Synchronizing the BMV - Another Victron question

Deb55 · August 2020

Since we just had rooftop solar and a Victron battery monitor installed in our 400, I've been doing a little testing. For 6 days, I've left my trailer unplugged from shore power and with the battery on. I've had the refrigerator on at the lowest setting, and I've off and on been inside just to hang out. That involved the fan, occasional lights, and a little bit of radio use. When I check the battery monitor, it shows 73% charged, but it also says around 12.88 volts with the sun shining and the fridge cycled off. Isn't 12.88 volts closer to the 90% charged range? What am I not understanding?

(Title edited for search purposes. Moderator)

pthomas745 · August 2020

The sun is charging your battery through the panel, which is what the 12.88 is showing you. The fridge is taking power from your battery. You really cannot get the proper state of charge of a battery while it is in this charging/discharging status. A battery monitor will always read low when it is being discharged (by your fridge, in this case).

Turn off your fridge. Watch the volts from the solar panel move higher. And watch your battery monitor almost immediately move higher.

Here is a shot of my Victron. On the left, the fan is running, and the battery is showing 12.09! But, that is not a "rested" battery. It is showing low because the battery is in use. On the right, the fan is off, and the battery has been "resting" for 10 minutes, and is showing a much more accurate reading of 12.46.

PS: You say "fridge cycled off". Does that mean the fridge is turned off? And, is your monitor telling you if you are in "Bulk" charge status? Where is the sun hitting your panel today?

Image: https://us.v-cdn.net/5021717/uploads/editor/jo/nyyn2w8xk9sx.jpg

Deb55 · August 2020

@pthomas745 this I understand. What I don't understand is why the state of charge shows 73% while the voltage reads 12.88 (with the fridge cycled off, and about 12.74 while it's running). Shouldn't the state of charge be higher than 73% if I'm showing 12.88 volts?

Denny16 · August 2020

Did you set up the Victron battery monitor for your specific battery? If not, the monitor may be using 13.x VDC as a fully charged battery set point.
cheers

pthomas745 · August 2020

When a battery is being charged, it can read above 13 for voltage, but that might not tell you much. What you are replacing are the amp hours of the battery, not the voltage. Does your monitor show the "Consumed AH" like in my photos above?

So, you are seeing that you have 73 percent of your battery available. There are many variables here: how much sun have you been getting, etc. If you have the two way fridge, and it has been on for 6 days, you might not have enough sun to replace the amps that the fridge, etc. have used each day. (After 6 days of fridge use, this is not too bad, I don't think). Have you checked the reading as the afternoon has gone on?

Deb55 · August 2020

@pthomas745 mine says -73.8 for consumed amp hours. Does this mean consumption since the last full charge?

pthomas745 · August 2020

Yes, that is what my manual says. So, in six days, you have used about 12 amp hours per day. (I'm not sure if that balances what the solar panels have added back in.....I never use that much per day!) If you can, show us some screen shots from your Victron.

Deb55 · August 2020

@Denny16 I thought I set it up for my battery. I followed the settings of someone else who has the same batteries as I do, at least I think I have those batteries. I've never actually removed the plastic case to look at them. The person whose settings I copied has the same year and month T@B 400 (March 2018) as I have. It's set to 14.1 as the fully charged set point. Does that sound too high?

pthomas745 · August 2020

Yes, the 14.1 sounds pretty high. Do you have a Victron manual? They suggest a 13.2 setting for the charged voltage. Find out why your friend decided on 14.1...here is a link to the manual, and the related setting is in paragraph 7.1.2

https://www.victronenergy.com/upload/documents/Manual-SmartShunt-EN-NL-FR-DE-ES-SE-IT-PT.pdf

Deb55 · August 2020

@pthomas745 changing it to 13.2 didn't make a difference. I'm stumped. I'm probably overthinking this whole battery thing, but I want to understand it enough to not be caught short while dry camping.

pthomas745 · August 2020

Find the section in the manual on "manual synchronisation." Para 5.3.2. Changing the charged voltage might require a complete reset of your Victron.

And, yes, it is confusing. Driveway practicing is always good! I would turn the fridge off and let your panel charge for the afternoon and see how your display responds.

pthomas745 · August 2020

And......open up that battery box. Find out exactly what your battery is. Put the proper settings into your Victron. If you "copied" the wrong charged voltage settings from the other owner, there may be more surprises in your other settings, and your Victron will never give you the proper readout.

