BMV702 Config

[andrew9484]

Hi All. 

Please see the graph attached. This is a view of battery state while charging from solar. I have had this more than once.I was informed this is likely to be result of a configuration of the Victron BMW 702? Is there someone who could assist by informing me chat to change to correct the problem?

 

Many Thanks

[Don]

Hi @andrew9484. I had the same issue. The problem you have when you have new batteries. The SOC jumps from 96% to 100% with the BMV default values.

New batteries achieve these values easily and thus the jump from 96 to to 100%. You need to make it as difficult as possible for your batteries to achieve the 100% target. Therefore your tail current need to be as low as possible and for as long as possible before it goes to 100%. 

To fix it, change the following values on your BMV:

1.) Go set your Tail Current (Setting 03) to 0.5%, the lowest value.

2.) Go set Charge Detection Time (Setting 04) to Maximum value of 50 Minutes. 

With these settings your batteries need to maintain a tail current of less than 0.5% for at least 50 minutes before it goes to 100%. If the tail current value is breached during this time, the timer would reset. 

I am not sure if this would make a difference, but set (Setting 06) to you your current charge efficiency. The default is 95% (New Batteries), I set it to my actual charge efficiency which is sitting at 91%.

3.) Go set (Setting 06) to your current charge efficiency. 

Currently my SOC jumps between 98.0-98.5% to 100%. Not perfect, but a lot better.

 

 

[andrew9484]

Hi Don. thank you for your comments. it is appreciated! .One question (sorry if it is a stupid question)...where do i get current charge efficiency from? 

[Don]

@andrew9484. There is no such thing as a stupid question.

Have a look at your BMV outputs. Your Charge Efficiency = Battery Discharge Energy / Battery Charge Energy x 100.

What it comes down to is that if your batteries are fully charged and you discharge them 10 kWh, you need to charge them say 11 kWh (91% charge efficiency) to get them fully charged again. The older your batteries, the more charge you have to put into them to get them fully charged again. You might find that if your batteries are a few years old, you might need to put 15 kWh (66% charge efficiency) back into your batteries to get them fully charged in the above example. 

These are these values to look for:

If you are using ICC, you can find them from the Inverter /Battery Values Tab:

I asked @Manie to add the Charge Efficiency here in the future versions of ICC, that way you can read it daily without doing any calculations.  

If those value are posted on Emoncms, you can always create a Virtual feed to calculate your charge efficiency:

 

[ebrsa]

@Don would you please have a look at my screenshot below. Looks like my Trojan T105 flooded batteries are super efficient. Considering that I cannot equalise charge them as I require 64.8V for that and the Axperts can only manage 58.4V max, I have to rely on the HA02 equalisers that @Chris-R kindly supplied at cost to many of us and hope for the best. So far I have been unable to find a battery charger on the Internet that can supply 64.8V or a bit more. Perhaps my BVM700 needs a reset so more studying to do unless you can point me in the right direction.

[Don]

1 hour ago, ebrsa said:

 

@Don would you please have a look at my screenshot below. Looks like my Trojan T105 flooded batteries are super efficient.

 

Hi @ebrsa. I think here is a problem with your figures. They should be the other way round. Yours show a charge efficiency of 130%. That means whatever energy you put into your batteries, you magically get and additional 30% more energy going into your batteries.

If you use Victron Connect, what values are displayed there?

You have a charge efficiency of 76.9%. You therefore need to charge them an additional 23.1% more than you discharged them to get them back to 100%. How old are your batteries? My batteries are about 5 months old and my Charge Efficiency is currently around 91.4%

[ebrsa]

Hi @Don. Thanks a lot for your reply. My batteries are almost 6 months old. It would be nice if the figures were correct as I would just have magically somehow acquired perpetual energy. I guess the best is to reset everything to zero and start again. Question is where is the data stored, the Pi database perhaps. If you know I would appreciate the answer. Otherwise I suppose the easiest solution would be to just reprogram an SD card with the ICC-Pi image and start from scratch. I have never used Victron Connect. Is that available on the Victron website. Guess I just have some learning to do.

