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Akif

Setting up Victron BMV-712 Smart Battery Monitor With Axpert Inverters

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Hi there,

a bit of a newbie to solar but being educated swiftly thanks to @Chris Hobson whom I keep bugging on WhatsApp! And to spare him from typing on his phone thought I would put up my first post and interact with him and all the other pros out there!

So after Chris's recommendation on my Axpert parallel inverter system have taken the plunge and purchased the Victron BMV-712 Smart which has built in Bluetooth so as to make sure my second set of batteries don't die as quickly as the first, which I am certain is due to the installer not setting up the low voltage cut out and leaving it at 42 volts hence killing them due to extended load shedding in our part of the world at the moment. Anyways as he had guaranteed the system he has since replaced the first 8 x Shoto 180Ah batteries with  8 x 200Ah Sunlight Batteries as per the pic attached which to be honest don't look like 200Ah batteries (smaller and lighter than other 200Ah batteries) and I have never heard of this brand but since it is under his guarantee I guess I just have to wait and see how they perform (he has guaranteed them for 18 months).

Since installing these new batteries i have changed the following settings on the Axpert inverters:

05 - set to user defined

26 - Bulk charging voltage - 56.4 Volts

27 - Floating charging voltage - 54.0 Volts

29 -  Low DC cut-off voltage - 48.0 Volts

I have also attached the initial settings of the BMV-712 in a screen grab below.

Unfortunately i don't have any specs on the batteries apart from what is printed on them which isn't much so I hope my settings on the Axpert are correct and would love some feedback on this.

I also wanted to know if my settings are OK on the BMV-712. One thing I noticed on the Axperts is that there is always about a +/-1.0A charging current going into the batteries and it never seems to drop down to zero, hence is the current threshold setting of 0.10A correct? How would the BMV-712 know when the batteries are charged fully if this +/- 1.0A current never drops down?

Looking forward to everyone's comments and help thank you.

BMV-712 Initial Settings.jpeg

Sunlight 200Ah Solar Battery.jpeg

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17 minutes ago, Akif said:

26 - Bulk charging voltage - 56.4 Volts

27 - Floating charging voltage - 54.0 Volts

29 -  Low DC cut-off voltage - 48.0 Volts

  • 26 - 12v Gel batteries in CYCLE use would typically have charge voltage of 14.4 to 14.7v, I would go in at 14,4V x 4 = 57.6V
  • 27 - Use the typical standby voltage as your float voltage, so 13.8 x 4 = 55.2V (if too low then you will struggle to fully charge),
  • 29 - 48V cut-off sounds about right, you may need to drop slightly if you have heavy loads which cause short term voltage drops.

Note the above is my opinion, others may differ.

 

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Thanks pilotfish,

In terms of the bulk charging voltage how would one know if it were set too high, would the batteries get very warm?

Are the BMV settings OK? I'm guessing if the float charge voltage is set to 55.2V as you have advised i would have to change the charged voltage setting to say 54.9V, how about the tail current, charge detection time and current threshold?

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3 hours ago, Akif said:

In terms of the bulk charging voltage how would one know if it were set too high, would the batteries get very warm?

I believe that the bulk charge voltage that I gave you is correct for most AGM/GEL batteries, so I doubt it is too high. The batteries will warm depending on the amount of current passing through them, but should not be hot to the touch which indicates problems. The battery room should be well ventilated to try and keep a temp as close as possible to 25C - above which battery life drops off fast.

@plonkster is the right guy to ask regarding the BMV - he is PAID to supply quality support:D

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The low DC cutoff (setting 29), I thought, was the point at which the inverter will switch off (in the absence of grid power). The point at which it would return to grid (i.e.  start recharging the batteries) is setting 12.

 

Off course I might be misunderstanding the question.

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@pilotfishThanks will try this out and the batteries are well ventilated so no issues there.

@OomD That is correct was looking at the point the inverter will switch off (setting 29 in the absence of grid power so as to ensure that the batteries don't get discharged beyond the point of no return.

@plonksterSir would appreciate your insight into the ideal settings for the BMV based on my initial post above. Just want to be sure that after the investment in it, I am setting it up correctly. 

