What is Balancing? What is Active Balancing?
The term balancing refers to BMS’s preventive actions to equalize the charge levels of each cell in a battery pack. Most of today’s batteries are charged using constant current up to the point that the battery reaches a terminal voltage. Then the constant voltage method kicks in and battery is charged to the point where current falls to 3 – 5% of the battery capacity.
How ever it is a “cliche” and true fact that not all cells even from same production batch are not the same. The cells eventually reflect different characteristics and some lose capacity more than others. This is critical especially in charging operation since the lowest capacity cell hits terminal voltage earlier than others. At that point, if the charger continues to push in constant current, that cell’s voltage will go beyond voltage limit and if not protected will fail and probably disintegrate.
Since the BMS’s function is to prevent such failures, before an overvoltage protection kicks in, the BMS tries to equalize cell charges whenever a difference is detected.
There are 2 widely known technique for cell balancing known as “passive” or “resistive” balancing and “active” or “charge transfer” balancing.
The passive balancing or resistive balancing is the technique in which BMS comprises a resistor in series with a switch (a MOSFET) for each cell connected to their terminals. When an imbalance is detected, BMS finds out the out of balance cells, which is indicated by higher cell voltage, and activates their balancing resistors. The resistors bleed energy from corresponding cells, thus equalizing cell voltages.
The active balancing or charge transfer balancing is the technique in which the BMS is capable of charging or discharging cells individually and transferring charge to/from another cell or battery pack.
When an imbalance is detected, BMS calculates average SOC and tries to equalize cell charges by transferring charge among cells. The cells with low SOC are charged and cells with high SOC are discharge, meeting all cells at the same SOC level.
Passive balancing is a low-cost technique since it requires very little components added to the BMS. However, as shown in figure 3, its nature is to equalize all cells SOC to that of the lowest one and is a highly inefficient technique.
Furthermore, the bleed energy is converted to heat and the level of balancing current determines the generated heat and temperature rise on BMS. For this reason, the balancing current is limited at most at 1 ampere per cell. Any higher current will lead to high heat generation and safety problems.
On the other hand, balancing should cope with an on-going charging process and prevent unsafe cell voltage. Balancing current should be comparable to charging current. Ideally, if balancing is always enabled, balancing current should be at least 3 – 5% of charging current even for high quality low cycle cells. That means if you have the most powerful passive BMS with a high-quality battery pack, you should not push in more than 30 – 35 Amperes of charging current.
Why We Need Active Balancing?
To cope with this limit, designers take the advantage of communication capabilities of BMS to alter charging algorithm. To leverage over all charging current and reduce charging time, the charger tracks the cell voltages over BMS and pushes in a high current to a point when a cell voltage hits terminal voltage. The charger then reduces charging current (even if it is still in constant current stage, not constant voltage state) to allow BMS to balance cells. When cell SOC’s are equalized, the charger re-increases charging current and top up the cells to the terminal voltage. The cycle of low current & high current modes can be repeated several times depending on the states of the cells. As all cells are equally brought to terminal voltage, the charger goes to constant voltage mode and charging stages are completed.
Aforementioned interactive charging allows charging speed to increase substantially. Nevertheless, it is still dependent on the health of each individual cell of the battery.
On the other hand, active balancing is a highly efficient technique in the sense that it does not dissipate excess charge but transfer it. The heat generation is only due to inefficiency of the charge transfer circuit. The conversion may be held with an efficiency of 85 – 92%. Thus, much higher balancing currents are possible. Moreover, since the charge transfer is bidirectional, it also halves the balancing time. For instance, V-ACT active battery management system provides 5 Amperes of bidirectional balancing current per cell. Despite its huge balancing capability, the 6-cell module is only fit to a 6 Module DIN rail enclosure.