[Don]

1 hour ago, ebrsa said:

Question is where is the data stored

It is stored on the BMV itself. 

 

1 hour ago, ebrsa said:

 I have never used Victron Connect. Is that available on the Victron website.

Yes, you will find it on this download page. (V2.4) - Windows Installer. Install the program on your PC and connect the BMV to your computer, then open Victron Connect. It should automatically pick up your BMV and display the values. Otherwise click on Scan.

 

1 hour ago, ebrsa said:

Otherwise I suppose the easiest solution would be to just reprogram an SD card with the ICC-Pi image and start from scratch.

No, this will not be necessary. First check the outputs on Victron Connect. It gives you a lot of useful information regarding your batteries. I would not reset the values, unless they are clearly incorrect. 

[Mark]

A quick question:  What size battery bank do you have and has the BMV been set to this size?

[ebrsa]

Thanks a lot @Don. I will give it a go tomorrow and report back. Your help is much appreciated.

[Don]

It is a pleasure.

[ebrsa]

Hi @Mark,  My battery bank is 450AH and the BMV has been set up accordingly. For the other settings I have used those of @Don as a guide. I suppose it is about time to check the SG of all 48 cells to really ascertain that all is well. Wish I had a digital hydrometer but at about $4000 they are way above where I am prepared to go.

[___]

There is also a VictronConnect for Android (and I think for iOS), but that requires either a USB-OTG connector (a little dongle that allows the Android device to be a USB host) or the bluetooth dongle, which is 50 Euro ex vat. The BT dongle is very very nice to have if you're an installer.

Something I might want to look into in the future is direct interfacing from the Rpi-3 to the bluetooth enabled vedirect devices, but that is far far in the future.

[ebrsa]

I just came across a Youtube video regarding setting up the BMV700/702. For solar installations they recommend setting the charged voltage at slightly below the absorption voltage which I guess is what Trojan calls the bulk charging voltage. Since the Axpert can only reach 54.8V, I guess a setting of 58.6V would be a good starting point. Tail current suggested is 4% which would be fine at 54.6V I would assume. I tested the SG on one of my cells today and it was 1.265 instead of 1.277 even though the BMV showed 100% charge. Would appreciate what other members who participated in the discussion thinks about this. It appears to be a Victron video although I am not too sure about the background scenery.

Here is the link.

 

[Don]

2 hours ago, ebrsa said:

For solar installations they recommend setting the charged voltage at slightly below the absorption voltage which I guess is what Trojan calls the bulk charging voltage. Since the Axpert can only reach 54.8V, I guess a setting of 58.6V would be a good starting point.

@ebrsa, no, the charged voltage they are referring to is the float voltage which in your case is 54.0 volts. They recommend setting the charged voltage setting to slightly below this value. Say 53.7 Volt.

 

2 hours ago, ebrsa said:

Tail current suggested is 4% which would be fine at 54.6V I would assume.

Your battery bank capacity = 450 Ah x 4% = 18 amps is your tail current at 4%.

Therefore, the BMV would deem your batteries to be fully charged once the current going into your batteries are less than 18 amps and at a voltage constantly above the set value of 53.7 volts for a period of 3 minutes. That is if you set your BMV as per the video.

As I posted earlier above, I still had the premature jump from 96% to 100% when I did the setup according to the video. That is why I reduced the tail current on my 500 Ah batteries from 4% to the lowest value of 0.5% which is 2.5 amps and I extended the detection time from 3 to 50 minutes. The float voltage on my batteries are 54.8 volts. I also have the charged voltage only reduced by 0.1 volt. I did not reduce it with 0.3 volt as in the video. Once my batteries are full, they float on exactly 54.8 volts. Therefore, in my case, my batteries need to charge at less than 2.5 amps for a period of 50 minutes and at a voltage of 54.8 volts, before my batteries are deemed to be fully charged and goes to 100% SOC.

