Can a Victron BMV-712 monitor be used to turn on/off an inverter as a low voltage protect?

[Jsh700r]

Hello all,

Can the Victron BMV-712's be used as a low voltage disconnect by using its relay function to turn on/off an inverter?

I plan on upgrading my 48VDC system to lifePO4 with a 16 cell string of 280AH cells, 100A BMS, Victron 150/35 Charge Controller and a BMV-712 monitor with temp sensor. I haven't figured out the best way to operate a low voltage disconnect but I am thinking the BMV-712 has a relay function that could turn on and off my Cotek 2000W inverter (SK2000-148) that has a three position toggle switch (On, Off and Remote). Would that work and be a good solution for a low voltage disconnect?

Any advise would be much appreciated,

Thank you,

JSH700R

 

[Zil]

Yes the BMV-712 has a programmable relay that could be used to be N/O or N/C on low voltage. The question is what do you control with the light weight relay. If you've access to the inverter switch it could be that. Is it recommended to switch the inverter off with that switch while under load? My inverter instruction is switch off load first. I'm using the BMV-700 connecting to a latching relay to control the loads on the inverter and the RV thermostat before a HD LVD relay kicks off. There are somethings that need to clear before pulling the plug.

 

[Jsh700r]

SolarKimmy – I agree and originally I planned on using the Victron 48V LV disconnect. Recently though, Will said Victron made an update saying not to use their LV disconnect with inverters. You can see this under comments on his youtube video for:

Complete LiFePO4 Solar Battery System Design (Updated Blueprint in Description)

Zil – Good points, though now I am more unsure of the best direction. I was speculating that turning off input power to the inverter, under load, would hurt the inverter and that is why Victron said not to use their LV disconnect directly connected to an inverter.

 

[Traveler0612]

My Cotek SD2500 does indeed have programmable disconnect. It has a built-in ATS and AC circuit breaker as well.

 

[Jsh700r]

SolarKimmy - I may eventually upgrade to a Victron. For now I think I will continue to use my Cotek until it eventually fails. I think for my application I will configure the BMV-712 to trigger turning off the inverter in low voltage situations. If I can’t get that to work I may purchase the Victron disconnect.

Traveler0612 - I think your SD2500 has more features than my SK2000-148, mine doesn’t have an ATS or an AC circuit breaker. I don’t have any complaints about the SK2000-148, been running it at our cabin for 4 years without an issue. When it eventually dies I will look at something larger and more configurable.

 

[HaldorEE]

You could drive a high currect contactor relay with the output from the BVM712. That is how the Electrodacus BMS handles load disconnect.

 

[JoeHam]

Voltage (V DC): 9.5-32 for the BEP

 

[Zil]

I have a Samlex BG-200. The instruction for my inverter says ok to use this LVD. However I will switch off the load on the inverter before this LVD kicks off.

 

[Yurtdweller]

Hey folks. I'm running 12v lifepo4 with a bmv-700. I want to use the bmv to trigger a switch to a 12v water heater at about 14v. Basically create a dump load switch. I get solar wiring, but real electronic design is new for me. If I understand relays correctly, it is a micro-voltage switch that can somehow trigger a higher voltage switch. Is that correct? Can y'all point me to some info?

 

[mikefitz]

The BMV 700 built in relay has low power contacts that could be used to control a more powerful relay, that would in turn switch power the heater. You could use either voltage levels or state of charge as the trigger value. You may have to experiment with values to obtain a working differential between switch on and switch off values. ( every time you switch on the heater, depending on available solar power and heater power required, the system voltage and state of charge will possibly drop below the 'on' settings. Thus the 'turn off' settings would need be to be lower by a margin to prevent continual switch on and off.

Typical 12 v water heaters have two elements that can be used in parallel for maximum power, individually for half power or in series for 1/4 power. In my setup with the elements in parallel, its a 600 watt heater, so around 50 amps. This is just about practical with a high current (80) amp automotive power relay carrying the switching current, with the BMV powering the relay coil.

You also need some temperature sensing and control circuit to limit the maximum temperature in water. This needs to be 'in circuit' with the power relay control, to switch off power when the required temperature is reached.

You are saying 'dump load', if this is the only requirement, rather than heating water for use, then there is no need to 'dump energy' as the solar controller takes care of things.

Mike

 

[Yurtdweller]

Thank you. I realize I do not require a dump load with solar. I will be doing this for usable hot water, for washing, as a thermal storage, and may possibly run the water through a floor mat, as my feet can get cold in my uninsulated yurt in winter.
I intend to use a product from Missouri wind and solar(mwands.com). They have a dc element with a built-in adjustable thermostat. It runs@200w.
Part of my incentive is this: I just got a lifepo4 with bms. I cannot set my cc low enough, even though I thought I could, to avoid the bms triggering high-voltage cutoff Therefore, hvco cycles constantly all day long as soon as my batt hits 100%, then drops below threshhold.
I believe this will result in unecessary wear and tear on the bms?
It is my intention to use the water heater to help reduce the number of bms cycles by keeping the system voltage lower. I will set the bmv-700 voltage threshhold below the bms ovdc. At least until I get a new cc, and I probably will not get a new cc until I switch to 24v, and that may take some time.

So, to my original question, I will need this automotive relay rated at 200w+output? And I should look to my bmv documentation to find the triggering power? This is where my knowledge is lacking.

I suppose I could also, somehow, use the bmv-700 to trigger an hvdc that is more healthy for my battery? Thus far the bmv has simply been used as a monitor. Thanks for all the input.

