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jcarnouil avatar image

Using BMV-702 relay to power a boiler when battery is almost charged ?

Dear community,

I've setup an off-grid system (while finishing to build my house) and would like to automatize the powering of a small boiler (100 liter / 1200 W) when battery charge is near 100 %, and to stop powering the boiler when battery SOC falls to 85 %.

I've tried to play with the different settings on the BMV-702 screen, but did not find the right config for now. (the BMV-702 relay with command a bigger relay .. of course !)

So any help is welcome !

Thanks in advance

- Jean-Claude

PS/ my config : MultiPlus II 48/3000/32 - 4 x 100 Ah 12v carbon-lead batteries - 6 x 300 Wc panels - MPPT 150/35 - BMV-702 - Venus GX controller. (see it on VRM : https://vrm.victronenergy.com/installation/25746/share/e8b68720)

BMV Battery MonitorRelay
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What type of relay are you switching the power to the boiler on/off with. I assume you are using the BMV to turn the high power relay on/off.

2 Answers
Mark avatar image
Mark answered ·

For a simple ON/OFF control based on SOC only, you could use either the BMV or the CCGX (or even the Multi using an assistant).

With the BMV you can use the 'Low SOC Relay' function under 'Settings' > 'Relay'. Just note that the BMV Low SOC relay 'set value' is also used as the BMV 'discharge floor', so changing this from your REAL discharge floor will effect the 'time to go' calculation. You also need to activate the BMV 'Invert Relay' toggle to obtain the desired relay behavior (ON/closed circuit >99% SOC & OFF/open circuit <85% SOC).

The issue with the CCGX is that you would also need to invert the logic or the relay operation, but currently there is no option in the CCGX to do this with the 'Generator Start/Stop' functionality (only with the alarm relay).

However, you could work-around this by purchasing a 240V relay with 2 output contacts (NO & NC), then wiring the load to the NC contact to obtain the desired ON/OFF behavior. Just note that this option is a bit risky, as if the relay is not energized for some reason the load will stay on continuously & quickly drain your batteries...

I too have been looking into this for some time & do have a very good idea of how I want to structure the logic - incorporating SOC, Battery Current & Solar Voltage. But I do need a few small tweaks/additions in the CCGX to make it work. When the dust settles with the new MPPT & Venus firmware releases I will create a post with my logic proposal & requested updates to make it work (which are quite basic additions, 80% of it is already there).

Another idea for you - for better control of the water heater, you could also use the relay in your MPPT & wire it in series with the BMV relay so that BOTH need to be ON for it to operate. Using VictronConnect you could select the MPPT relay to function based on 'high solar voltage' (which is configurable when physically connected to the MPPT, but not in the Demo for some reason) - that way the heater will not operate if there is no or little sun. The only thing is that I don't think that your MPPT (being one of the smaller models) has the relay & terminals built in & as you are using the VE.Direct port to connect to the VenusGX you also cant use the VE.Direct adapter cable.

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Thanks Marc for your explanations, I'll take a try ASAP and let you know.

My MPPT 150/35 is not the Bluetooth one, so I'll try to configure it with the display buttons (and a lot of patience ;). No relay either of the 150/35 ..

Paul B avatar image
Paul B answered ·

Hi Jean,

I would suggest that you get a bluetooth dongle to set it up as its MUCH easyer than using the screen, etc


also you could use the MP2 to also do that as well or even better use the VENUS unit and use Gen autostart to turn the relay on and off on the Venus GX that way you dont have to buy the Bluetooth unit

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Thanks Paul for your help.

The Venus GX way would be great .. if I'was able to invert the way the relay works.

I tried with Genset but did not find the way to do it work as I wanted :

- let say SOC is 80 % on morning, the PV charger charge the battery up to 95 % and at that time, I want to start the boiler.

- if there is enough sun, the boiler will not draw the battery so much, so that the battery can even go to 100% with the boiler always ON.

- if not enough PV power, the boiler will draw the battery, until the SOC reaches 85 % .. and then the "system" stops powering the boiler.

- so during the next hours, the battery is able to charge to almost 100 % .. until the sun falls ..

I do not see how I can program that with the Venus GX functions .. but may be, it's possible . who knows ?