Typically, a BMS’s oversight consists of:
- keeping an eye on the battery
- Providing protection for batteries
- Calculating the operational status of the battery
- Continually improving battery efficiency
- Notifying external devices of the functioning status
Although the term “battery” in this context refers to the entire pack, the monitoring and control capabilities are specifically applied to individual cells or groups of cells known as modules within the overall battery pack assembly. Since they offer the maximum energy density, lithium-ion rechargeable cells are typically used in battery packs for a variety of consumer goods, including laptops and electric cars. Despite their excellent performance, they can be quite harsh if used outside of a relatively narrow safe operating area (SOA), with effects ranging from compromised battery performance to outright dangerous outcomes. The BMS’s job description is undoubtedly difficult, and its overall complexity and supervision reach may encompass a wide range of disciplines, including electrical, digital, control, thermal, and hydraulic.
How Do Systems for Battery Management Operate?
There is no set of requirements that must be followed by battery management systems. The implemented features and scope of the technology design typically connect with:
- The battery pack’s price, complexity, and dimensions
- Use of the battery and any issues with longevity, safety, and guarantee
- Government rules necessitate certification, and if insufficient functional safety measures are implemented, costs and penalties are crucial.
Battery pack protection management and capacity management are two of the many crucial BMS design elements. Here, we’ll talk about how these two aspects function. The two main areas of battery pack protection management are thermal protection, which uses passive and/or active temperature regulation to keep the pack inside its SOA, and electrical protection, which refers to preventing the battery from being harmed by use outside of its SOA.
Protection of Electrical Management: Current
The path to electrical protection involves keeping an eye on cell or module voltages and battery pack current. Any battery cell’s electrical SOA is constrained by voltage and current. A well-designed BMS will safeguard the pack by prohibiting operation outside of the manufacturer’s cell ratings, as shown in Figure 1, which depicts a typical lithium-ion cell SOA. To extend the battery’s longevity, further derating may frequently be used to stay within the SOA safe zone.
Both charging and discharging lithium-ion battery modes can withstand larger peak currents, albeit for brief periods of time, and have differing current limits. In addition to peak charging and discharging current restrictions, battery cell manufacturers typically give maximum continuous charging and discharging current limits. A maximum continuous current will undoubtedly be applied by a BMS that offers current protection. To accommodate for an abrupt change in load conditions, such as the sudden acceleration of an electric car, this might be preceded. By combining the current and delta time, a BMS may integrate peak current monitoring and determine whether to cut the available current or stop the pack current completely. This enables the BMS to be tolerant of high peak demands, provided that they are not excessive for an extended period of time, while also having virtually instantaneous sensitivity to extreme current peaks, such as a short-circuit condition that has not been detected by any resident fuses.
Protection for Electrical Management: Voltage
A lithium-ion cell must function within a specific voltage range, as seen in Figure 2. The intrinsic chemistry of the chosen lithium-ion cell and the temperature of the cells at any given moment will eventually decide these SOA bounds. Furthermore, these SOA voltage restrictions are typically further limited to maximize battery lifespan because every battery pack undergoes a considerable amount of current cycling, draining as a result of load demands, and charging from a range of energy sources. The BMS needs to be aware of these boundaries since it will make judgments based on how close these thresholds are. For instance, a BMS may ask that the charging current be gradually reduced as it approaches the high voltage limit, or it may ask that the charging current be stopped completely if the limit is reached. To avoid control chatter regarding the shutdown threshold, this restriction is typically complemented by extra intrinsic voltage hysteresis considerations. On the other side, a BMS will ask important active offending loads to lower their current needs as the low voltage limit approaches. This can be accomplished in an electric vehicle by lowering the traction motor’s permitted torque. Naturally, in order to safeguard the battery pack and avoid irreversible damage, the BMS must prioritize the driver’s safety.
