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Choady avatar image

Using BMV-712 relay to trigger the MPPT 100/20 VE Direct Rx Pin to disconnect a load

Hi there,

I currently have a 4WD setup with a BMV-712, 130AH AGM rear battery and 1500W inverter. I have standard 4wd type accessories like a 12v fridge, water pump, lights, 12V outlet for phone charging etc connected through individually fused circuits from a central 6 port fuse block. The total load of everything if everything runs at once (except inverter) is around 13A.

The rear battery currently gets charged from the engine alternator through an electronic voltage sensitive relay connecting the two batteries. At the moment all loads are routed through the BMV 712 Shunt, including the inverter (which incidentally only ever gets used for short periods of time for the sole purpose of making toast!)

I am considering fitting a Smartsolar MPPT 100/20 (or 75/15) so I can go off grid for longer without running the engine and I really like the concept of using the load control functionality to protect my rear battery. I would fit all but the inverter to the load connections on the MPPT (through the single +ve cable that currently runs to the fuse block), and the inverter would remain connected direct to the battery. I turn the inverter on and off manually through its own switch.

The problem is when the battery is under load from the inverter (900W to run the toaster through short length 2B&S cables) the voltage often drops below all the MPPT pre set voltage triggers and would therefore to disconnect the load to my fuse block. When the battery is at a low SOC to start with, the load may never actually be reconnected as it may not recover to above the 'reconnect' trigger voltage.

To avoid this I would like to use the BMV-712 SOC setting to trigger the load disconnect signal, rather than the MPPT inbuilt voltage settings.

I have read several posts and understand that that I can easily use the BMV to control a separate high current relay that would disconnect the battery for SOC triggers, but this seems to defeat the purpose of having the 100/20 and its excellent load connection & control abilities.

From my research I understand that the MPPT VE direct port has 4 pins (1 GND, 2 RX, 3 TX, 4 PWR)

I also understand that the VE direct RX port on the MPPT can be set through Victron connect software to act as either a remote on/off (for the MPPT itself), or as a load output on/off (including an inverted mode) to the load connections on the MPPT.

Where I fall down is working out how to actually achieve this.

I have read that the VE direct Rx port is 5V, and I understand the BMV relay has a COM, NC, & NO connections that can also be inverted through the Victron connect software.

So was wondering if I connect a 5V source to the BMV-712 relay COM terminal, can I then connect the BMV relay NO output terminal (which would also be 5V) direct to the MPPT VE Direct Rx port ? I could configure the MPPT such that when the BMV low SOC triggers, voltage at RX on MPPT hits zero and then triggers the load output to off.

Can I do this with my own wiring (ie BMV NO to MPPT RX, plus an earth cable) at the MPPT - or do I have to purchase the VE direct non isolating cable to make this work ?

Or perhaps my understanding from all my research is flawed and this is not actually achievable ?

I know I could just fit the high current relay, just space is at a premium and I am keen to explore the realms of possibility.


BMV Battery Monitor
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4 Answers
craigp avatar image
craigp answered ·

AS a note, I can confirm the 100/20 MPPT goes into reset everytime my fridge turns on when connected to the 'output' of MPPT. My fridge draws 3.6amps when running. Inrush is obviously way above that.

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ripper answered ·

If it is a compressor fridge, be prepared that the load out on the Mppt might consider a start of that compressor a short and hence disconnect. That seems to also depend on the software version and is an ongoing problem. I recommend testing it on low battery voltage with everything switched on for highest current draw or consider using the load to trigger a big relay, SSR or victron battery protect. The bp will be able to supply the inverter too if that makes for ease of wiring.

If you are using an inefficient peltier element or gas/12/230V fridge then you are good. No high start up current here.

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Great point about the compressor. Maybe I will just get the 20A MPPT which will definitely cover off on it.

Fridge max draw is 4.5A, water pump 6A and lights a couple A. Never watched to see what the spike of the fridge (if any) is. Its a good quality fridge so I would hope it is minimal.

Looks like a BP is the way to go for guaranteed performance for my configuration, but I don't think I will splurge on it and I only have just enough room to squeeze in the MPPT.

I was hoping to use the features within the MPPT load control, but I might still have a play around with it though using your suggestions. I have an old AGM battery that chokes when you apply a load to it. Might use that as a test case !

If it doesn't work, then no big deal. I haven't had load disconnect functionality so far and managed to get away with it !

The 20 is better but I had trouble with it too. Compressors are notorious for the start up current. Inrush current of a usb port might be enough to upset it too.. It monitors the current flank. If its too steep, it doesnt like it it and cuts the power. But just try, you dont break anything on the Victron side.

Choady avatar image
Choady answered ·

Thanks @WKirby for the response. But if I set it to Remote on/off, won't the entire unit switch off ? I still want charging to occur, just want the load output to turn off.

The manual describes pin numbering from left to right on the 100/XX models, and from right to left on the 75/XX models.

The pin numbers you referred to are different to the pin number references I found in the VE direct Protcol manual. Wouldn't I connect to pin 4 (pwr) from MPPT to COM on BMV, then NO (or NC) depending on how I set it back to pin 2 (RX) on MPPT?


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WKirby avatar image
WKirby answered ·

This is easy to to.
Connect BMV relay COM to pin 1 on the MPPT, connect BMV relay NO (or NC) to pin 2 on the MPPT.

Use VictronConnect to configure the Rx pin to "Remote On / Off".

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