DenisP · August 2020

https://m.youtube.com/watch?v=Bq_eDTSzRsE

https://m.youtube.com/watch?v=4ReljfO3k6M

I have attached two YouTube’s. One is a Victron instruction on setting parameters for BMV 700 series and the first one is a nice overview for the 400, batteries and monitoring.

Deb55 · August 2020

@DenisP thanks for posting these. The first one was helpful, even though I didn't didn't install the monitor myself. It helps to be able to see what connects to what under the bed! The second video was a bit over my head, but interesting nonetheless. I've changed my settings, brought the battery up to 100% charge, and synchronized the Victron monitor. I think I should have accurate readings now.

elbereth · August 2020

@Deb55, @pthomas745’s first response was on the nose. The reason you get a Victron battery monitor (those of us who have installed it ourselves before it was more standard!) is that the voltage meter is only an approximation of state of charge. It just reads the voltage difference across 2 points, and that is affected by inputs and outputs to those 2 points - a charger will raise the voltage difference (that’s part of how it works!) and anything pulling charge from your battery will lower it.

So to know what is happening with the battery itself, rather than the current “net” voltage of your whole electrical system, you have to read the voltage with nothing flowing to and from the battery. That’s whats meant by a voltage reading from a “rested” battery not under load, and is what those % charged charts are based on - but even that’s approximate.

The BMV-712 uses a shunt between your battery and anything that would add or take away charge, and actually “counts” the flow of electricity. So setting your parameters accurately in the app, and “synchronizing” when you know the battery is fully charged, is important in getting that % charged reading to be accurate, because otherwise it doesn’t “know” how big your battery is or where it’s at in its charging cycle. It’s “smart” but it’s still kinda dumb!

One of the “dumb” parts is that setting the “fully charged” number is just where the app will automatically reset itself to 100% charged when it reads the system at that voltage or above for more than 3 minutes. It kinda overrides the Amp Hours counter that you are setting with other parameters, and again using voltage as a marker of state of charge. So if you were to make that “fully charged” number 12.2 V, for instance, the app would be constantly deciding the system was at 100% and you’d never get an accurate reading.

Many chargers will actually charge above 13.2V, especially in an equalization or even in a bulk portion of the cycle. So setting “fully charged” at a voltage above where your charger will hit, like 14.2, will keep it from falsely reading “fully charged” when it’s not.

It’s most important to set the battery capacity in Ah correctly, and then just to hit synchronize when you know it’s fully charged. I keep mine plugged in at home, so I just hit “synchronize” each time we pull it off the charger to go camp - I know after a week or two at home that battery is “full.”

Deb55 · August 2020

@elbereth are you suggesting that is preferable to set the battery capacity higher? I had it at 14.1, then after reading up on it, watching a video, and seeing some suggestions here, decided that it really should be 13.2. I've reset it to that, fully charged the battery on shore power, and resynchronized. Is it your opinion that it needs to be set back at 14.2? Sometimes I feel like I'm overthinking this...

pthomas745 · August 2020

Yes, you are overthinking this. You would have to ask the person who had their setup at 14.1 why they chose that value. Did it work for them? What kind of battery did they have? What size? For your stock battery, the 13.2 number (which Victron recommends) works just fine.

Deb55 · August 2020

@pthomas745 I did find out that the 14.1 was an error that he corrected and passed that info along to me. So, hopefully since I've corrected it, recharged and synchronized, I'll be all set.

elbereth · August 2020

I was running into issues with the 712 synchronizing automatically too early when I had it on a Noco charger, so I bumped the charged voltage setting to 14.2 as suggested in section 2.3 of the manual. It doesn’t automatically synchronize now very often, but that’s a better problem than having it show 100% when it’s less than that.

@Deb55 I agree with @pthomas745, though - just use the 13.2 as your charged voltage setting for now, as the manual suggests, with the correct amp hours for your battery (if it’s the stock battery it’s probably somewhere between 210 and 235Ah) for the capacity. If later you’re finding that it is showing 100% charged when you know it shouldn’t yet, then you can fiddle with the voltage setting.