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BMV settings for lead acid, pretty much:

Charged Voltage: At least 54V (recommended 0.3V below your float voltage). It is not too sensitive, it must just be a voltage that is higher than the resting voltage of the batteries, but lower than float.

Tail current: 1% (default is 4%, which is a bit high for large lead acids).

Charged detection time: 15 minutes

Peukert factor: 1.25

Charging efficiency: 95% is the default. I find that one has to play with this one a bit. If the BMV reaches 100% while still accepting lots of current, this figure needs to be turned down a bit.

Also see Marianka's video here.

Edit: Just need to add something about the recommendations in the video regarding the charged voltage. For A Victron system I would absolutely recommend that you set the charge voltage about 0.4V below the absorption voltage. If you do that, the BMV will only ever reset if the charge current drops low enough while its still in absorption charge. When the tail current drops below 1% under absorption... you know that battery is full.

Unfortunately I cannot advise this setting on the Axpert because even with the modified firmware it simply doesn't stay in absorption long enough.

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Thanks plonkster, very informative, will amend my settings as you have recommended. 

In terms of tail current percentage is this the residual current after the batteries have reached float or is it the difference between bulk/absorption currents? Also for lead acid batteries why is 4% too high?

Is the current threshold at 0.10A OK? Is this setting for when absorption charge is over?

I did watch the video and your edit has me a little confused. When you say set the charged voltage 0.4V under the absorption voltage, did you mean float voltage?

So with the Axpert absorption voltage issue will I have an accurately synchronised monitor ever?

Another issue I am facing is connection over Bluetooth, which works seamlessly on my Android phone but not at all on my iPad Pro, it just keeps saying no devices found in the VictronConnect iOS app. Any idea why this is?

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8 hours ago, plonkster said:

Unfortunately I cannot advise this setting on the Axpert because even with the modified firmware it simply doesn't stay in absorption long enough.

I agree with this statement, but I have recently come to believe that it is as a result of incorrect advice on the forum and not necessarily an Axpert fault.

If you look at any number of AGM/GEL battery spec sheets for example the advice is fairly similar (but please use your own spec sheet);

  • From Vision 6FM100P-X spec sheet
  • Cycle Use = 2.4 to 2.45 VPC = 14.4 to 14.7 per 12v battery = 57.6 to 58.8 per 48V bank
  • Standby use = 2.2 to 2.3 VPC = 13.2 to 13.6 = 52.8 to 54.4 per 48V bank

The thing is that we are (mostly) using our batteries in Cycle Use and not Standby, but for some reason we all charge out batteries to Cycle Use voltage and then drop off to Standby Use voltage, not allowing the time to fully charge and equalize. Nowhere on the spec sheet does it mention dropping to standby voltage when in cycle use!

So why not set bulk and float BOTH to 57.6V (the lower end of the scale to minimize gassing and stay away from 60V capacitor limit), then your batteries will get charged properly - problem over!

 

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2 hours ago, pilotfish said:

The thing is that we are (mostly) using our batteries in Cycle Use and not Standby, but for some reason we all charge out batteries to Cycle Use voltage and then drop off to Standby Use voltage, not allowing the time to fully charge and equalize. Nowhere on the spec sheet does it mention dropping to standby voltage when in cycle use!

Hi Pilotfish - when I  use to have AGMs I would do exactly as you describe. Before I changed to Coulomb's firmware due to the charging bug we would go to float early and I noticed that the batteries would still accept 15-20A at the lower float voltage and then tail off. Only after I changed the firmware did it behave correctly with <3 A going into the batteries at float. I mention this since it seems that batteries not near fully charged it still can accept a fair charge rate even at the lower float voltage. At 25°C the gassing voltage is 57.4V but this drops to 56.8V if batter temps reach 30°C. I have found my AGMs were about 3-4 degrees above ambient temperature while charging so it is something to consider with VRLAs.

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3 hours ago, pilotfish said:

So why not set bulk and float BOTH to 57.6V (the lower end of the scale to minimize gassing and stay away from 60V capacitor limit), then your batteries will get charged properly - problem over!

