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jeffb avatar image
jeffb asked

SmartShunt Settings and State of Charge

I was required to select a topic, and BMV seemed most relevant, as my main question is about the SmartShunt - which should be included in that category.

I've been using a SmartSolar MPPT 100/30 for a couple years now, and have a good understanding of the normal electrical usage on our RV. I recently added a SmartShunt to be able to monitor battery power more closely, especially when charging from sources other than our solar panel - like shore power or a generator or from the tow vehicle. I've only used it a few days, so still figuring out the proper settings. I have a 210A-hr lead-acid battery bank. The default settings of 13.2V Charged Voltage and 4% Tail Current had the State of Charge resetting to 100% much too early - well before bulk charging was finished. So I tweaked both of those for the expected charged conditions from the MPPT charge controller, but then realized that when recharging from shore power, I'll never get over 14V, so the Tail Current will probably be the main thing which determines 100% charged. The second day I was watching the charging progress carefully, and the MPPT switched from Bulk to Absorption at 14.4V (about 92% indicated SoC with about 20A-hrs to go) and then switch to Float right as the SmartShunt indicated 100% SoC and 0A-hrs consumed. For networked devices - how come you don't have the charger tell the battery monitor (over the VE.Smart network) when the battery is fully charged? The charger switching into Float mode seems a more accurate indication of 100% charged than parameters in the battery monitor.

Now the main question I have is with the State of Charge calculation in the SmartShunt - which I assume works the same as all the BMV700 series monitors. The next morning when our SmartShunt showed -72.4A-hrs, the SoC indicated 72%. That doesn't work out - there seems to be some other factor in the SoC calculation -72A-hrs should be about 65% of my 210A-hr battery bank. And yes - the Battery Capacity is programmed to 210A-hrs. And usage of 70A-hrs is quite normal for us.

So far I'm liking the SmartShunt and hope the get the settings dialed in so I can keep closer tabs on our trailer batteries.

BMV Battery Monitor
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2 Answers
JohnC avatar image
JohnC answered ·

Hi @JeffB

The trouble with Amp-Hours is that they mean nothing without a Voltage applied to them. So the Ah going out is going to mean something different discharging at lower V than charging at a higher V. And I'm not sure what Victron might have coded in if they actually manage to return to a balance. So I don't look at them - at all..

There are some kWh figures in the History section which are worth a look though, because they aren't subject to 'non-definition'.

General comments..

All batts are different, so you'll need to determine your own settings.

Peukert Exponent.. try to find your batt capacity rating at two different C levels, say C5 and C20. Then feed them into the Victron calculator. Mine worked out at 1.16.

Charge Efficiency you can fiddle with, ideally to return you to the same SOC you saw the previous days at the same Tail. Eg if you're drifting downwards, raise the CE.

The Charged Voltage I set a little lower than Float, but bear in mind that Float V can vary with the seasons (temp comp), so allow for that.

Tail default is too high for me. I use 0.7% to sync, and about 2.7% to drop my mppt from Abs to Float. You may not have that second option available, but I found it reduces my Abs time markedly, and my SOC when that happens is ~99%. Surprised me too, but my batts are fairly reactive floodeds. So I was overcharging them a little, and that revelation alone made the Smartshunt a most worthwhile purchase.

Have fun tuning it in, but be warned - this can be an addiction hard to shake. And frustrating if hot/cold spells throw everything slightly out.. :)

Good luck.

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greekgeek avatar image greekgeek commented ·

your answer has so much compressed knowledge i need to study hard to fully understand everything!! i would like to ask something about the smart shunt that keeps bothering me. I have an mppt charger and a smart battery sense gadget working as a system at the moment. i want to add the smartshunt to monitor my battery bank better; is it a good idea to make a network with all three (battery sense, shunt and charger)? should i install the battery shunt and not include it in the network?

appreciate any response, cheers.

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JohnC avatar image JohnC ♦ greekgeek commented ·

Hi @greekgeek

Sorry, I'm unfamiliar with Smart networking. I use a wired GX with DVCC, and the Smartshunt is certainly worth having there. It sorta brings the whole system 'together'.

Maybe someone else can help. Or keep up the study.. that doesn't hurt, hey? :)

Try it either way.

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jeffb avatar image
jeffb answered ·

Thanks @JohnC

OK - I think I've figured out what's happening with my State of Charge readings. Most think of the Peukert factor as a negative, but this is a case where it's a positive. Most RVers actually discharge their batteries fairly slowly - around C50 or C100. This (supposedly?) yields more battery capacity than the advertised C20 rate. I've recalculated the Peukert Exponent based on my battery specs, and adjusted that value. I'll see if my State of Charge vs Amp-hrs readings change on our next trip.

BTW, amps are amps - regardless of the of the battery voltage. That's what the battery shunt measures. Although it does help to convert to Watt-hrs - that makes it easier to compare power flow at different voltages. Unfortunately, my MPPT charger reports status in KW-hrs, while the SmartShunt reports current status in Amp-hrs. Only the cumulative power numbers are expressed in KW-hrs. Surprised there isn't more consistency.

It will probably take a few trips to get all the settings fine-tuned. As well as experimenting with different charging sources. Most all my charging is solar, so that's first.

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