When utilising a battery pack for a specific application, there are a number of things you need to be aware of, particularly when it comes to maintaining and ensuring that your battery pack works correctly. So, with that in mind, AceOn explains what cell balancing is, why it’s important, and the different techniques available to battery pack manufacturers and technicians.
What is cell balancing?
Cell balancing is an integral technique when it comes to battery efficiency. Balancing the cells within a battery pack ensures that each individual cell connected achieves maximum efficiency, ensuring the optimal operation of your application. In battery pack manufacturing, various cells are combined to form a pack, and it’s important for them to be of the same chemistry and voltage value. However, as soon as the battery pack is installed and connected to an application, the voltage values of the cells tend to vary. As a result, cell balancing is integral to ensure the optimal functioning of the battery pack.
If left unchecked, the unbalancing of the cells in a battery pack can lead to a number of issues, including:
Thermal runaway
Thermal runaway is one of the biggest problems that you can encounter with a battery pack. As you might be aware, the lithium cells that comprise a battery pack are sensitive to overcharging and discharging. If the cells within a pack are unbalanced, the cells with the lower charge will not be able to be fully charged, while those that are of a higher voltage will overcharge. Thermal runaway will occur as a result, which affects the efficacy of the entire battery pack.
Cell degradation
If you overcharge a lithium cell slightly above its recommended value, its efficiency and life cycle will be negatively affected. As an example, an increase in charging voltage of 0.25 volts can degrade the battery 30% faster, which is bad news for the durability of the battery pack in question.
Incomplete charging
As a battery pack ages, it’s common for some of the cells to weaken. This is a problem, as they will charge and discharge more quickly than cells that are still optimal. This can result in incomplete charging, which further adds to the issues surrounding the durability and effectiveness of the battery pack.
Why does cell unbalancing occur?
As explained, cell balancing is crucial – but how do cells become unbalanced in a pack in the first place? Some of the main causes of cell unbalancing in battery packs are:
SOC imbalance
Measuring the SOC (state of charge) of a battery cell is difficult, but it’s important. However, since it is practically so difficult, cells are often only matched on voltage (OCV) as opposed to their SOC, which can lead to complications later down the line.
Internal resistance variation
The internal resistance of cells essentially determines the current flowing within them. It’s particularly difficult for manufacturers to find cells with matching IR, which means that the current running through each of them might be varied, which in turn, varies the voltage of the battery pack.
Temperature
Finally, the temperature of a battery pack can cause cell imbalances. For instance, in a big solar battery pack, there might be a temperature difference within the pack itself. This can cause one cell to discharge faster than the others in the application. This can also happen when a battery pack is poorly stored.
Cell balancing techniques: Four options
It’s now time to look at the four cell balancing techniques you can employ to ensure your battery pack works optimally:
Passive cell balancing
This is the simplest method and is helpful to most people. There are actually two types of passive cell balancing – charge shunting and charge limiting. Fundamentally, charge shunting uses a dummy load like a resistor to discharge the excess voltage and equalise it with the other cells in the pack. Charge limiting is less efficient, as all the cells are monitored continuously. It involves revising the charge of each of the cells during a charge before disconnecting the pack when certain cells reach a specific charge.
Active cell balancing
In active cell balancing, the excess charge from one cell is transferred to another cell with a lower charge. This equalises the cells within the application and is an extremely efficient way of redistributing charge within a battery pack application.
Lossless cell balancing
As a recently developed cell balancing technique, lossless cell balancing makes use of a matric switching circuit that provides the capability to add or remove a cell from a pack during charging and discharging.
Redox shuttle
Finally, chemical engineers might employ a redox shuttle as a way of balancing the cells within a battery pack. The idea behind the redox shuttle is that the chemistry within a lithium cell is altered, which should prevent the cell from becoming overcharged when connected to the application.
More information about cell balancing
As mentioned, cell balancing is an integral part of battery pack maintenance, as it ensures that your pack works optimally when connected to your various applications. At AceOn we provide custom battery packs to a broad range of clients and can assist with cell balancing techniques. Reach out today and give us a call if you have any further questions – 01952 293 388.