That's probably better than dropping to float too soon. I'm just talking from my own observation after moving from a Microcare MPPT to a Victron charger. They stay in absorb much longer. Then I did a full charge with a Multiplus, goodness, it remains in absorb even longer! In the beginning I thought it was overcharging the batteries... then I noted how the tail current continues to drop... right down to below 1%. And then the inverter went to float. That's when I realised what it really takes to get a full charge on lead acids...

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So gents my current set up is more for backup during the frequent power cuts we get in Malawi. However of late they have installed some emergency generators and instead of 4 days a week where we have no power for up to 20 hours a day we are now down to 1 or 2 days a week. So would you say I should set up the charging voltage parameters for cycle or standby use?

Depending on the reliability of our utility power in the future I may decide to swap to a Solar priority from Utility priority which is what the inverters a set to on program 01 currently.

 

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If Utility priority leave the setup as 57.6 bulk and 54.4 float (or there about, try and find your battery spec for standby voltage), if you keep your batteries at a high voltage for a long time they will dry out and fail prematurely.

I can do it because on SBU setting my batteries don't spend a long time at [57.6/56.8] - some days not reaching there at all. On average my BMV will sync every second day.

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Thanks pilotfish so basically a higher bulk voltage to ensure that the batteries are being charged up fully and equilised if necessary but because it is programmed for utility/standby a lower float voltage (but higher than 54.0V) so I don’t kill the batteries if we don’t have consistent power outages and they remain in float for excessive periods. Unfortunately this is the issue, don’t have any specs for these batteries apart from what’s printed on them as per the pic above and I have tried searching for them on the net but cannot find anything on them whatsoever so am guessing since they are gel batteries your recommendations should work for them?

 

@plonkster would love your feedback on my post below cheers

5 hours ago, Akif said:

Thanks plonkster, very informative, will amend my settings as you have recommended. 

In terms of tail current percentage is this the residual current after the batteries have reached float or is it the difference between bulk/absorption currents? Also for lead acid batteries why is 4% too high?

Is the current threshold at 0.10A OK? Is this setting for when absorption charge is over?

I did watch the video and your edit has me a little confused. When you say set the charged voltage 0.4V under the absorption voltage, did you mean float voltage?

So with the Axpert absorption voltage issue will I have an accurately synchronised monitor ever?

Another issue I am facing is connection over Bluetooth, which works seamlessly on my Android phone but not at all on my iPad Pro, it just keeps saying no devices found in the VictronConnect iOS app. Any idea why this is?

 

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1 hour ago, Akif said:

I did watch the video and your edit has me a little confused. When you say set the charged voltage 0.4V under the absorption voltage, did you mean float voltage?

Most chargers use three stage charging. Bulk, followed by Absorption, followed by float. For example, let's say you set absorption to 57.6V and float to 54V.

Bulk: the charger will charge with the maximum configured current until the voltage rises to 57.6V.

Absorption: The charger holds the voltage at 57.6V, while the charge current tapers off on its own as the battery charges.

Float: The battery is about 90% full at this point. The charger drops its charge voltage to 54V in order to reduce off-gassing. Because the battery isn't full yet, it needs to spend quite a bit of time at float level.

Now, the battery will always accept more charge current at the absorption voltage than at the lower float voltage. So a tail current of 1% at the higher voltage is a better indication of a full battery than the same tail current at the lower voltage.

So, the recommendation from Victron, for solar systems that are cycled daily, is to set the BMV "charged voltage" to just a little bit below the higher absorption voltage. If you do this, the BMV can never reset once the charger goes to float. It can only do a reset if the tail current drops really low while it is still in absorption charge.

This is not a problem when using other Victron kit: They don't go to float too easily. Their absorption phase is calibrated according to how long bulk took.

But the Axpert goes to float really easily. If you use the Victron advice, it might never reset because it doesn't spend enough time at the higher voltage. So I'm saying you then need to use a little bit less than the lower float voltage and hope for the best.

Of course, it is not a problem on systems that are not cycled too often.

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