Relay Solutions in Battery Protection Boards: Principles, Advantages, and Applications

In battery management systems, the core function of a protection board is to cut off the circuit promptly when abnormalities such as overcharge, over-discharge, overcurrent, or short circuits occur, ensuring the safety of both the battery and the user. Currently, the two mainstream protection approaches are the MOSFET solution and the relay solution. This article focuses on the working principles, technical characteristics, and suitable applications of the relay solution.

The relay solution refers to a battery protection board that uses an electromagnetic relay as the main circuit switching device, controlling the battery pack's connection and disconnection by energizing or de-energizing the relay coil. Unlike the MOSFET solution, a relay is a mechanical switch that relies on electromagnetic force to open or close its contacts.

A typical relay-based protection board consists of a battery management chip (or MCU), a driver circuit, the relay itself, current-sensing resistors, and other auxiliary components.

The protection board continuously monitors parameters such as battery voltage, current, and temperature. When all parameters remain within normal ranges, the MCU or protection chip energizes the relay coil through the driver circuit, the contacts close, and the battery pack can charge or discharge normally.

When an abnormality is detected:

• Over-discharge: voltage drops below the lower threshold

• Overcharge: voltage rises above the upper threshold

• Overcurrent/short circuit: current exceeds the set limit

The control system immediately cuts off power to the relay coil, the contacts open, and the main circuit is disconnected. Once the fault is cleared, the system can re-close the relay through an external charging signal or a reset command.

Relay contacts have extremely low resistance (typically in the milliohm range), generating minimal heat under high-current conditions. This makes relays particularly suitable for applications requiring over 100A, such as electric forklifts, RV energy storage, and low-speed electric vehicles. In contrast, MOSFETs experience significant heating due to voltage drop at high currents.

There is physical isolation between the relay coil and its contacts, requiring no additional isolation circuitry. This naturally meets high-voltage and low-voltage side safety insulation requirements, simplifying design.

Relay contacts can withstand substantial surge currents during short circuits without being easily damaged, whereas MOSFETs are prone to avalanche breakdown under overcurrent conditions. Relays have an inherent advantage in terms of "ruggedness."

When the relay contacts are open, the air gap between them results in negligible leakage current, making relays safer and more reliable for battery packs that need long-term storage.

Relays are insensitive to current direction. The contacts provide a purely resistive path, unlike MOSFETs which require back-to-back series connection to achieve bidirectional cutoff. This results in a simpler structure.

Of course, the relay solution is not without drawbacks:

• Limited mechanical/electrical life: Mechanical contacts wear out with repeated switching. Electrical life is typically in the range of thousands to tens of thousands of cycles, making relays unsuitable for frequent switching applications.

• Slower response time: Relay pickup and release times are typically several to tens of milliseconds, significantly slower than MOSFETs (microseconds). For extremely fast short-circuit protection, fuses may be needed.

• Risk of contact welding: Under very high short-circuit currents, contacts can fuse together, causing protection failure.

• Size and cost: High-current relays are relatively large, and their cost can exceed that of equivalent MOSFET solutions.

• Audible noise: Relays produce an audible "click" when switching, which may be undesirable in noise-sensitive applications.

Given these characteristics, the relay solution is best suited for:

1. High-capacity energy storage batteries (e.g., home energy storage, telecom backup power): High operating current but low switching frequency, less demanding on cycle life.

2. RV/marine auxiliary batteries: Require high-power inverter support and very low static power consumption (relays consume no power when open).

3. Low-speed electric vehicles (e.g., electric golf carts, sightseeing vehicles): High current and relatively harsh operating environments.

4. Industrial equipment batteries (e.g., AGVs, forklifts): High reliability and impact resistance are critical.

5. Battery repair or testing fixtures: Relays provide a simple and reliable way to manually or remotely disconnect the main circuit.

If you are designing or selecting a relay-based protection board, keep the following in mind:

• Contact rating: Allow sufficient margin. Choose a relay rated for at least 1.5 times the expected continuous current. Short-circuit withstand capability should also match actual conditions.

• Coil power consumption: DC relay coils typically consume between 0.5W and 3W. Proper drive circuit design and heat dissipation are important.

• Contact material: Silver-alloy contacts suit general applications; silver-tin oxide (AgSnO₂) and silver-nickel (AgNi) offer better anti-welding performance.

• Sealing rating: For humid or vibrating environments, choose sealed or epoxy-encapsulated relays.

• Auxiliary contacts: Some relays provide auxiliary contacts that can signal the actual contact status, improving system safety.

The relay solution remains irreplaceable in applications requiring high current, low switching frequency, and high safety isolation. While perhaps less "smart" and "fast" than MOSFET solutions, relays continue to be widely used in energy storage, industrial equipment, and specialty vehicles due to their simplicity, reliability, and durability.

The choice between relay and MOSFET ultimately depends on your product's requirements. For small batteries in the hundred-watt range, MOSFETs are more suitable. For large battery packs in the kilowatt or even tens-of-kilowatt range, relays are often the more pragmatic choice.

I hope this article provides you with a comprehensive understanding of relay solutions for battery protection boards. If you have practical design experience or questions, feel free to leave a comment.