Why does BMV-712 SoC report 100% when MPPT still bulk charging, and BMV voltage at 13.5V?

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Tim Rohrer asked Dec 10, '19

Why does BMV-712 SoC report 100% when MPPT still bulk charging, and BMV voltage at 13.5V?

I'm still making adjustments, so perhaps something isn't quite right yet. My battery bank is 600Ah of Lithium. My Charged Voltage is set to 14.5; Peukert Exponent is 1.05; Current Threshold is 0.10A; Charge Efficiency Factor 99%.

My 150/100 MPPT indicates a battery bank voltage of 13.7, and remains in bulk charging mode.

My load consists of basically the refrigerator and a little bit of electronic gear. BMV is showing 0Ah consumed, and that +450A or so is being applied to the batteries.

There seems to be a mismatch between voltage and tracked current. Am I misunderstanding something?

I had synchronized the BMV just a couple weeks ago shortly after installing it when I was on shore power. We stayed on shore power until just a couple of days ago. My first run down of the batteries when to 28% a couple night ago. Solar has been doing the over the past couple of days; I would not have thought the BMV synchronization would be lost that fast.

Thoughts on what I should check/adjust next?

MPPT Controllers BMV Battery Monitor SOC solar

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edgargw22 commented · Dec 11, 2019 at 06:49 PM

Hi Tim. I wish I had an answer for you, but I have the same problem with SOC showing higher levels than what is actually still left in the batteries. It seems to get worse the longer I am away from a fully charged scenero.

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Justin Cook answered · Dec 11, 2019 at 09:47 PM

@Tim Rohrer, a couple of things: I'm hoping that your statement of "...and that +450A or so is being applied..." is a typo? 450A into a 600Ah LFP bank is far higher than most manufacturers recommend (standard recommended LFP charge rate is .5C or less) and is obviously far higher than the MPPT is capable of putting out, so I'm hoping that it's a typo.

What is the charging voltage set to in your MPPT, and how far is the MPPT away from the battery bank? If the MPPT is far away from the battery bank, it could be reading 13.7v as a result of voltage drop over a great distance or through undersized cabling - or as the result of a poor connection somewhere along the line between the batts and the controller. A poor crimp somewhere along the line could easily be heating up and increasing resistance on the line, which would cause a voltage drop.

I would start by measuring the battery voltage at the terminal screws of the MPPT, then measure the battery voltage at the battery; if there is a significant difference between the two, you know it's an issue of voltage drop or a bad connection somewhere along the line. Then compare your readings to what the BMV is telling you the battery voltage is; the BMV reading should be nearly identical (+/- maybe a tenth or so of a volt depending on your multimeter) as the voltage reading at your battery. If it isn't, then you know that there's an issue with the BMV. There are a number of possible causes for this, so I'd start with getting those measurements and reporting back; those measurements will help us further diagnose the system.

EDIT: I'll add, in case I misread the original post, that if you have loads running while the controller is charging, the controller will -depending on the loads- stay in bulk mode because it's effectively powering those loads. Technically it's capable of putting out up to 100A of charge, but even if you're only running ~10+A of loads, the controller won't generally switch to absorption until the loads go away; you're effectively powering your loads straight off the solar controller. Obviously this depends somewhat on how you have everything connected; if you wouldn't mind posting a photo of the wiring/battery/shunt portion of your setup, that would also be helpful in diagnosing what -if any- misbehavior of your components might be occurring.

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swampbush answered · Dec 28, 2019 at 01:49 AM

It seems your BMVis resetting too early. The Victron mppt manual explains you have to change it for solar. Otherwise one cloud can see the current drop and the bmv thinks the system is in float. A high current shore charger won't cause it to reset, but a low power solar will. The default setting I think is 13.2 volts and 4% end amps.

Increase the voltage to slightly less than that absorption level say by 0.3 volts.

My LifePo4 settings are in the screen shot.

screenshot-20191228-124359.png (104.0 KiB)

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Modifications Space

Node Red Space

Answer 1 · 2019-12-11T18:59:40Z

yannick answered · Dec 11, 2019 at 06:59 PM

Try lowering the charge efficiency. 99% is too high for lithium batteries. Try somewhere around 90%

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Tim Rohrer commented · Dec 11, 2019 at 09:06 PM

How does that influence state of charge calculations? My reading indicates lithium batteries should have a charge efficiency of near 100%. I will do more research, but would like to better understand your suggestion as part of that.

Thank you.

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Justin Cook ♦♦ commented · Dec 11, 2019 at 09:36 PM

@Yannick, @Tim Rohrer, Ahhhh no, charge efficiency should always be set to 99% when using LFP batteries. Charge efficiency 99%, Peukert exponent to 1.05%, current threshold 0.10. This is per Victron, per Battle Born, per LiFeBlue, per (effectively) every reputable LFP battery manufacturer on the market. 95% charge efficiency and downward is only applicable for Pb based battery banks.

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Tim Rohrer Justin Cook ♦♦ commented · Dec 13, 2019 at 10:38 PM

Thanks @Justin Cook. That was my understanding as well.

So, does the Victron BMV really lose sync with 100% charged that quick? I'd read that one needed to get try an get the batteries fully charged once a month or so to keep the sync calibrated. I wasn't more than a few days since having been fully charged.

Once again I'm showing 100% SOC and I'm pushing in nearly nearly 700W in bulk charging. Batteries voltage reported at nearly 13.6V.

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Answer 2 · 2019-12-11T19:13:09Z

fairline answered · Dec 11, 2019 at 07:13 PM

I have flooded batteries and my charge efficiency is 99%, when voltage is over 13.2 volt the efficiency is 100%. Is that correct? So batteries is fully charged at 13,2 volt?

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Justin Cook ♦♦ commented · Dec 11, 2019 at 09:59 PM

@fairline, a brand-new set of FLA batteries will start out around 92-95% charge efficiency (Peukert exponent should be set to 1.25%) and will, with age, dwindle downward. A 99% charge efficiency setting in the BMV should only be applied when you have a lithium-base battery bank.

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Answer 3 · 2019-12-27T21:24:23Z

svnova answered · Dec 27, 2019 at 09:24 PM

Hi @Tim Rohrer - Did you ever get this sorted out? I'm having a similar experience. I suspect it has to do with the "tail current" setting when using a charging source that fluctuates. In our case, solar.

I have 1200Ah of lithium, and 2100W of solar, using three 100/50 MPPTs and a BMV 712. I have not yet put them into a smart network.

To the best of my understanding, if your battery capacity is set to 600Ah, and your tail current is set to 4%, the measured value will be 24A. The BMV considers the batteries to be full when the charge current drops below the tail current value. Well with solar as the charge source, that's going to happen multiple times throughout the day! Even if you set it to the lowest value (0.1%), on a cloudy day it's still going to fluctuate enough to trigger "full" according to tail current.

If both Charge Voltage AND Tail Current have to be met simultaneously, that is a different matter. But I want to just disable the "Tail Current" value completely on my BMV.

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