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BMV & Relay to disconnect batteries


viceroy
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So this weekend we had a bit of a scare when the house did not switch over to grid at the scheduled time, and by the time we realised, the battery bank had discharged to nearly 50%.

Now the BMV alarm would have gone off, but if we had not been home or by chance didn't hear the alarm, it could have been a disaster.

Now I have no experience with relays, but I now need to install one to disconnect the bank if the BMV triggers it.

Please advise what I need in order to do this.

Thanks.

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Hi Viceroy

It is easier to disconnect the load than the batteries. The load will be 230 AC and contactors are readily available. A DC contactor is more expensive. The disadvantage is that the system does not totally shut down. Everything that is downstream of the contactor is shut down but the inverter remains powered. The inverter draws 50W. So in you case 100W. This has the advantage that if there still is solar available you will continue to charge your batteries. The Axpert manual warns not to disconnect under load so I am not sure what will happen if you suddenly remove battery as a source.

 

A lengthy discussion was precipitated by TTT but it appears the relay on the BMV can handle 230AC.

Addition: There is a good video on this forum demonstrating the difference between disconnecting loads - AC versus DC. After watching that you will probably plum for a AC contactor.

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40 minutes ago, Chris Hobson said:

A lengthy discussion was precipitated by TTT but it appears the relay on the BMV can handle 230AC.

Been doing it since 2011. ;)

BMV relay is triggered based on i.e SOC of 90% and released again at say 95% SOC. You can set it any SOC you want.

Left is inverter and right is Eskom. BMV triggers relay, and switches power from inverter to Eskom and back.

One irritation in my system is that at the time I did it, no relay timer could go below 1 second. So I have to use a UPS to carry the 1 sec break.

Why? You cannot swap over instantaneously from inverter to Eskom. UPS'es are doing it as fast as 8ms.

 

Changeover (6).jpg

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50 minutes ago, The Terrible Triplett said:

no relay timer could go below 1 second

So, like, don't use a relay timer. :-)

Then again, I have to tell you, I don't know if there are technicalities involved with swapping one source for another, such as maybe syncing up the waveforms before you do the swap. A slow UPS (and a Victron inverter) will swap between 16ms and 20ms, so that's between one complete AC cycle. With a space that big, perhaps you can just turn off the one and turn on the other even if they are out of sync (how does an induction motor feel about that?). With 8ms, the only way you're doing that is by syncing frequencies. I don't know this for sure, just call it a gut feeling. Perhaps the more clever power electronics people here will care to comment.

On my Multiplus, I can certainly report that the delay between giving the signal to switch, and it actually switching, is not constant. It takes a few seconds, but sometimes it is faster and sometimes it is slower. You can also audibly hear the hum change a bit as it prepares to switch. I don't know this for a fact, but I half suspect it syncs up the waveforms.

In other words, without significantly more intricate electronics, 1 second is perhaps a very safe compromise :-)

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I spoke to a few UPS experts at the time. There are technicalities involved. You cannot swap instantaneous, it will work, but things like caps will degrade over time, and go long before their due date.

Newer UPS'es, the more expensive ones, are 8ms. Victron is just playing it very save with all types of power source they need to cater for in the marine industry.

What annoys me at 20ms changeover is that my 42" Samsung TV, which has a very small PSU, switches off when UPS swaps over that slow. Computers, laptops etc have no issue at all at 20ms. Therein the cheaper option at 20ms.

And also therein my infatuation with online UPS'es, after having tried many makes of UPS'es back in 2008/2009.

Why solar and UPS inverter manufacturers do not share on Github, true Linux style, I have no idea. Take a good online UPS, the real ones, operate 24/7/365 off inverter. You can even do a bypass on them if you need to hotswap the battery pack, with no load interruption.

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O, lest I forget, had a lovely online 3000VA UPS that once saved all my devices like fridge, computers, routers, phones system ... all that stuff you want powered during Eskom failures.

Electrician somehow connected neutral and earth in the main DB board. Not once, but 3 times before he saw his mistake.

The resultant DB board tripping each time was epic! Blue lightning like light came from the DB board.

Never knew there was a problem until about 10 minutes later when the UPS died due to flat batteries. Charger circuitry was fried.

That day I go a LOT of respect for online UPS'es.

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48 minutes ago, The Terrible Triplett said:

What annoys me at 20ms changeover is that my 42" Samsung TV, which has a very small PSU, switches off when UPS swaps over that slow. Computers, laptops etc have no issue at all at 20ms. Therein the cheaper option at 20ms.