 

[Yurtdweller]

 

[DeeK]

You could drive a high currect contactor relay with the output from the BVM712. That is how the Electrodacus BMS handles load disconnect.

FWIW, I tried driving a standard 12V contactor with the 712 and could not get it to operate. The contactor requires just under 1A when it's engaged (I measured it at 0.95A), which the 712 should be able to drive since it's barely within its stated 1A spec. But, it may be that the surge current required to initally activate the contactor is too much for the 712. So, I need to find some intermediate switch that the 712 is able to drive and then that would connect a circuit to engage the contactor. Any advice would be appreciated.

 

[mikefitz]

Any advice would be appreciated.

Use a MOSFET as a switch activated by the MBV,

https://www.electronics-tutorials.ws/transistor/tran_7.html

Mike

 

[DeeK]

Use a MOSFET as a switch activated by the MBV,

https://www.electronics-tutorials.ws/transistor/tran_7.html

Mike

OMG Mike, thanks for sending that link but it was way over my head! I wouldn't know where to start to buy the right one.

 

[mikefitz]

but it was way over my head

OK. use the BMV tiny relay to switch a standard automotive relay. Use the switch contacts on the automotive relay to drive the contractor.

Mike

 

[DeeK]

Thanks Mike, I had already ordered these from Amazon when I saw you had posted: https://www.amazon.com/gp/product/B072QXDZRD/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
Seems like they should work and the cost is low if it doesn't pan out. Says they use 154 mA which should be low enough that the Victron can drive them. What do you think?
I also found a few old car relays in my garage that I thought might work but only one still seemed to work and it was a Normal Closed and I think I need Normal open for this.

 

[HaldorEE]

FWIW, I tried driving a standard 12V contactor with the 712 and could not get it to operate. The contactor requires just under 1A when it's engaged (I measured it at 0.95A), which the 712 should be able to drive since it's barely within its stated 1A spec. But, it may be that the surge current required to initally activate the contactor is too much for the 712. So, I need to find some intermediate switch that the 712 is able to drive and then that would connect a circuit to engage the contactor. Any advice would be appreciated.

I would use one of these. Switch 30VDC at 10A. Only takes 5mA input current to switch it. And the buffer circuit is optocoupled (galvanic isolation). Can't beat the price either.

https://www.amazon.com/HiLetgo-Channel-Isolation-Support-Trigger/dp/B00LW15D1M/ref=asc_df_B00LW15D1M/?tag=hyprod-20&linkCode=df0&hvadid=167156423255&hvpos=&hvnetw=g&hvrand=9929169542500190365&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9029990&hvtargid=pla-343992463804&psc=1

Important caution: Be sure to add a reverse biased diode across the coil of the bigger relay you are controlling. This is to avoid creating a huge voltage spike on your 12V bus every time the relay opens.

This article talks about it.

http://www.idc-online.com/technical_references/pdfs/electronic_engineering/Basic_Relays.pdf

Any switching diode will work. A 1N4001 is fine. $6 per hundred.

Amazon.com: RLECS 100-Pack 1N4001 IN4001 Diode Standard Recovery Power Rectifier 1A 50V DO-41 DIY Diodes: Toys & Games

Amazon.com: RLECS 100-Pack 1N4001 IN4001 Diode Standard Recovery Power Rectifier 1A 50V DO-41 DIY Diodes: Toys & Games

www.amazon.com

 

[DeeK]

I would use one of these. Switch 30VDC at 10A. Only takes 5mA input current to switch it. And the buffer circuit is optocoupled (galvanic isolation). Can't beat the price either.

https://www.amazon.com/HiLetgo-Channel-Isolation-Support-Trigger/dp/B00LW15D1M/ref=asc_df_B00LW15D1M/?tag=hyprod-20&linkCode=df0&hvadid=167156423255&hvpos=&hvnetw=g&hvrand=9929169542500190365&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9029990&hvtargid=pla-343992463804&psc=1

Important caution: Be sure to add a reverse biased diode across the coil of the bigger relay you are controlling. This is to avoid creating a huge voltage spike on your 12V bus every time the relay opens.

This article talks about it.

http://www.idc-online.com/technical_references/pdfs/electronic_engineering/Basic_Relays.pdf

Any switching diode will work. A 1N4001 is fine. $6 per hundred.

Amazon.com: RLECS 100-Pack 1N4001 IN4001 Diode Standard Recovery Power Rectifier 1A 50V DO-41 DIY Diodes: Toys & Games

Amazon.com: RLECS 100-Pack 1N4001 IN4001 Diode Standard Recovery Power Rectifier 1A 50V DO-41 DIY Diodes: Toys & Games

www.amazon.com

Thanks for that, unfortunately I already ordered a relay (4 actually) that I am hopeful will do the job. The contactor has two sets of tab connections, one of which is diode protected so I will use that set. It's nice to know of the option you provided if the relays I ordered don't work.

 

[HaldorEE]

Thanks for that, unfortunately I already ordered a relay (4 actually) that I am hopeful will do the job. The contactor has two sets of tab connections, one of which is diode protected so I will use that set. It's nice to know of the option you provided if the relays I ordered don't work

For $5, with Amazon Prime free shipping, I wouldn't mess around. You could be damaging the relay output of your BMV-712 (burning the relay contacts).

Definitely make sure you have diodes across any relay coils or you could cause other electronic failures in your van. Understand that interrupting current flowing though a coil is how cars turn 12 volt into 40,000 voltage for the spark plugs.