Sharon_is_SAM · April 2021

I, too, have had BMV readings that were out of whack. In my case, I had a persistent 0.2 A Current with the battery off that I could not identify. I found some good marine sources. I discovered that not only is it necessary to adjust the BMV settings for solar use, but it is important to “re-zero” the BMV to avoid inaccurate assessments of the SOC. So, my newly added task before our infrequent camping trips is to:
fully charge the battery via charger, manually synchronize the BMV, then with the battery switch off to discontinue any load - do a Zero Current Calibration. My battery voltage was confirmed against a multimeter and was fine. I kept track of BMV readings during our Battery Tender charge cycle and found that, just like @elbereth, my SOC was 100% as it entered bulk charging. I ended up altering the Charged Voltage, Tail Current and Charged Detection Time - all to prevent premature 100% SOC readings. I lowered the Battery Efficiency to 85% for our 4 year old Lifeline battery. Hope to test all this very soon. I made a note to adjust the Charged Voltage based on the absorption voltage and further reduce Tail Current for when we use solar.

Dutch061 · April 2021

Okay, so I have been working through this for the BMV-712 and the MPPT 75/15 on our Tab 400 BDL with factory solar so that it will provide accurate information without me manually synchronizing things etc.

These are 3 things that I can assure you that need to be looked at, based on hours of testing, reading Victron Blogs, and manuals.

1. The Charged voltage CAN NOT be set at 13.2 when Solar is involved, clearly stated in the manual in more than one place. 13.2 is the default value when the BMV-712 detects it is connected into a 12 volt system. It is NOT the recommended setting when solar is involved.

2. Tail current, I believe the default is set to 2% and the manual suggests 4%. This is 4% of the AH Capacity of your batteries, so in the case of 224 AH (factory Harris 6 volts) this comes to 8.96 Amps.

3. Charged detection time, default is set to 3 minutes.

How all of this works together when set at Charged voltage 13.2 volts, 4% Tail current, and 3 minutes Charged detection time goes like this.

When the battery voltage reaches 13.2 volts and the charging current flowing into the battery is LESS THAN 8.96 Amps for a period of 3 minutes, the State of Charge (SoC) is reset to 100%, Consumed Ah is set to zero and on the history tab you will see the Time since last charge reset to zero. This will NEVER work with Solar if you want any type of accuracy.

The issue I was having is that the SoC was being reset to 100% in error.

Currently I have mine set as follows:

Charged voltage 13.6 (not the Victron recommended 14.2 which is .2 volts below the Absorption voltage)
Tail current .5% (this is as low as it can be set and equates to 1.12 Amps). If I could set it lower, I would
Charged detection time is set to 5 minutes

So, I need to see 13.6 volts with less than 1.12 Amps passing into the battery for 5 minutes to reset SoC to 100% etc.

So far, in my testing in NC this seems to meet my needs but I am still evaluating this as I type.

In the case of the original poster, they are not ever meeting the requirements of Charged voltage vs Tail current vs Charged detection time. That is why they are showing high consumed Ah, if it never synchronizes, the numbers continue to get weirder with each passing day.

I think some of these settings also can be affected by several factors including (but not limited to):

1. Latitude
2. Sunlight intensity IE cloudy, partly cloudy
3. Usage case of TAB IE shade, angle, direction
4. Battery size and type IE AGM, Wet Cell, Lithium, number of batteries

Meaning that each of you will need to determine what best works for you and what if any errors you are willing to live with.

Sorry, I wish there was a "one size fits all answer" but there simply isn't.

Even in the Victron blogs, it is kind of like "here are some settings that may help, good luck".

Brad

TomCanada · April 2021

+1 to Brad - him and I have been exchanging some emails and data on this topic, and it's quite the rabbit hole once you jump in..

I've collected a couple of days of data myself and found that 13.6V charged voltage will likely not work in a solar-enabled scenario for me. The reason is that when I get a cloud on a sunny day, I could quickly drop in current to below tail current settings (even really low ones like 0.5%), while voltage remains elevated (above 13.6V). Victron's Johannes explains this in a recent video from this year, and I now I see why their latest recommendations for BMV settings in a solar environment are:

- Charged voltage = Absorption minus 0.2V (14.4V - 0.2V = 14.2V)
- Tail current = 5-6% (of full capacity, ie. 224Ah for Tab 400)

These settings ensure we won't prematurely sync when a cloud passes by.

Brad has suggested a longer measuring period (5 minutes instead of 3) could compensate for this and allow voltage to drop lower, avoiding the premature sync, and I agree - however, I still feel there's risk in case it's a large cloud and your individual battery's voltage doesn't drop as fast as someone else's - there's definitely variability and this changes throughout the battery's life as well.

