Managing the voltages of the cells by cycling the battery to maintain balance, or close to equal voltages, at all State of Charge (SOC) levels is referred to as “balancing,” which is the act of matching the capacity and voltage of the cells. Cell balancing takes place both before and after a battery is manufactured, and the battery must function at its peak throughout its lifespan.
For instance, four LiFePO4 cells make up a 12.8-volt battery (each with 3.2 volts). Before the battery is put together, all of the LiFePO4 cells must be balanced and matched for capacity, voltage, and internal resistance.
To keep the battery balanced, or roughly equal voltages, at all State of Charge (SOC) levels, the capacity and voltage of the cells must be matched, and their voltages must be managed by cycling the battery.
A battery’s balancing circuit merely balances the cell voltages while the battery is charging. The battery is said to be balanced when all cell voltages are within a minimal tolerance of one another.
The high voltage cells are utilized to charge the low voltage cells in the case of active balancing. This cycle is repeated until the battery is fully charged and all of the cells reach the same voltage range.
Bypassing the cells through a predominantly dissipative channel, the passive cell balancing technique seeks to discharge the cells. The bypass is simpler and easier to deploy than active balancing methods and keeps the system more cost-effective in both cases because it can be external or integrated. 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.
A lithium battery shuts down when the lowest voltage cell exceeds the discharge voltage cutoff. If the cells are not balanced, some of them will still have some energy when charging begins, and charging will cease as soon as any cell with voltage reaches the cut-off voltage, leaving some of the cells partially charged.
Regularly charging and discharging an imbalanced battery will eventually cause it to lose capacity. Additionally, this implies that certain cells will be fully charged while others are not. As a result, the State of Charge of the battery may never reach 100%.
Balanced cells turn off at the same voltage since they discharge at the same rate. A balancing circuit ensures that the battery cells are perfectly matched while charging because this isn’t always the case, preserving the battery’s capacity and enabling full charging.
A protective circuit module controls the battery’s parameters by shielding it from overcharging and over-discharging. It consists of a balanced circuit and additional circuitry. During charge and discharge, it measures current, voltages, and temperatures and compares them to preset limits to achieve this. The battery will stop charging or discharging if any of its cells approaches one of these limits until the release mechanism is satisfied.
There are a few alternatives for restarting the charge or discharge after the protection has been tripped.
The first is value-based, and to be released, the value must meet certain thresholds. For instance, when cell voltage exceeds the predetermined threshold for a predetermined amount of time, over-charging prevention is activated. When the voltage of that cell drops below the overvoltage recovery threshold, it is let go. This can occur immediately after the release criterion has been satisfied.
The second type of protection is activity-based protection, which needs to be carried out to be released. For instance, the action can be to add a charge or to remove the load. Similar to the value-based protection release, this one may happen instantly or gradually. This can need waiting 30 seconds after the protection is off before removing the load from the battery. It’s important to keep in mind that these tactics can be applied in various combinations in addition to time and value or activity and time-based releases. For instance, once the cells have dropped below 2.5 volts, the over-discharge release voltage may be set, however, it might take 10 seconds to get there. All three categories are included in this type of publication.
A LiFePO4 battery shuts down completely when the lowest voltage cell hits the discharge voltage cut-off set by the BMS or PCM. If the cells are unbalanced during charging, charging will stop as soon as the cell with the highest voltage reaches the cut-off voltage, and neither the LiFePO4 cells nor the battery will be fully charged. If the cells were unbalanced during discharge, it is possible that some of them still hold energy and the battery isn’t truly “empty.”
What’s the big issue with it, then? In the beginning, an unbalanced battery has a lower capacity and a higher cut-off voltage. Additionally, continuously charging and discharging an unbalanced battery will only make matters worse in the long run. All cells must be matched and balanced due to the LiFePO4 cells’ typically linear discharge profile; the greater the voltage differential between the cells, the less capacity is available.
As a result of discharging uniformly, balanced cells permanently shut off at the same voltage, according to the theory. A balancing circuit (or PCM/BMS) ensures that the battery cells are completely balanced while charging because this isn’t always the case. This enables the battery to maintain its design capacity and reach full charge. Cell balancing is an important part of maintaining your lithium battery to ensure that it lasts as long as possible.