later, when the battery pack is to be recharged, the various cells might also recharge at different rates. The point of balancing is to redistribute charge from the battery pack such that power is evenly
Battery balancing involves redistributing charge around battery cells so that they have similar SOC.
What’s the danger in allowing one cell to have a dominant SOC in a battery pack? There are several reasons to manage this through charge redistribution:
- Extend total operational time between recharging
- Prevent overcharging in the higher SOC cells
- Prevent excessive heating in any cell with dominant discharging
Battery balancing techniques are implemented using a control system that requires sensing and routing charging current to different cells. The controller system needs to sense the charge in each cell and then implement an algorithm that routes a charging current between different cells. Sets of ASICs (battery sense chips and a system charge controller) are available that provide this capability, or the system can be built from discretes using ADCs and a microcontroller.
Passive Battery Balancing
In this method, a battery balancing controller allows one battery cell (that with the highest SOC) to discharge into other cells through a unique interconnect architecture. The discharge rate will be intentionally limited with some resistance in order to prevent high current discharge that can damage the battery or the controller. Eventually, the SOCs between each cell will come into balance naturally, although some power will be wasted during the process.
Active Battery Balancing
In active battery balancing, a charging current is intentionally routed between a high SOC cell and a lower SOC cell. This is done with an interconnection as in the passive case, but the charge is intentionally directed between specific cells rather than allowing the charge to balance naturally. Once the two chosen cells are brought into balance, other cells are selected until charge is balanced throughout the system.
At a high level, the following example algorithm implements an active battery balancing scheme.
In Line 3, the algorithm uses a 100 mV threshold as the allowed maximum voltage difference between any two cells as this deviation between any two cells will be typical for fully-charged battery cells that come off a manufacturing line. This value is also used in other battery balancing algorithms. Physically larger battery cells that output higher voltages can expect to see a ~1 V difference between cells.
The image below shows a systems level view of a battery balancing system. In this topology, the switches can be actively controlled by the controller IC to select pairs of batteries, and the monitors are used to track the SOC until the controller determines the balancing threshold is reached. The monitors can continuously sense the SOC of the system and supply their data stream to the controller, and the controller can then engage and begin system balancing as needed.
It’s also possible for the controller to regulate and implement balancing during the charging stage, not just after the device is charged to the required system voltage. Battery balancing during charging requires a more sophisticated controller and monitoring topology, and the balancing controller would need to interface with the battery charge regulator so that charging currents to different cells can be toggled on or off. While there can be significant design effort involved in these types of controllers, they help extend battery life and provide important safety functions, especially in electrified vehicles and appliances.
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