Why Do Dual-Coil Automotive Relays Provide Latching Functionality for Memory Apps?

2026-10-21

A vehicle electrical design engineer is calculating the dark current budget for a new model. The target is 20 mA total when the vehicle is parked and locked. The seat memory module alone is drawing 12 mA because it uses a standard single-coil Automotive Relay that must remain energized to keep the memory position stored. The engineer replaces the single-coil relay with a dual-coil latching relay. The holding current drops to zero. The seat memory module now draws only 0.5 mA in sleep mode. The engineer recalculates the total dark current: 8.5 mA, well within the target. This guide explains how dual-coil latching relays achieve zero holding current and why that matters for memory applications.

1. What Is the Dark Current Problem With Standard Single-Coil Relays?

A standard single-coil Automotive Relay requires continuous current through the coil to keep the contacts closed. For a memory application, the relay must stay closed for weeks or months while the vehicle is parked. The coil current is typically 100 to 200 mA for a 12V relay. Even with a duty cycle reduction circuit, the average holding current is 10 to 50 mA. This current drains the battery. The table below shows the battery drain for different holding currents over a 30-day parking period.

Holding current (mA)	Drain over 30 days (Ah)	Battery capacity remaining (60 Ah battery)	Engine start risk
50	36	40%	High
30	21.6	64%	Moderate
10	7.2	88%	Low
1	0.72	98.8%	Very low
0 (latching)	0	100%	None

In our factory, we manufacture Automotive Relay units for both standard and latching configurations. The dual-coil latching relay is designed for applications that require the contact state to be maintained without continuous power. Ningbo Forward Relay Corp., Ltd. has been manufacturing automotive relays for over 18 years.

2. How Does a Dual-Coil Latching Relay Work?

A dual-coil latching relay has two separate coils: a set coil and a reset coil. The set coil is energized momentarily to close the contacts. The reset coil is energized momentarily to open the contacts. Once the contacts are in position, they are held by a permanent magnet. No current is required to maintain the state. The pulse duration is typically 20 to 50 milliseconds. The table below compares the operating characteristics of a standard relay and a dual-coil latching relay.

Parameter	Standard single-coil relay	Dual-coil latching relay
Coil current to close	150 mA continuous	400 mA for 30 ms (pulse)
Coil current to hold	150 mA	0 mA
Energy per operation	1.8 J (continuous)	0.14 J (pulse)
Contact state on power loss	Opens (de-energized)	Remains in last state
Battery drain in sleep mode	10 – 50 mA	0 mA
Typical life	100,000 operations	100,000 operations

Design engineer's note: The pulse energy is much lower than the continuous energy of a standard relay. The dual-coil latching relay draws 400 mA for 30 ms, which is 0.14 joules. A standard relay drawing 150 mA continuously for the same 30 ms would draw 0.0054 joules, but it would continue drawing that current indefinitely. The key advantage is the zero holding current. In a memory application, the relay may be in one state for months. The zero holding current eliminates the battery drain entirely. In our factory, we test the pulse energy and the holding force of every latching relay batch to ensure consistent performance.

3. Which Memory Applications Use Dual-Coil Latching Relays?

Dual-coil latching relays are used in four common memory applications in vehicles. The first is seat position memory. The relay stores the seat position by maintaining the connection to the memory module. The second is mirror position memory. The relay maintains the mirror angle setting. The third is clock and radio preset memory. The relay keeps the memory backup circuit alive. The fourth is HVAC setting memory. The relay maintains the last temperature and fan speed settings. The table below shows the application, the typical relay configuration, and the holding current saved.

Memory application	Relay configuration	Holding current saved	Battery drain avoided (30 days)
Seat position memory	DPDT dual-coil latching	15 mA	10.8 Ah
Mirror position memory	SPDT dual-coil latching	8 mA	5.8 Ah
Clock and radio preset	SPST dual-coil latching	5 mA	3.6 Ah
HVAC setting memory	SPDT dual-coil latching	10 mA	7.2 Ah

In our factory, we manufacture dual-coil latching Automotive Relay units in SPST, SPDT, and DPDT configurations. The relays are sealed and rated for the engine compartment temperature range of -40°C to +125°C. Ningbo Forward Relay Corp., Ltd. supplies these relays to seat module manufacturers and body control module suppliers.

