Cell Balancing LFP

Lithium batteries are built up of cells. Cell balancing is required in not just the lithium batteries but any battery that is made up of cells, but what exactly is cell balancing? How does cellular balancing take place? What effect does this have on performance?

A 12.8-volt battery, for example, is made up of 4 x LiFePO4 cells (each with 3.2 volts). Before the battery is assembled, all of the LiFePO4 cells must be matched in terms of capacity, voltage, and internal resistance, and they must also be balanced afterward.

What is Cell balancing?

The phrase “balancing” refers to the process of matching the capacity and voltage of the cells and managing their voltages by cycling the battery to maintain balance, or close to equal voltages, at all State of Charge (SOC) levels. It’s important to remember that cell balancing occurs both before and after a battery is assembled, and the battery must perform at its best throughout its life. 

What is Lithium Balancing Circuit? 

A balancing circuit in a battery simply balances the voltages of the battery’s cells while it is charging. When all of the cell voltages are within a tiny tolerance of each other, the battery is said to be balanced.

In the case of Active Balancing, the high voltage cells are used to charge the low voltage cells. This process is done till all the cells attain the same voltage range and the battery is fully charged.

The passive cell balancing technique aims to discharge the cells through a bypass route that is primarily dissipative. Because the bypass can be external or integrated, it is simpler and easier to deploy than active balancing solutions, and it keeps the system more cost-effective in either case. However, because all of the excess energy is wasted as heat, the battery’s run time suffers and it is less likely to be used during discharge.

The sole difference between active and passive cell balancing is that active balancing is faster and more efficient.

What is the significance of cell balancing? 

When the lowest voltage cell in a lithium battery reaches the discharge voltage cutoff, the entire battery shut down. If the cells aren’t balanced then some cells will have some energy left in them and when charging will start, as soon as any cell with the voltage hits the cut-off voltage, charging will stop, and not all of the cells will be fully charged.

Charging and discharging an unbalanced battery regularly will reduce its capacity over time. This also means that certain cells will be fully charged while others will not, resulting in a battery with a State of Charge that may never reach 100%.

Balanced cells discharge at the same rate and, as a result, cut off at the same voltage. Because this isn’t always the case, a balancing circuit guarantees that the battery cells are fully matched during charging, protecting the battery’s capacity and allowing it to fully charge.

What is a Lithium protective circuit module? 

A Protective Circuit Module is made up of a balanced circuit and additional circuitry that regulates the battery’s parameters by protecting it from overcharging and over-discharging. It accomplishes this by measuring current, voltages, and temperatures during charge and discharge and comparing them to pre-set limits. If any of the battery’s cells reaches one of these limits, the battery will cease charging or discharging until the release mechanism is met.

After the protection has been tripped, there are a few options for turning the charge or discharge back on.

The first is value-based, in which the value must recover threshold criteria to be released. For example, over-charging protection is triggered when cell voltage reaches beyond the set threshold for a certain period. It is released when the voltage of that cell falls below the overvoltage recovery threshold. Once the release criterion is met, this can happen right away.

The second is activity-based protection, which requires an action to be completed to release the protection. For instance, the action could be to remove the load or to apply a charge. This release, like the value-based protection release, may occur immediately or over time. This may necessitate removing the load from the battery for 30 seconds before the protection is withdrawn. It’s worth noting that, in addition to time and value or activity and time-based releases, these strategies can be used in different combinations. The over-discharge release voltage, for example, maybe set once the cells have gone below 2.5 volts, but it may take 10 seconds to reach that level. This type of release encompasses all three categories.

Why do balancing LiFePO4 cells matter?

When the lowest voltage cell in a LiFePO4 battery reaches the discharge voltage cut-off set by the BMS or PCM, the entire battery is shut down. If the cells were unbalanced during discharge, it’s possible that some of them still hold energy and the battery isn’t truly “empty.” If the cells are not balanced when charging, charging will be terminated as soon as the cell with the highest voltage hits the cut-off voltage, and neither the LiFePO4 cells nor the battery will be fully charged.

So, what’s the big deal about that? An imbalanced battery has a lower capacity and a greater cut-off voltage at the battery level, to begin with. Moreover, charging and discharging an imbalanced battery repeatedly will compound the problem over time. Because of the generally linear discharge profile of LiFePO4 cells, all cells must be matched and balanced; the bigger the voltage differential between the cells, the lower the capacity available.

According to the principle, balanced cells all discharge at the same rate and, as a result, always turn off at the same voltage. Because this isn’t always the case, a balancing circuit (or PCM/BMS) guarantees that the battery cells are fully balanced during charging, allowing the battery to maintain its design capacity and become fully charged. Cell balancing is an important part of maintaining your lithium battery to ensure that it lasts as long as possible.