If you are familiar with lithium batteries, you know they are made up of cells. This concept is not too different from sealed lead acid (SLA) batteries, which are also made up of cells. Both types of batteries require cell balancing, but what exactly is cell balancing? How does it happen and how does it affect performance?
Before we dive into lithium iron phosphate cell balancing, let’s quickly review the structure of a battery. Different applications require different voltage and amp-hour specifications, so batteries come in single-cell and multi-cell versions. For example, a 12.8-volt battery comprises four LiFePO4 cells, each at 3.2 volts. Conversely, a 12-volt lead-acid battery consists of six 2-volt lead-acid cells.
It is critical to ensure that all LiFePO4 cells are matched in terms of capacity, voltage, and internal resistance before the cells are assembled, and that they are balanced after production.
What is battery balancing?
Battery balancing is the process of matching cells by capacity and voltage, and controlling their voltage by cycling the cells to maintain balance or near-identical voltages at all states of charge (SOC). It is important to note that battery balancing occurs before and after the battery is assembled and must continue throughout its life to maintain optimal performance. This concept is similar to maintaining balance when connecting individual cells in series.
Balancing Lithium Iron Phosphate Cells
LiFePO4 battery packs (or any lithium battery pack) are equipped with a circuit board with a balancing circuit, protection circuit module (PCM), or battery management system (BMS) circuit board that monitors the battery and its cells. In batteries with balancing circuits, the circuits balance the voltage of the individual cells as the cells approach 100% – the industry standard for lithium iron phosphate is to balance the cell voltage above 3.6 volts. In a PCM or BMS, hardware usually maintains balancing with additional protection or management features beyond the balancing circuitry, such as limiting charge/discharge current.
SLA battery packs are not monitored like lithium batteries and therefore are not balanced in the same way.SLA batteries are balanced by charging them at a slightly higher voltage than normal. Since it lacks internal monitoring, it must be monitored externally via a hydrometer to prevent thermal runaway. This is not automatic but is usually part of routine maintenance.
Active vs. Passive Cell Balancing
Passive cell balancing discharges current into a cell through a resistor. When a cell is “full,” the resistor converts energy into heat, which reduces the current to full cells until all cells are balanced.
Active cell balancing first directs current to the least charged cell and uses the “full” cell to charge the “low” cell. During this process, the current will be focused on filling the less charged cells until they are equal to the other cells. Once equalized, the current is distributed equally until all cells are fully charged and equal.
Active and passive balancing accomplish the same thing, but active balancing is faster and more efficient.
Why is it important to balance lithium iron phosphate battery cells?
In lithium iron phosphate batteries, once the cell with the lowest voltage reaches the discharge voltage cutoff point, the performance and life of the cell will be affected. Therefore, maintaining cell balance is critical to optimize cell function and extend service life. For an in-depth look at the different types of lithium batteries, please refer to our complete guide to AGM batteries for optimal performance.
When commanded by the Battery Management System (BMS) or Protection Circuit Module (PCM), the entire battery is shut down. If the cells are unbalanced during discharge, this may mean that some cells still have unused energy, i.e. they are not really “depleted”. Similarly, suppose the battery pack is unbalanced during charging. In that case, the charging process will stop when the highest voltage cell reaches the cut-off voltage, resulting in not all Li-FePO4 cells being fully charged, thus undercharging the battery.
Why does this problem occur? First, cell imbalance will result in lower capacity and higher cut-off voltage of the cells. In addition, continually charging and discharging an unbalanced cell will worsen the situation over time. Given the relatively linear discharge curve of LiFePO4 cells, matching and balancing all cells is critical – the greater the difference between cell voltages, the lower the cell capacity.
Theoretically, balanced cells will discharge at the same rate so that the current can be cut off simultaneously at the same voltage every time. However, this is not always the case in practice, so the balancing circuit (or PCM/BMS) is very important. It ensures that the cells are fully balanced during the charging process so that the designed capacity of the battery is maintained and a full charge is achieved. Proper maintenance is critical to prolonging the life of lithium batteries, and battery balancing plays an important role in this process.
If you have any other questions about battery balancing, lithium batteries, or related topics, please feel free to contact us.