Active Balancing, Still a Big Deal?
Back in 2011, my wild idea came to life, evolving into a benchtop prototype. Its essence was to dynamically alter the battery configuration, for example, 8S4P to 4S8P and vice versa, or any other combination you could think of, given the number of cells. This idea boasted several benefits, including the ability to bypass cells and active balancing, to mention just a couple, aside from the versatile configuration changes. I might delve into these details if there’s an opportunity in the future. The reason I mention this is because I want to discuss active balancing. Sure, it may seem like a cliché, but I believe it’s still significant.
For those looking to refresh their knowledge, active balancing refers to the process of redistributing energy within the battery to ensure that all cells or modules charge evenly. Without it, cells with higher charge levels could potentially overheat or become damaged.
Basics of cell balancing
For a primer on cell balancing, my BatteryAlchemist offers a more thorough explanation…
- Objective of Cell Balancing: The primary goal of cell balancing is to ensure a balanced system condition, taking into account the actual system parameters. This involves maintaining a deviation of the State of Charge (SOC) below 3%. The balancing process aims to correct differences in internal resistances and capacities of the battery cells to avoid asymmetry due to varying cell characteristics.
- Balancing Algorithm and Process: The balancing algorithm must account for differences in internal resistances and capacities of the cells. It calculates the time required to achieve a balanced system condition. Cell balancing can occur both during active Battery Management System (BMS) operation and in sleep mode, with active charging operation being preferred. A significant aspect is the stand-alone balancing (SAS) process, where the BMS wakes at regular intervals to maintain or achieve balance. The process should allow for a reduction of cell charge by 5% within 16 hours.
- Requirements and Implementation: Each cell or string within the battery should have its balancing unit, normally off and activated upon reaching a voltage threshold. The BMS should control each unit individually, monitor, and report the status of each balancing unit via LIN communication, and detect balancing faults. During the lifetime of the vehicle, the balancing strategy should consider the vehicle’s power net management operational strategy.
- Safety and Reliability: The documents emphasize the importance of designing the cell balancing system to be safe, reliable, and by automotive industry standards. This includes regular monitoring of SOC levels, continuous adjustment based on operational strategies, and ensuring that all components, including the cell balancing units, meet stringent quality and performance criteria.
- Collaboration with OEMs: The specifications for the cell balancing algorithm, including its detailed implementation, are to be agreed upon with the Original Equipment Manufacturer (OEM). This collaborative approach ensures that the battery system’s design and functionality align with the broader requirements and standards of the automotive industry.
All these insights make sense to me. Well done, BatteryAlchemist.
Balancing Strategies
I recall that implementing cell balancing required a series of discussions about various factors: What to monitor, when to trigger balancing, which conditions to meet, how long to maintain activation, and what dependencies to consider. Yes, it’s complex. I believe these considerations are still relevant in today’s battery technologies.
What to Monitor
Terminal Voltage and State of Charge (SOC) are crucial indicators. Cells with higher voltage are typically more charged and are selected for charge equalization. However, terminal voltage can fluctuate significantly with battery aging, also influenced by the depth of discharge (DOD).
Conversely, SOC is used to balance cells based on capacity. Cells with higher SOCs are more charged and are thus selected for balancing. While this approach aligns with the ultimate goal of balancing, SOC-based balancing faces challenges due to potential inaccuracies in SOC estimation.
Ultimately, a battery management system that maintains high SOC estimation accuracy across different DOD levels tends to use SOC-based balancing.
When to Trigger It
Passive and active balancing differ in their triggering points. Passive balancing, which dissipates excess energy as heat, is limited in its activation and typically begins as the battery nears full charge. It also depends on battery chemistry; for example, LFP batteries have a flat charge curve where balancing is less effective. The timing of the balancing process is crucial as the dissipated energy becomes heat, requiring a slower process.
Active balancing is more adaptable, potentially occurring at any DOD level during both the charging and discharging phases, ideally without energy loss.
Which Conditions to Meet
Balancing may occur if the difference between a chosen indicator's maximum and minimum values exceeds a certain threshold. The DOD is also a critical factor in this equation. For passive balancing, a worst-case scenario might involve one cell being significantly less charged than the others, possibly indicating a short circuit in the low-voltage cell.
Active balancing involves transferring energy from the most charged cells to the least charged ones, continuing until the difference falls below the threshold.
How Long to Maintain It
The duration of activation varies. Passive balancing is limited by the rate at which energy is dissipated as heat, inversely related to the reliability of the balancing circuit. As BatteryAlchemist notes, passive balancing might take hours, spread out over multiple cycles, or occur during sleep mode.
What Dependencies to Consider
The balancing circuit is often embedded into the voltage measurement circuit. In practice, passive-balancing FETs are used to measure voltages. So, these two features cannot happen at the same time. Balancing is activated during the duty cycle of an interval, and the voltage measurement occurs at the off-duty cycle in the interval. Make sure to put an idle time to avoid interference between the two functions.
Summary
Exploring the area of passive and active balancing, we delved into the battery cell-balancing strategies, covering aspects like what to monitor, the ideal timing for triggering balancing, essential conditions to meet, the duration of activation, and the major dependency to keep in mind. Balancing strategies are not only highly specific to their implementation but also deeply influenced by the chemistry of the battery used. Moreover, there are a couple of balancing-related topics we haven’t touched upon in this discussion. Please feel free to reach out if you have any questions or thoughts you’d like to share.
