How to properly set charged voltage on BMV-712?

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Tomasz Czernecki asked Dec 30, '20

How to properly set charged voltage on BMV-712?

We have 2 x 6V 224Ah batteries in series hooked to a WFCO converter and while the converter/charger's bulk voltage is 14.4V (which it only enters after deep discharge), the absorption voltage output on this charger is only 13.6V and float is 13.2V. Meanwhile, my parallel solar charger (Victron 75/15) is set to 14.4V absorption and 13.8V float. Does this mean that the battery will only be fully charged if I have solar charging present? And if so, I'm wondering how to properly configure a BMV-712 monitor in terms of "charged" voltage - if I use 13.2V then the monitor might think the battery is full when it's not, but if I use for example 14V, the monitor may never show a full battery if I'm just charging off the converter (ie. Cloudy day / no solar).

Thank you for your help!

MPPT Controllers BMV Battery Monitor monitoring battery system voltage

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seb71 commented · Dec 30, 2020 at 10:53 PM

The main measurement the BMV (the shunt) does is "Ah in" / "Ah out" (to/from the battery).

It also knows the actual battery voltage over time.

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Tomasz Czernecki seb71 commented · Dec 30, 2020 at 11:43 PM

Yes I know those things - that wasn't my question though :)

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seb71 Tomasz Czernecki commented · Dec 30, 2020 at 11:51 PM

"but if I use for example 14V, the monitor may never show a full battery if I'm just charging off the converter (ie. Cloudy day / no solar)."

If enough Ah get into the battery, SOC will reach 100% , even if the "charged voltage" set in BMV is not reached.

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JohnC answered · Dec 31, 2020 at 11:15 AM

Hi @Tomasz Czernecki

It appears your WFCO uses different terminology than Victron. The 14.4V Victron calls 'Absorb', the 'Bulk' a transition stage on the way there. The 13.6V is 'Float', and the 13.2V 'Storage', in Victron-speak.

They'll accept charge at all those V's, but the-lower-the-slower. When the Tail Current stops (or comes close to) tailing off then your batts are full. So you set your Charged V and Tail to resync to 100% when that happens.

Your batts sound a little like GC2-pattern floodeds, and if they are let me know and I'll post up my own settings & explanation.

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Answer 1 · 2020-12-31T04:54:23Z

goldspace answered · Dec 31, 2020 at 04:54 AM

I recently installed my BMV712 and had a similar question. The instructions describe that the auto synchonization of the BMV is set by "tail current". That, I believe, is the small current the batteries take temporarily after 100% charge. Tail current is an adjustable parameter in settings; BMV default tail current is 4% of capacity. Your capacity is 224 a/h; by default, a tail current of 9A for a period of time (3 min above a voltage you select) would reset the BMV to 100%. The instructions are in the manual. Refer to paragraphs 2.2 (manual, pg 10), and 4.2.1. (manual pg 21)

The other important aspect here is to determine the manufacturer's recommended float voltage. Then I suggest playing with the auto sync settings, including tail current, to find an acceptable combination of parameters in which you can be confident that you're completely charging your batteries.

Good Luck!

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seb71 commented · Dec 31, 2020 at 09:55 AM

The automatic synchronization of SOC to 100% does happen when 3 conditions are met (true). Current being under the "tail current" value set in BMV/SmartShunt is just one of them.

The conditions are:
- battery voltage must be greater than the "charged voltage" set in BMV/SmartShunt;

- current (into battery) must be smaller than the "tail current" set in BMV/SmartShunt;

- the above two conditions must be simultaneously true and must remain true for the time period set in BMV/SmartShunt.

The charging algorithm and charging voltages for various charging stages from MPPTs and grid chargers are another matter, and do not depend on the BMV/SmartShunt settings.

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Answer 2 · 2020-12-31T14:36:31Z

Tomasz Czernecki answered · Dec 31, 2020 at 02:36 PM

Thank you all! @JohnC here is the datasheet of the battery - not sure if this is a GC2 though people have referred to it as such in some forums:

https://drive.google.com/file/d/1VWbKPs3r3bNxp0eHl0ekIga29l7vO7IE/view?usp=drivesdk

It sounds like the tail current is the more critical parameter than the charged voltage in the BMV-712. Something like a setting of ~13V should cover me for both solar and WFCO converter cases, and I'll just tweak the tail current (another user of this battery recommended 2%) for appropriate behavior. I wonder, any disadvantage to setting a really low tail current, like 1%, to ensure battery is fully charged before the BMV synchronizes? What happens if it synchronizes at a tail current of say 2% but then the battery keeps charging. Does the BMV indicate >100% charge, or will it just say 100% but not go to 99% until there is net amps lost compared to synchronization point?

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JohnC ♦ commented · Jan 01, 2021 at 03:47 AM

@Tomasz Czernecki

Yeh, those are close to GC2, same size but sealed and slightly different chemistry. I took the liberty of running the capacity specs through the Victron Peukert calculator. It bobbed with 1.19, so you could use that rather than the 1.25 default. (I use 1.16)

The Tail is indeed important, and it can even vary between winter and summer as batt reactivity varies. I used 0.7% in winter, but now in summer with my batts up over 30 degC and longer days, 0.5% works ok.

The Trends graph in VictronConnect is good to watch this on. This is mine today, and if you look closely you'll see a jump from 99.7% to 100% as it synched. I could set the Tail lower, but practicalities rule - I may run out of daylight on bad days.. :)

It won't go above 100%, even though the scaling suggests it might. And you must accept that 100% as a practical level, achievable in the daylight you have available, and is not a lab figure. I just accept that the 0.3% I've 'jumped the gun on' will be achieved anyway over the rest of the afternoon.

And oh, (with 48 > 12V conversion) I'm using 13.5V Float (+/- realtime temp comp) and 13.2 Charged V. Also, I found 90% Charge Efficiency best, as distinct from 89% or 91% (or the default).

Have fun tuning..

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Tomasz Czernecki JohnC ♦ commented · Jan 01, 2021 at 04:19 AM

Thanks again John - so if you keep charging beyond sync point (say, another 2 amp-hours), and then disconnect from the charger and run on battery, will the BMV not drop below 100% until you've used those 2 extra Ah beyond the sync point?

Happy new year to you and yours!

Tom

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JohnC ♦ Tomasz Czernecki commented · Jan 01, 2021 at 07:27 AM

@Tomasz Czernecki

The 100% is a true sync, and there's no 'reserve' considered. The BMV is just doing what you've told it to do with the settings you've applied. And of course it can't know about batt self-discharge and suchlike.

I've always considered SOC as a bit of a mushy figure, subject to a number of issues. I grabbed a SmartShunt just to show me Amps into the batts and control my mppt. But then found I could get a reproducible SOC with a little care, and now I'm fairly confident in it. To the point where I'd be happy switching loads based upon it. Mine would have paid back it's purchase price alone in generator fuel. It changed the way I manage my batts. Great little devices, love mine.

And a Happy 2021 to you too..

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