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Pylontech RT12100G31 12V 100Ah LiFePO4 battery with Victron MultiPlus II

Battery Communication: Closed vs. Open-Loop Communications

When searching for "communicating battery" on Google, you'll likely come across the terms open and closed-loop communication. We would like to shed some light on these terms and the implications for system performance and longevity. 

Open-loop communication is what we commonly see in systems with lead-acid batteries. In this setup, the inverter uses tools, such as a shunt, to estimate the battery's state of charge (SOC) from an external perspective by measuring the change in voltage as the battery charges and discharges as well as the amount of current that has passed into or out of the battery. A typical open-loop system must be calibrated by bringing the batteries to a full SOC (determined by the manufacturer's recommended “full” voltage) and repeating this calibration every few weeks to ensure accuracy. While this method has a proven track record with lead-acid batteries (as their voltage curve clearly indicates their SOC), it faces challenges when applied to modern lithium chemistries.

Lithium batteries are more difficult to monitor in an open-loop setup due to their unique voltage curve.

Lithium vs. Lead Acid Discharge Curve

As shown in the chart above, cell voltage in a lithium battery plateaus between 90% to 20% SOC (State Of Charge.) This makes it difficult to accurately determine battery SOC within this range using a shunt. To compensate for this uncertainty, the inverter must incorporate large margins of error to prevent overcharging or discharging of the batteries (remember; lithium batteries are far more sensitive to overcharging than lead acid). These error margins, often up to 30% of the battery's capacity, inhibit inefficiency for the sake of safety. The true potential of lithium chemistries is their ability to accept a high-current bulk charge to full capacity while also allowing discharge down to the low single digits. Without precise knowledge of the battery's state of charge, these capabilities remain locked away, and overall system efficiency is limited. A lithium battery in an open-loop configuration is essentially non-communicating: it does not share its state of charge or any other information gathered by the internal battery management system (BMS).

Is a lithium battery with a Bluetooth app a communicating battery?

A battery with Bluetooth can talk to an app, but the information shared ends there. If a battery has no hard-wired communications cables via CAN bus or Modbus, we would still classify it as a non-communicating battery. Apps can be convenient for monitoring, adjusting settings, or troubleshooting basic issues when physically near the battery. However, if the information is not being shared with the inverter or gateway device, then the system as a whole fails to benefit as these two systems are isolated from each other - neither truly knows what is happening within the other’s system.

In contrast to open-loop communication, Closed-loop communication is the modern solution to the issues with lithium outlined above. In a closed-loop system, a line of communication is opened from the battery to the inverter/charger, allowing measurements to be taken directly from the battery's internal BMS sensors. When done properly, this eliminates the need for voltage-measuring shunts and provides an accurate baseline for charge/discharge decisions to be made. As a result, performance can be greatly enhanced, and the battery will last longer, as it will never risk being drawn outside its optimal operating parameters. A good closed-loop communicating battery with a compatible inverter can take full advantage of available capacity with fewer moving parts and a simplified commissioning process. (Read more about the importance of inverter-battery compatibility here).

The mechanics of closed-loop communication are generally defined as follows: Player A asks a question of Player B, and Player B acknowledges the question and provides an answer, closing the communication loop. Our team at Intelligent Controls finds this description misleading and far too general to describe what's happening. The reality of the communication path in a closed-loop system is less like a back-and-forth Q&A and more like a one-directional stream of information from the battery to the gateway device, such as a Victron Cerbo GX device. If the system is prepared to understand and act upon the information received, everything can operate efficiently and without guesswork.

Easy peasy, right? Well, before we go and buy the first closed-loop communicating battery we can find, let's dive a little deeper and discuss the nuances that can be the difference between a robust system that just works and a tragedy of broken dreams and error lights.

Related Articles: 

>>> Bad, Better, Best: Battery-Inverter Communications & Compatibility

>>> Active Management: Expect More From Your BMS

>>> Adding Capacity to a Pylontech Lithium Battery Bank

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