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Yueqing City,Zhejiang Province,325604,China-October 22, 2026

What is a dry contact signal?

A dry contact, also known as a passive contact, is a switching signal that does not actively output a voltage. It typically transmits “start” or “stop” commands to external devices by closing or opening relay contacts.

Unlike control signals with voltage output, a dry contact generally does not supply power to the generator set itself but functions as an independent switch. Upon receiving a signal that the contacts have closed, the generator controller starts the engine according to its own control logic; when the contacts open, the generator executes a shutdown or cool-down procedure.

Since dry contacts do not rely on specific communication protocols, they generally offer good compatibility between lithium-ion battery systems and generator sets of different brands and models. However, before actual wiring, it is still necessary to confirm the interface definitions, contact capacity, and control logic of both systems.

How does a lithium-ion battery system determine whether to start the generator?

Lithium-ion battery systems typically use a BMS, EMS, or inverter controller to monitor battery status and load conditions in real time. Common startup conditions include:

Battery SOC falls below the set startup threshold;

Battery voltage falls below the set value;

On-site load power continues to rise;

Insufficient PV generation or continuous lack of sunlight;

Entry into standby or emergency power mode;

The system detects a need for auxiliary power from an external source.

For example, when the battery SOC drops to the preset startup value, the EMS can control an internal relay to close the dry contact. Upon receiving the remote startup signal, the generator set automatically completes preheating, ignition, and speed ramp-up, and begins supplying power to the load or charging the energy storage battery.

Automatic Startup Process

A typical dry-contact startup process usually includes the following stEPS.

Step 1: Continuous System Monitoring

The lithium-ion battery system collects real-time data on battery voltage, current, SOC, temperature, and load power. The controller uses preset parameters to determine whether the generator needs to be started.

Step 2: Meeting Start Conditions

When the battery charge level falls below the start threshold, or when the load consistently exceeds the system’s power supply capacity, the EMS sends a control command to the dry-contact relay.

Step 3: Dry-Contact Closure

The relay contacts close, forming a remote start circuit recognized by the generator set. At this point, the lithium-ion battery system outputs a switching signal rather than directly driving the generator motor.

Step 4: Generator Set Startup

Upon receiving the signal, the generator controller starts the unit according to its own program. Some generator sets also perform actions such as preheating, lubrication, restart attempts after a failed start, and operational status checks.

Step 5: Energy Storage System Recovery

Once the generator is running, it can supply power to the on-site load and recharge the lithium-ion battery via charging equipment or a Hybrid Inverter. As the battery’s SOC gradually recovers, the system enters standby or reserve mode.

How Does the Generator Shut Down Automatically?

To avoid frequent generator start-stops, the system typically does not shut down immediately when the battery has just recovered a small amount of charge; instead, it sets a shutdown threshold and a delay time.

When the battery’s SOC reaches the set shutdown value and the load is within the permissible range, the EMS deactivates the dry contact signal. Upon receiving the shutdown command, the generator set typically does not shut down immediately; instead, it first enters a no-load operation or cooling phase and automatically shuts down only after the engine temperature and operating status meet the requirements.

This control method—characterized by a “low start threshold and high stop threshold”—creates a reasonable hysteresis range, reducing frequent generator starts and stops while lowering fuel consumption and mechanical wear.

Key Advantages of Dry Contact Control

1. Simple Wiring

Dry contacts primarily transmit control status through the closing and opening of contacts, eliminating the need for complex communication protocols and making on-site installation and commissioning relatively convenient.

2. Strong Compatibility

As long as the generator set is equipped with a remote start interface and the interface logic matches that of the lithium-ion battery system, automatic interlocking can be achieved.

3. Reduced Need for Manual Monitoring

In applications such as remote areas, unmanned power rooms, ships, and RVs, the system can automatically start and stop the generator based on the battery status, reducing the frequency of manual inspections.

4. Improved Power Supply Continuity

When photovoltaic or other renewable energy sources are insufficient, the generator set can promptly take over to provide backup power to the load, reducing the risk of power outages caused by low battery levels.

5. Easy System Expansion

Dry contacts can be used in conjunction with EMS, BMS, inverters, automatic transfer switches, and generator controllers to build a multi-energy coordinated power supply system.

What Should Be Considered During Installation and Commissioning?

Although the control principle of dry contacts is relatively simple, the following issues still require special attention in practical applications.

First, the remote start method for the generator set must be confirmed. Some units require closed contacts to start, while others use open contacts; therefore, the control logic cannot be directly applied.

About us

Bangzhao Electric Co.,Ltd( Sandi Electric) is a company who was established in 2008 and specialized in developing and supplying of power products. The high performance of its products stems from the cutting-edge technology developed by the domestic top 100 engineers. During the past 10 years, we have been providing the best system solutions of Lithium battery storage system, PCS bi-directional energy storage power supply, Pure sine wave inverter, Marine inverter, Vehicle Inverter, Solar pump inverter, Explosion-proof inverter, PV Inverter-Controller Integrate, PV combiner box, Solar charge controller, AC-DC charger, Static phase converter etc.

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