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BMS Theory | Closed-Loop Communications
In this piece, we discuss the importance of closed-loop communication between the battery and an inverter/charger in modern energy storage systems. It explains how this two-way communication link allows for dynamic real-time control and monitoring of the battery system, leading to enhanced safety, performance, reliability, and increased lifespan of the batteries. We compare closed-loop communication with open-loop communication and highlight the potential issues with the latter. Overall, the integrated approach of closed-loop communication is seen as a crucial feature of modern ESS.
Understanding Closed-Loop Communication
On modern ESS systems, Closed-Loop communication is an essential feature of system design. Closed-loop communication with the Battery Management System (BMS) involves a two-way communication link between the BMS and a compatible inverter/charger. This functionality allows for real-time data exchange between the battery and inverter, allowing for dynamic system control, which enhances the safety, performance, and reliability of the battery system.
With closed-loop communication, the BMS continuously sends data to the inverter/charger. This data includes crucial parameters like the state of charge (SoC), battery voltage, current temperature, charge/discharge rates (current), and charge/discharge voltage. The BMS uses this data to command charging and discharging functionality and provide the user interface with accurate battery-related information. Based on the data received from the BMS, the inverter/charger can adjust its charging and discharging strategies; for instance, if the BMS indicates that the battery is nearing full charge, the BMS can reduce the charging current or voltage to prevent overcharging and allow optimized cell balancing. Conversely, if the battery temperature rises above a safe threshold, the BMS can lower the charge/discharge rate or stop it altogether, the same goes for if the battery gets too cold.
Understanding Open-Loop Drawbacks
An open-loop, or "dumb battery" system, lacks this level of communication and control. The inverter/charger operates based on preset parameters and does not adjust in real time to the battery's condition. This can lead to less efficient charging, potential overcharging or deep discharging, reduced battery life, and a higher risk of safety issues due to the lack of active monitoring and adaptive control.
The largest issue with an open-loop configuration is it is very difficult to reliably and consistently fully charge lithium batteries without overcharging them. The effectiveness of charging depends on the precision and calibration of the charging equipment, and most equipment that’s incapable of closed-loop is also not likely to be very accurate or precise with respect to voltage monitoring of the battery bank.
Lithium-ion batteries operate within a narrow voltage window; even a small overshoot in voltage during charging can be considered overcharging, requiring BMS intervention. To prevent constant BMS intervention, Open-Loop inverter configurations are often programmed for conservative charge values. This helps to reduce the balancing stress of the BMS and the likelihood of BMS intervention. However, this can cause other issues, for example, very slow charging rates at the upper end of the charging profile, potential over-discharge, and the biggest issue is inconsistent BMS cell balancing. Long term, these issues could significantly reduce expected battery life.
Closed Loop Conclusion
Closed-loop communication between a battery management system (BMS) and an inverter/charger is crucial for modern energy storage systems. The two-way communication link allows for dynamic real-time control and monitoring of the battery system, leading to enhanced safety, performance, reliability, and increased lifespan of the batteries. On the other hand, an open-loop system lacks this level of communication and control, leading to potential safety issues, reduced battery life, and less efficient charging. Therefore, closed-loop communication is the preferred approach for modern energy storage systems.
To learn more about lithium batteries:
- Lithium Batteries: BMS Theory
- Lithium Battery Theory | Fundamentals of The Main Components
- Lead is Dead | Lithium Iron Phosphate Batteries are Now the Norm.
- Lithium Batteries: Are They Worth the Cost?
- Lithium Battery Cell Quality - Everything You Need to Know
Shop Our Lithium Batteries Here.
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