4. What Are the Design Considerations for Using Latching Relays?

The design engineer must consider four factors when using a dual-coil latching relay. The first is the pulse drive circuit. The relay requires a short, high-current pulse. The driver must be able to supply 400 mA for 30 ms. A capacitor discharge circuit or a dedicated driver IC is typically used. The second is the state feedback. The relay does not provide a direct indication of its state. The designer must use an auxiliary contact or a software flag to track the state. The third is the fail-safe behavior. If the vehicle battery is disconnected, the relay remains in its last state. This is desirable for memory applications but may not be desirable for safety-critical functions. The fourth is the coil resistance tolerance. The pulse current depends on the coil resistance, which varies with temperature. The driver must be designed to supply the required current over the full temperature range. The table below summarizes these considerations.

Design consideration	Requirement	Typical solution
Pulse drive	400 mA for 30 ms	Capacitor discharge or driver IC
State feedback	Determine if relay is set or reset	Auxiliary contact or software flag
Fail-safe	Defined behavior on power loss	Use latching only for non-safety functions
Coil resistance tolerance	Maintain pulse current over temperature	Constant current driver or PWM

In our factory, we provide the coil resistance versus temperature curve for each latching relay model. We also provide a recommended driver circuit and a state feedback circuit. Ningbo Forward Relay Corp., Ltd. offers application support for designers who are transitioning from standard relays to latching relays.

Frequently Asked Questions About Dual-Coil Latching Relays for Memory Applications

Question 1: What happens if the vehicle battery is disconnected while the latching relay is set?

Answer: The dual-coil latching Automotive Relay uses a permanent magnet to hold the contact state. If the vehicle battery is disconnected, the relay remains in its last state. This is the intended behavior for memory applications. The seat position, mirror position, and radio presets are retained. When the battery is reconnected, the memory module reads the stored position and restores the settings. In our factory, we test the holding force of the permanent magnet after 1,000 hours of temperature cycling. The holding force remains within 5 percent of the initial value. We also recommend that the memory module include a non-volatile memory backup in case the relay is accidentally reset during service.

Question 2: Can a dual-coil latching relay be used for safety-critical functions like airbag deployment?

Answer: No, a dual-coil latching Automotive Relay should not be used for safety-critical functions such as airbag deployment. The latching relay maintains its state even when the power is removed, which is not desirable for a safety function that must default to a known safe state. For safety-critical functions, a standard normally open relay or a solid-state switch with a fail-safe default is required. The latching relay is intended for memory and convenience functions where maintaining the state is the desired behavior. In our factory, we clearly mark the latching relays with a warning label that states "not for safety-critical applications." We also provide a separate product line of safety-rated relays for airbag and ABS functions.

Question 3: How do I test a dual-coil latching relay to verify that it is working correctly?

Answer: To test a dual-coil latching Automotive Relay, apply a 30 ms pulse to the set coil and verify that the contacts close. Then remove the pulse and verify that the contacts remain closed. Apply a 30 ms pulse to the reset coil and verify that the contacts open. Then remove the pulse and verify that the contacts remain open. Measure the coil resistance of both coils and compare to the specification. In our factory, we perform a 100 percent functional test on every latching relay. The test includes a set pulse, a reset pulse, a contact resistance measurement, and a holding force check. We provide a test report with each batch. For field testing, we recommend a simple test box that provides the set and reset pulses and indicates the contact state with an LED.

Summary for Design Engineers

Dual-coil latching Automotive Relay units provide latching functionality for memory applications by using a permanent magnet to hold the contact state after a short pulse. The zero holding current eliminates the dark current drain that would otherwise deplete the battery over time. The relay is suitable for seat memory, mirror memory, clock memory, and HVAC memory applications. The design engineer must consider the pulse drive circuit, the state feedback, the fail-safe behavior, and the coil resistance tolerance. By selecting the right latching relay and driver circuit, the engineer can achieve a reliable memory function with minimal battery drain. Ningbo Forward Relay Corp., Ltd. has been manufacturing automotive relays for over 18 years and provides full application support for latching relay designs.

Ningbo Forward Relay Corp., Ltd. manufactures dual-coil latching Automotive Relay units in SPST, SPDT, and DPDT configurations, sealed for engine compartment use. We provide coil resistance curves, driver circuit recommendations, and test reports for all of our products.

Need help designing a latching relay circuit for your memory application? Contact Ningbo Forward Relay Corp., Ltd. for a free consultation. We will review your dark current budget and recommend the optimal latching relay configuration.

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