So for now I am currently sticking with the Victron recommendation above + 95% efficiency, but based on my results I'm comfortable lowering my tail current to 4%. The only downsides to these high tail currents, based on my data, are:

- we'll sync to 100% SOC when we're actually only 95-96%. Not that big a deal - and I'll be asking Victron what happens to extra charge after a sync point (can we somehow be greater than 100% SOC internally?)
- in the case of charging from shore power only, and no solar top-up, we will never synchronize to 100% automatically. I'm ok with that - because we'll always have some sort of top-up from solar even on cloudy days, and I can always manually sync to 100% if I see the converter has mostly charged the battery

Tom

Sharon_is_SAM · April 2021

@Dutch061 - I think I read some of the same material that you did.😊. I read your previous post regarding BMV settings here: https://tab-rv.vanillacommunity.com/discussion/13362/bmv-712-proper-settings-with-solar-panels#latest.

I read that having multiple sources of charging can contribute to inaccurate SOC, part of why I am taking the advice to recharge, sync and zero. We don’t have installed solar, but will use a suitcase at intervals, so, my experience will be somewhat limited to one charging source at a time. We have a 150Ah AGM. I watched the BMV as our Battery Tender charger entered Absorption (14.4V) and then Float (13.2V) to arrive at my Charged voltage numbers (-.2). Not sure if that is how you determine the BMV settings specific to your individual battery, but at least Lifeline agreed with my Absorption voltage. I need to charge it via the converter now and see how things correlate. Because our WFCO converters don’t bulk charge unless the battery is > 50% depleted, I don’t know if that will ever happen.

Regarding tail current, we know the TaB converters do not enter/stay in Bulk mode long enough for AGM batteries. WFCO starts Absorption at 13.6V and the BMV Charged voltage parameter set to 13V is met immediately. The Tail current for converter charging at the recommended 4% seems too high. Seems like a too high set Tail Current will still have the BMV reading 100% prematurely. So, I lowered the BMV Tail Current to 2% to more fully charge. I see you lowered yours to 0.5%. I do not understand why the Victron video said to raise the Tail current to 5-6% with solar. If you raise the current amp setting, won’t end of Absorption be defined prematurely? I put the Charged detection time at 10 min. Longer is better for solar and I suspect won’t matter for charging via converter.

@TomCanada - big rabbit hole. It seems like the industries (battery, charger, battery monitors) would play nice and use set standards. No such luck.

I think I understand the rationale for raising the Charged voltage for solar (a higher voltage prevent the BMV from labeling a full charge just because the voltage temporarily dips) but raising the Tail current to 5-6% seems counter intuitive. If a cloud passes by, the current can temporarily drop faster than the voltage and a full charge would be noted I thought lowering the Tail Current you would prevent meeting the set parameter too early. What am I not getting?

TomCanada · April 2021

@Sharon_is_SAM the issue causing the need for a high tail current is that the MPPT solar controller used on the Tabs has an algorithm where the time spent in absorption (14.4V) is as little as one hour, and a max of four hours, depending on SOC at the beginning of the day. As a result, using a lower BMV tail current setting might not work, because with a BMV "charged voltage" setting of 14.2V, the MPPT could drop down to float (13.8V) before we meet that lower BMV tail current threshold during absorption mode.

The alternative is to use a lower 'charged voltage', but that will result in premature syncs during passing clouds (I've collected actual data showing that is a very high probability of happening).

So the best we have, as I can ascertain, is what Victron recommends - for solar, use absorption minus 0.2V as the charged voltage, and 5-6% as the tail current. I actually have my tail current set to 4% now as I think that will work with the MPPT, but other controllers may need higher (or could get away with lower, if they stay in absorption mode longer and you have confidence you'll hit that low tail current threshold before dropping to float).

Hope that helps!

Denny16 · April 2021

Thanks Tom, that is how we have our’s set, and it seems to work fine, battery is solar charging to 14.2 VDC, as tested with power off. This has been an interesting discussion to follow.
cheers

Sharon_is_SAM · April 2021

@TomCanada - Ok. I get it. Boy, between the WFCO and the Victron controller, you can’t win.

We are using a different charge controller, so I will need to check the parameters on it - a Renogy PWM Wanderer.

MarkAl · May 2021

We rarely use the WFCO even when home for two reasons: I see the charge state in absorbtion far too much and we have a remote solar input that (thanks to another member of this forum) I use a 100 watt old 20V laptop supply to keep the battery in float. So have not had problems re-syncing the battery level- sun's always out. Note there's enough power that I can also turn the refridge ON the day before to pre-cool it without losing battery charge.

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