[___]

40 minutes ago, Don said:

tail current at 4%.

On my AGMs, 4% would be too high. My tail current comes down to below 1% (1.5 ampere, 200Ah batteries) when it is properly full. With the Trojans I'd expect the tail current to be a little higher. If you log charge current, you should be able to see it on chart somewhere, eg the attached one showing only 24 watts towards evening (on a 24V bank, so about 1 ampere).

[Don]

When I go from absorb to float, I am sitting on 2.5 amps. 

[Czauto]

Off topic....anybody knows where i can find 2 pole 230v contactors and transfer switches tomorrow? Any electrical wholesalers that might be open on public holidays? I'm in Gordon's bay and am planning to get going back to beaufort west by latest 11am

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[___]

1 hour ago, Czauto said:

I'm in Gordon's bay and am planning to get going back to beaufort west by latest 11am

Frontline (Victoria Road, Somerset West) and AC/DC is probably going to be closed, but you can try them to be sure, and I'm sure Voltex and Cronlec will be too (same general area, Gantz Plaza in Strand) but once again, close enough that you might try anyway. But Builders Warehouse should be open and they have a small selection of breakers there.

Interestingly I'm going the OTHER way tomorrow, from Prince Albert back that way.

Edit: I doubt Builders will have a transfer/changeover switch. For that, Frontline remains the best option.

[Czauto]

Thanks Plonks i'll go see if they might be open before i go otherwise i'll just have to order online.

Sent from my S60 using Tapatalk

[___]

If I was home I could offer you the rubbish AC/DC changeover I still have in a box there... might be better than nothing? I won't be home until late afternoon though, and my neighbour won't know where to find it in my garage :-)

[Czauto]

Lol. Not that much of a hurry. I'm experenting a bit to need some switches, relays, etc. I'll see if i find anyone open and if not just order online on tuesday. Wind is driving me crazy this side. No wonder you guys don't bother with wind turbines here. Thet would disintegrate within the first week.....

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[___]

9 minutes ago, Czauto said:

No wonder you guys don't bother with wind turbines here. Thet would disintegrate within the first week.....

Oddly enough... we don't have enough wind. What you need for a turbine isn't strong wind, but a consistent fresh breeze... like what they've got up on the west coast :-)

[Czauto]

Exactly. The gusts here would rip anything apart.

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[ebrsa]

Hi @Don, On the video that I posted, the  presenter recommends that for solar installations, the charged voltage on the BMV be set slightly below the absorption voltage just after recommending the normal settings. The BMV manual also recommends slightly below the float voltage. Trojan does not recommend an absorption voltage, only a bulk charging voltage. Yesterday I turned everything off including the BMV and then restarted. I have been having a problem that with your settings, the Axperts stop charging when the batteries reach about 96%. Turning on grid charging did not help either. As I said before, I measured the SG of just one cell and it was at 1.265 instead of 1.277 for a fully charged battery.

I agree that 4% tail current is way too high in my case and probably for many. To the best of my knowledge Victron started as inverters for yachts where the chargers used to be generators before solar panels became affordable. That may explain the charged voltage being set to marginally below the float voltage as the generators would produce a constant current.

Today I will set the BMV charged voltage at 58.6V which is 0.2V below the maximum the Axpert can produce and the tail current at 1% and see what happens as it is a sunny day. Since I will spend most of the day in front of the computer, I will be able to monitor MQTT, which runs on my CCTV Windows 10 computer and see what happens. 

Thanks for all the help everyone. I am just trying to optimise my system to see how long the batteries will last given that the Axpert bulk charge voltage is 0.88V lower than the Trojan specs. It may also prove @Chris Hobson 's view, which sounds logical to me, that current is the important factor to determine whether batteries are fully charged.