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New latching relay driver (LRD)

Latching (bi-stable) relays have the great advantage to maintain their contact position indefinitely without any power being sent to the coil. They save a lot of energy compared to standard contactors that can consume up to 0.4 A to maintain their contacts closed (10 Ah per day / 300 Ah per month). But they need to be commanded by pulses on two different inputs.

The LRD makes it easy to use a power saving latching relay.

  • converts standard output from a BMS, a relay or a switch into the proper signals to drive a dual-coil latching relay
  • it is reliable: built with discreet components (no micro-controller)
  • it resists humidity: the components are coated with an epoxy resin
  • the spring clamp terminals are coated with an anticorrosion molecular compound

What is a latching (bi-stable) relay?

In a latching relay the position of the contacts (power circuit) is maintained by a magnet. It just needs a short pulse on one of the coils to change the position of the contacts. Once the contacts are in the desired position the drive power to the coil can be removed.

When the “ON coil” is powered the power circuit is closed

When the “OFF coil” is powered the power circuit is opened

The key characteristics of a latching relay:

  • maximum switching current (the maximum current in the power circuit)
  • coil voltage (the range of voltage to apply to the coils to change the state of the contacts)
  • maximum coil current (the maximum current drawn by the coil when powered)
  • coil suppression (a circuit to avoid high voltage spike when the power is removed from a coil)
  • coil pulse duration (ms)
  • high / low side drive (whether the coil is controlled on the positive or the negative terminal)

The LRD drives the coils on the high side, which is compatible with most latching relays on the market.

Examples of latching relays

LRD connections

LRD top view
  • Power
    • power supply should be sourced directly from the battery (before the relay) or from a separate battery (starter battery)
    • the voltage of the power supply (vcc) should match the coil voltage of the relay (9 – 30 volt)
    • should be protected with a fuse rated to match the relay coils
  • BMS
    • input for the driving command (BMS output or the output of other command source like a relay)
  • Input for an override switch
    • use a switch with 3 positions to set the relay ON, OFF, or have it set by the driving command
  • Outputs
  • ON
    • a pulse is sent on that output when the driving command transitions to vcc or when the override switch is flipped to the ON position
close the relay: trace 1 is the input (driving command)
trace 2 is the ON output / trace 3 is the OFF output
  • OFF
    • a pulse is sent on that output when the driving command is disconnected from vcc or when the override switch is flipped to the OFF position
open the relay: trace 1 is the input (driving command)
trace 2 is the ON output / trace 3 is the OFF output


  1. Maybe I’ve to add a third thing?
    3. Why does the VCC not match the LFP 48VDC (means a bit under 60VDC in reality). Would make things much much easier…


    1. Because most latching relays we have seen on the market propose only 12 or 24 Volt command circuits. But I see the TBS relays have an option for up to 68V. We may want to reconsider in the future…

  2. Hi Philippe,

    its me, Dirk, again 😉

    I’ve two things to add / to ask:
    1. The manufacturer TBS seems to be expensive but very reasonable. Please be so kind and give other the chance to get or to reject it… The second thing about that relay is the undervoltage protection, which ist independently from all other measurements. Regardless that overvoltage protection is as important as undervoltage protection – it is most likely that some unnoticed drainage might kill your battery bank than overcharge voltage would kill them. Take a look into it again (please): https://tbs-electronics.com/product/battery-protect-relay-500a/
    2. This is a bit more, mmmmh, weired to explain. So, give me a chance please.
    I’ve an automatic controll system for my roller shutters for one of my houses. Sometimes, totally random, the controller gets a command to close or to open – altough it is I2C bus controlled. Simple sollution: Only a double command is accepted as valid. Result: Not a single single mailfunction since 7 years! And here comes my question: Wouldn’t it be sensefull to send a double open or close command to a latching relay to avoid any “misinterpretation” or “oops – didn’t recognize due to whatever”. In my opinion – would significantly decrease random failures. Just from my experience. Just my 2 cts.

    Hope you’re doing well?!


    1. Hi Dirk,
      Thanks for sharing about the TBS relay. Might be worth starting a thread on the forum to discuss about it as comments on articles are less visible – we can add a link to the forum thread as a comment.
      I like the independant low voltage cut-off and also the modular series equipments to make a clean installation.
      I understand this relay has a feedback loop to confirm if it is open or closed. This is something we will consider using in a future version of the TAO BMS that will have the possibility for all kind of inputs that can be used by the BMS (additional voltage, current, temperature sensors, external switches / remote commands, relay outputs.. and also inputs from a network / Wifi connections…)
      I think your suggestion on your second point is a good one to reduce the risk of a command being lost or parasit commands driving the relay in the wrong state. This idea is now recorded as an improvement for the next version of the LRD. Some more thinking will be required as requiring a double command to command a latching relay is a function of the relay itself – not the driving device (correct me if I am wrong or misunderstood what you meant). But we can use the feedback loop to check the relay is in the desired position.

  3. Hi Jens,
    I had the same question a few years ago and I called Blue Sea support to understand.
    Actually the Blue Sea manual say: “Amperage Operating Current when changing state <7A (40 ms)"

    As you can see on the wiring diagram https://d2pyqm2yd3fw2i.cloudfront.net/files/resources/wiring_diagram/7700_7702.png , you need to have two fuses. The 7A switching current is taken from the supply that is protected by a 10A fuse.

    The Latching Relay Driver takes the place of the “Control Switch” and is protected by the 2A fuse – You can use the LRD to drive the Blue Sea 77xx relay. I did some test with early LRD prototypes.

    Here is a link to the wiring diagram: https://www.taoperf.com/wp-content/uploads/2021/12/LRD_1.drawio-1.pdf

  4. Hello Phil
    I just bought a Blue Sea ML-RBS to control the Under Voltage Protection of my Lithium Battery.
    I want to command this relay with the Tao LRD.
    In terms of current to run the coil of the Blue Sea ML-RBS I cannot find matching information.
    The specification in the manual of Blue Sea shows: “Amperage Operating Current when changing state max command current 1A

    The maximum output for the LRD given in the specification “up to 5 A”

    What is the correct current for commanding the Blue Sea relay?
    Can I use the Tao LRD for the Blue Sea relay with out damaging anything?
    Did you test the Tao LRD on this Blue Sea 77xx relays?

    Thanks for your help


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