Protection from Thermal Management: Temperature
Although lithium-ion cells appear to have a broad temperature operating range on the surface, their overall battery capacity decreases at low temperatures due to a significant slowdown in chemical reaction rates. They do outperform lead-acid or NiMh batteries in terms of low-temperature capability; nonetheless, temperature control is prudently necessary because charging below 0 °C (32 °F) presents physical challenges. During sub-freezing charging, the anode may experience the phenomena of metallic lithium plating. This is irreversible damage that not only lowers capacity but also makes cells more prone to failure under stressful circumstances like vibration. By heating and cooling the battery pack, a BMS can regulate its temperature.
The size and cost of the battery pack, performance goals, BMS design specifications, and product unit—which may involve taking the intended geographic area into account (e.g., Alaska versus Hawaii)—are all factors that affect realized thermal management. Regardless of the type of heater, it is usually more efficient to use an alternative resident battery or an external AC power source to run the heater when necessary. On the other hand, energy from the primary battery pack can be drained to heat the electric heater itself if its current use is minimal. When a thermal hydraulic system is employed, the coolant is heated by an electric heater before being pumped and dispersed throughout the pack assembly.
In order to include heat energy into the pack, BMS design experts surely have some tricks up their sleeve. For instance, the BMS’s capacity management-related power electronics can be activated. It can be used even though it isn’t as effective as direct heating. In order to reduce a lithium-ion battery pack’s performance loss, cooling is especially important. For instance, a battery may function best at 20°C; if the pack temperature rises to 30°C, it may lose up to 20% of its performance. Continuous charging and recharging of the pack at 45°C (113°F) can result in a significant 50% reduction in performance. If batteries are constantly subjected to high heat generation, especially during rapid charging and discharging cycles, they may also experience premature aging and degradation. Both passive and active approaches can be used to accomplish cooling, which is typically accomplished in two ways. The battery is cooled by passive cooling, which depends on air flow. This suggests that an electric vehicle is just traveling along the road. It might be more advanced than it seems, though, since deflective air dams could be deliberately auto-adjusted to maximize air flow by integrating air speed sensors. At moderate speeds or when the vehicle has stopped, the installation of an active temperature-controlled fan can be beneficial, but it will only aid to balance the pack with the ambient temperature. This could make the initial pack temperature higher on a very hot day. An electric motor-driven pump circulates ethylene-glycol coolant with a specified mixture ratio through pipes and hoses, distribution manifolds, a cross-flow heat exchanger (radiator), and a cooling plate that is positioned against the battery pack assembly. Thermal hydraulic active cooling can be designed as a supplementary system. In order to ensure optimal battery performance, a BMS keeps an eye on the temperatures throughout the pack and opens and closes several valves to keep the battery’s overall temperature within a specific range.
Management of Capacity
One of the most important battery performance characteristics that a BMS offers is probably optimizing a battery pack’s capacity. A battery pack may eventually become worthless if this maintenance is neglected. The fundamental problem is that a battery pack’s “stack,” or series arrangement of cells, is not quite equal and has somewhat varied leakage or self-discharge rates by nature. Although it may be statistically affected by small changes in the production process, leakage is a feature of battery chemistry rather than a manufacturer problem. A battery pack may contain well-matched cells at first, but as time goes on, the cell-to-cell similarity deteriorates even more. This is caused by a variety of factors, including charge/discharge cycling, high temperatures, and general calendar aging, in addition to self-discharge. Having said that, keep in mind that lithium-ion cells work incredibly well, but they can be harsh if used outside of a strict SOA. Since lithium-ion batteries can not handle overcharging well, we already learnt about the necessity of electrical protection. After they are fully charged, they are unable to take in any more current, and any more energy that is forced into them is converted to heat. As a result, the voltage may rise rapidly, even to potentially hazardous levels. The cell is not in a healthy state, and if it persists, it may result in irreversible harm and dangerous operating circumstances.
The entire pack voltage is determined by the battery pack series cell array, and while trying to charge any stack, mismatches between adjacent cells cause problems. This is demonstrated in Figure 3. Every cell in a properly balanced set will charge equally, and when the upper 4.0 voltage cut-off level is reached, the charging current can be stopped. The charging current must be stopped for the leg before the other underlying cells have been fully charged in the unbalanced case because the top cell will reach its charge limit early.
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