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svaeolus avatar image

Relay recommendation for BMS to shut down alternator regulator

I have an alternator that charges a Victron LIFePO4 house battery bank. The alternator is regulated by a Balmar 614 external regulator.

My understanding is that the recommended method for shutting down the regulator is to have the BMS charge disconnect trigger a relay that will open the connection between ignition and the Balmar regulator.

I'm looking for specific recommendations for a relay that works for this purpose, or other known good solutions to this problem.

What I've found are other threads referencing problems activating common relays given the very low 10ma current sourced by the BMS charge disconnect, which is also being relied on to disconnect a Cyrix Li Charge that the charge load goes through (sources are 3 solar panel controllers and the alternator). Hard to find a relay that is triggered at, say less than 5mA. Or is it? What's your favorite relay for this purpose?

Then I thought maybe use a MOSFET or a transistor in front of the relay to boost the charge disconnect current up to where it would trigger a relay. But that seems to be going a bit off-script, and this isn't an area where I want to "innovate" but would rather "stand on the shoulders of giants" who have already implemented a proven solution. If you've got one, please share!

I have also seen references to a solution where instead of the BMS charge disconnect, someone used the BMS pre-alarm to trigger the relay. Although I can't find a spec for its current, presumably it is higher than 10mA if it is expected to trigger a light or buzzer.

And I've seen solutions that just throw more Victron products at the problem. I'm OK with that if it is the best answer, but it seems pretty heavy handed for the meager ignition circuit. Either another Cyrix Li Charge solely for the ignition wire (seems like 1000% overkill but should work?) or a Victron Battery Protect wired up in reverse. Again seems awfully beefy for the measly current going through the ingition wire. Also raises the question of how many relays can you successfully hook up to one charge disconnect terminal on the BMS and still have them all trigger from its 10mA current.

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1 Answer
Stefanie avatar image
Stefanie answered ·

Hi @svAeolus,

I'm using a optocoupler relay Wago 859-744, see: Wago Optokopler for the same purpose from the VE.Bus BMS charge disconnect. The optocoupler drives a automative relay or the one you could buy from balmar (which really is just an ordinary automative relay but comes with a harness). Works great.

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Did you try having the optocoupler alone disconnect the Balmar's connection to ignition? My understanding is the current is very low, under 1 amp. I am asking why the automotive relay was necessary. Maybe I don't understand how the optocoupler works and need to read up on it.

No I did not because I had the automative relay already installed and wired. But yes, the optocoupler can easily handle the current.