question

tjorg avatar image

Using the BMV-700 relay

I have the BMV-700 installed in a solar powered camper van with AGM batteries. My understanding is that I can set the BMV relay to disconnect power so that I do not overdraw the batteries. I have set the floor parameters in the settings and done some tests. I set the SOC% floor and also the battery voltage floor and ran a bunch of power through to draw the batteries down. The BMV shows that the battery percentage and voltage are both lower than the floor that I set but it did not alert or use the relay to shut power down. I drew it all the way down to 11.4V and it read that as 35% remaining. That was as much as I felt comfortable draining them.


What am I doing wrong? Or am I misunderstanding the capabilities of this unit?


Thanks!

BMV Battery MonitorRelay
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1 Answer
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ripper answered ·

The manual and the quick hint in the app just describes it wrong. Took me hours to finally get there.

First of all the BMV does not disconnect the power itself in case you misunderstood. It needs extra devices such as a relay. The build in relay can be used to drive for example an external relay.


To use the internal relay you would have to select standard under relay preferences, probably inverse the relay, adjust the bell for soc for the low limit when it should shut off and adjust the bell with a bar through when usage is okay again. Unlike stated int the manual, the relay is not on while being between the two values, rather it is hysterisis. Means on till its hitting lower limit, then stays off till higher limit is reached, it then goes back on. For some reason if you put bell with bar to a hundred, it shows zero and does something stupid. You can do 99% max. Also make sure to disable high and low voltage underneath unless you want it. It stays on if you played around up top and selected charger mode or whatever and messes with your selection.


The alarm buzzer is indipendant and you can play around in the same mannor.

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Thanks for the help! I did misunderstand what this thing can do then. The product description says "Programmable relay, to turn off non critical loads". So what it actually does is send a signal to a separate external relay (at the SOC% or low voltage that I program)? Would this be something like the Victron Battery Protect? What relay setup did you end of using for your system?


I did get the alarm to work at least.


Thanks!

Yes and no: It does not send a signal as such it just has a tiny relay inside that can be manipulated (adjusted V and SOC by you) to shift its state and thereby not only but also sending a signal if wiring it up like so. The battery protect does two things. It autonomously shuts itself off by adjustable voltage but obviously it does not know SOC and it can be made to shut off/on via the remote in and out (glorified relay) from external input, for example the BMV. For that you just run a wire in a circle remote out, BMV in and out and remote in then yes it would work for you.


If you have the Victron App, have a look at the demos. There you can play with the adjustments to get a better picture.

I use the relay just to signal a high SOC for something else to happen, my system is not secured in that regard as I hardly go down that far or am very aware at least

@ripper

It appears from your answers, you have become very familiar with the BMV relay. I have a pretty good understanding of the relay except for the actual contacts. Are both contacts completely floating, or is one contact internally connected to power or ground ? I would expect they are completely isolated so that I could connect Power OR Ground to one contact as needed, and use the remaining contact to control a relay.

Any help will be greatly appreciated !

They are floating, you can manipulate in the the software whether it is normally open or closed by selecting "inverted" on the relay page. The relay can switch surprisingly high voltages but only low amperage. Check the manual for details.

@ripper

Thank you for your reply and answer !

After looking at the relay connections at the rear of the BMV-712 (COM, NC, NO), I see this as an SPDT switch ?

What I would like to do is connect +12v to the "COM" terminal, and see +12v at the "NC" connection until a "fault" condition flips the relay (for example, Low Battery Voltage). At that point, I would expect +12v to be present at the "NO" contact, which would be identical to the operation of an SPDT switch.

I don't have access to the BMV at this time, or I would just try it out, but I am guessing that whatever is connected to "COM" will connect to "NC" until the relay is triggered by the selected "fault" condition.

The Victron documentation on the relay operation is somewhat vague and confusing, especially in regard to the "Invert" mode, so.I would rather avoid using the Invert mode if the relay operation is as straightforward as an SPDT switch.

Again, thanks for your help, and any further information or comments you would like to add would be greatly appreciated.


I can only guess at this point I actually have the 702 und there are only two connectors. What you wrote makes sense and I would assume the same. Thats easily tested though once you have the unit. It is able to do what you want in any case...

@ripper

Thanks again for your reply !

Yes, I will need to just try it to be sure. The BMV-712 documentation describes the relay as "bistable", with "coil current" listed at 1mA in either direction (NO or NC position). So, that implies that the relay is always engaged, and that the COM terminal would always be connected to one contact or the other ?

Thanks again for your replies !

hello friend, i am building a system using 3 quattro inverter, 4 smart solar charge controller and CCGX device. when i purchase these devices and all their accessories. the engineer in the store told me that i must buy the bmv-700 device to run the system without issues. So i want to know is he correct? or can the system run normally without the bmv-700

I would say BMV 712 SMART