Battery balancing is also necessary to correct the imbalance within the battery itself. All batteries, including those in electric vehicles, will lose balance over time due to mismatches in the manufacturing process or operating conditions, resulting in uneven aging between batteries.
A battery can only provide charge before its weakest cell is fully discharged, even if the others may still have a significant amount of charge remaining. Therefore, battery balancing extends battery life by maximizing the capacity of the battery pack and ensuring all its energy is available, which, in the case of electric vehicle batteries, extends driving range. In addition to maximizing battery capacity, battery balancing also ensures safe battery operation by preventing overcharging and over-discharging, both of which can lead to accelerated battery degradation and potentially dangerous operating scenarios.
How cell homeostasis works
There are two common methods for cell balancing: active battery balancing and passive battery balancing. Active battery balancing uses a DC/DC converter to provide higher capacity to lower-capacity cells, thereby redistributing the charge of the cells. Today, battery manufacturing and sorting have been significantly improved, allowing for very low mismatches within a battery pack. Therefore, it is possible to avoid balancing large mismatches in the cells at the start of operation with a large balancing current. Frequent battery balancing with a smaller balancing current can manage any mismatches that gradually emerge during operation.
Passive balancing typically removes charge from larger batteries through heat dissipation until all batteries have the same amount of charge. The main difference between passive and active balancing is that passive balancing does not allocate energy but dissipates it until all batteries with higher initial charges eventually match the battery with the lowest charge. Passive balancing is a more popular method because it is simpler and less expensive.
Battery capacity is typically expressed as state of charge, which explains the battery's charge level relative to its capacity. Figure 1 illustrates the differences in battery balance types.
Figure 1: Battery charging status under various balance modes
Passive battery balancing in electric vehicle batteries
Passive balancing eliminates overcharged battery charge by switching a resistor connected in parallel with the battery and dissipating energy into that resistor. This energy dissipation results in heat in the battery, as well as in the switches and resistors used for dissipation. Maintaining the lithium battery temperature as close to room temperature as possible is crucial. Failure to do so can lead to thermal runaway when the rate of internal heat generation exceeds the rate of heat release.
Lithium-ion batteries degrade more rapidly at high temperatures due to structural changes and the formation of films on the electrode surfaces. Furthermore, excessive heat buildup can damage battery balance switches and resistors. Typical electric vehicles have numerous batteries and battery balance switches and resistors, which are often packaged close together, making it necessary to manage the heat dissipation of the battery and its battery management system during passive balancing.
Improving EV Battery Safety with TI Battery Monitors and Balancers
TI's BQ79616-Q1 performs passive cell balancing using internal switches. Due to these switches, heat is generated internally within the BQ79616-Q1 during battery balancing. Hot spots are located on the device's printed circuit board (PCB) and on the balancing resistors. The BQ79616-Q1 provides two thermal management features to prevent chip overheating and monitor PCB temperature.
One thermal management function monitors the die temperature, and another monitors the thermistor temperature. A high die temperature triggers a microcontroller (MCU) fault, suspending battery balancing to allow the integrated circuit (IC) temperature to drop. Once the IC temperature has decreased and the fault has cleared, the MCU can command the BQ79616-Q1 to resume battery balancing.
By monitoring with a thermistor, if the temperature exceeds the pause threshold, the BQ79616-Q1 will automatically pause balancing. When the temperature drops below the recovery threshold, balancing will automatically resume. In this case, the BQ79616-Q1 will pause and resume battery balancing without any intervention from the MCU. Figure 2 shows the temperature monitoring of the device and the thermistor.
Figure 2: Temperature monitoring location of BQ79616-Q1 on PCB
The cell balancing pause state also freezes all balancing timers and settings, which are restored once the device exits the pause state. To manage thermal increases caused by external balancing resistors, the BQ79616-Q1 can pause battery balancing on all channels if any active thermistor connected to the general-purpose input/output detects a temperature higher than a set overheat battery balancing threshold. Once overheat battery balancing detection is triggered, balancing on all enabled channels will resume once all active thermistors detect a temperature lower than a predetermined recovery threshold.
Autonomous battery balancing helps maximize battery life, a key advantage for electric vehicle batteries. The BQ79616-Q1 adds enhanced IC thermal management and fault indication capabilities to the MCU, enabling fast and safe battery balancing in a cost-optimized manner, thereby extending battery uptime between chargers and ultimately extending the lifespan of electric vehicle batteries.