I have absolutely no problem at all. Sometimes (very rarely) my Yamaha AV receiver will dip (it has a delayed speaker-on protection relay, and you'll hear it click in and out), but that is it. We literally don't notice when the switch happens. And this is with an ancient (in inverter years) Multiplus Compact :-)

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1 hour ago, plonkster said:

I have absolutely no problem at all.

Good for you.

You obviously have missed the family sitting there, tension running high, and then, right at the point where the movies hits a really high note (guys gets shot, lady gets kissed) ... whoeps, TV is off - and remote is nowhere to be found to switch it back on. :P

SWAMBO and Co. does not tolerate this too much. Worse than a cold shower.

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2 minutes ago, The Terrible Triplett said:

SWAMBO and Co. does not tolerate this too much. Worse than a cold shower.

Well, I just downgraded from Monochoice premium to a smaller package when I realised we practically watch only two or three channels, the majority of these involve Tom and Jerry and/or Mickey Mouse. So hulle moet maar deal :-)

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Well, I just downgraded from Monochoice premium to a smaller package when I realised we practically watch only two or three channels, the majority of these involve Tom and Jerry and/or Mickey Mouse. So hulle moet maar deal :-)

Get yourself a Roku. Best investment I have ever made. Well except for PV.

Sticking it to another ZA "man"...

Sent from my SM-N900 using Tapatalk

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While we're a bit off-topic, I had an APC 3000VA UPS with 4 x 102Ah batteries before I went the solar & infini route - both wired into the DB. Both APC and infini switches fast enough to never switch off or badly dip anything including any of my Samsung TVs, the Yamaha amp or the MEDE8ER - a transition is only sometimes noticed on certain lights e.g. fluorescent tubes, but not on CFLs and LEDs - the filter caps in their PSUs prevent a visible dip.

Maybe this bit of useless information might help someone in future.  :)

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12 minutes ago, superdiy said:

Maybe this bit of useless information might help someone in future.  :)

Most definitely.

Nothing so disappointing than when you just got this cool new inverter, only to find out 20ms is too slow for some of your devices.

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Get yourself a Roku. Best investment I have ever made. Well except for PV.

Sticking it to another ZA "man"...

Sent from my SM-N900 using Tapatalk

The Swambo watches Binneland. Any IPTV stuff us extra, it won't serve as a replacement :-)

Sent from my GT-I9195 using Tapatalk

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Whoah did this train derail or what:P

I really don't mind what I disconnect, be it AC or DC, whichever is better I will do, I just want a way to protect my batteries from discharging in the case that the SOC gets down to 50% (55% to provide a little safety net in the case of disconnecting AC, because as you say the Inverters will remain powered and drawing +-100W)

So looking at TTT's pic, there seem to be two types of devices installed. Could you clarify which is what, and what I'll need to just cut the power. I'm not interested in swapping over loads from one source to another. If the batteries get to 50% the load must go.

 

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Jip, we are a bunch of highly motivated individuals who all have bits and pieces to add, covering a very broad spectrum of thoughts. :D

Due to inverters switching off in your case, in my case swapping loads with a resultant little matter of a break in current, means we have struggled with related problems ourselves.

My plan was born from that fact that I never want to go below X% SOC unless I make it so.
Therefor the batteries are protected in my absence from the larger loads.
So that the off-grid loads can run for days with no problem

.

48 minutes ago, viceroy said:

So looking at TTT's pic, there seem to be two types of devices installed. Could you clarify which is what, and what I'll need to just cut the power. I'm not interested in swapping over loads from one source to another. If the batteries get to 50% the load must go.

Good point. +-100w load for inverters that just sit there is not cool. Huge waste on batts.

Can your inverters be remotely switched off, do they have relays built in for that?

Do they have on/off switches? Can they be rewired so that the BMV relay switches the inverters off?

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3 hours ago, The Terrible Triplett said:

Can your inverters be remotely switched off, do they have relays built in for that?

You can switch Victron inverters to AES mode via the comms interface, which will halve the power consumption. Does the Axpert have a similar low-power mode perhaps?

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The Axperts do have a low power mode but not when in parallel mode it seems.

They have power switches, which could be plugged into by the BMV...not a bad idea. I think I'll take the multimeter to them to see how exactly they function. The one thing I've found is that if the batteries are connected to the inverters, the inverters turn on (not fully on, but on enough to change settings etc). The only way I've found to properly turn them off is to disconnect the batteries.

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  • 4 weeks later...

Viceroy, I have used a 220v AC din rail contactor with a 12v coil and this is controller by the SOC on my BMV-702

Once SOC reaches 50% the relay switches over and drops the contactor out so the load is disconnected 

Then when the SOC reaches 70% the relay turns the contactor back on

Maybe not the best way to do it but it worked well for me

I am also based in Broadacres so if you are keen to have a look at my little setup feel free

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