bennypass.com - Open-Loop Communication (Passive)

Open-Loop Communication (Passive)

passive

Open-loop communication in battery systems refers to a setup where the battery management system (BMS) does not share real-time, dynamic data with the inverter or charger. In this passive mode, the inverter operates based solely on fixed parameters (such as voltage, current, and chemistry type) manually entered by the user.

This is often considered the "traditional" or "standard" approach, frequently used with lead-acid batteries or earlier generation lithium batteries.

Key Characteristics

No Data Feedback: The battery does not inform the inverter of its state of charge (SoC), temperature, or health in real time.
Fixed Parameters: The inverter relies on preset, rigid settings for charging and discharging.
Passive Monitoring: The inverter often uses a shunt to estimate SoC from an external perspective, measuring voltage changes and current flow.
"Drop-in" Compatibility: Often used for lead-acid replacements where the battery acts as a passive energy reservoir.

Advantages

Simple Installation: No specialized communication cables (CAN bus/RS485) are required between the battery and inverter.
Compatibility: Works with almost all older or simple inverters.

Disadvantages

Lower Precision: The inverter may have an incorrect estimation of the actual battery capacity.
Limited Protection: The system cannot automatically throttle charging/discharging based on battery temperature or cell health.
Manual Maintenance: Requires periodic, manual calibration to ensure accuracy.

Open-Loop vs. Closed-Loop

While open-loop works on fixed settings, closed-loop (active) communication enables the battery and inverter to communicate directly (via CAN/RS485), allowing the inverter to adjust voltage and current in real time, which increases safety, longevity, and efficiency.

Comparison At A Glance

Feature	Open-Loop (Passive)	Closed-Loop (Active)
Data Exchange	None; inverter operates independently.	Real-time; BMS talks to inverter.
SOC Accuracy	Estimated (can drift over time).	Exact (provided by the BMS).
Safety	Limited; relies on preset limits.	Dynamic; adjusts to cell temp/health.
Setup	Simpler; no data cables needed.	Complex; requires protocol matching.

Pro Tip: If you're using lithium batteries (LiFePO4) in an open-loop setup, it's highly recommended to use a Battery Shunt to get a more accurate reading of your power levels than voltage alone can provide.

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