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John Muntanga asked

Smart Shunt not automatically synchronising SOC

I wached Johannes Boonstra's video on setting up a smartshunt with lead acid and lithium batteries. I just installed a Smart Shunt on my offgrid system and followed his recommendation to use the absorption Voltage as the "Charged Voltage". On my MPPT, my float voltage is 27.4V, absorption Voltage 28.6V. I have set my Charged Voltage at 28.2V, Discharge floor at 80% as I only use 15% of of 220AH battery. I have set the Charge efficiency at 75% and charge detection time to 3 minutes and the tail current to 5.5% (12.1A). When my battery was full, I the current reamined at 1.5A, Could my tail current be set too high? Also, the shunt nuver sychronised automatically? Could there be something wrong with my settings? If yes, what should I adjust my settings to?whatsapp-image-2022-02-17-at-83445-am.jpegwhatsapp-image-2022-02-17-at-83445-am-1.jpeg

SmartShunt
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JohnC avatar image
JohnC answered ·

Hi @John Muntanga

Sorry if this sounds tough.. but you really should be careful changing these settings until you understand why. Great you're researching and asking though. In this case I see an important one, not so much the Sshunt as the mppt. This is my equivalent at 48V..

1645090652018.pngSee the *negative* sign? Without it you have Temp Comp working the wrong way, and risk damage to the batts. Easy to miss on a phone, but important.

On to the shunt. (Excuse the abbreviations).

Your 75% CEF is too low. Your SOC will continually drift downwards and frequent syncs will be essential, not 'as necessary'. Try 90-95%.

You say "When my battery was full, I the current reamined at 1.5A".

And that's a great start. Equal to ~0.7% Tail on your 220Ah of batt, and you'd use a Tail setting higher than this (the closer, the more accurate, but it must be reproducible regularly). But the important thing there is at what V? Absorb or Float? It's likely Float, and if so I'd start with a CV maybe 0.3V lower than your mppt Float setting (this to dodge the Temp Compensated setting in the mppt). Others, especially Li users, may disagree with whether to sync in Abs or Float. And you can choose, but the Tail % must be set appropriately to the V you have at the time you want to sync.

I won't rant on further now. Best to get the fundamentals under control first. You ain't at the fine tuning stage yet, but you'll get there. Come back as you need.


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John Muntanga avatar image John Muntanga commented ·

Thanks for that feedback @JohnC.


On the Temp Compensation, I picked that early this morning and added the "-" its -36 as shown in the screenshot below. I hope its not too late and hope the batteries are not yet damaged? This was on for 1 day only.

On the VC, im using the absorption voltage, I have set my VC 0.4 less my absorption voltage. In the video by Johannes Boonstra, this is his suggestion if there is no grid charge on the system and this is to manage the issue of cloud intermittent cloud covers were the votage drops slows and the current drops signifcantly causing the Shunt to sync to 100% SOC. Please see the link for the video below.


Regarding the charge efficiency, I only use 14% of my battery capacity and again in the same video, Johannes Boonstra advised a charge efficiency of between 75-80% for lead acid batteries if they are only cycled in the up range.

https://www.youtube.com/watch?v=mEN15Z_S4kE

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JohnC avatar image JohnC ♦ commented ·

@John Muntanga

I was worried long-term with the -ve thing. Only an issue really with hot batts, and 1 day no problem.

If you've decided to sync in Abs that's ok, as long as you know that sync can't then occur when in Float. To keep this under control you could look at using the Tail function in the mppt, which when networked to the shunt can terminate Abs always at much the same SOC (a great feature in any case). It's listed in Amps rather than %, but if you had sync arranged to happen just before the drop to Float then it would always comply. Depending on your batts this might happen 'fairly' close to 100% SOC anyway, but you'd need to learn to live with that. Ideally the 100% sync would be selectable to something lower, say 98%. Now that would be some feature too.

The false cloud syncs can indeed happen with 'sluggish' batts, and even on-the-way-up, very early in the charge cycle. The Charged Detection Time can be increased a little to prevent this.

Cycling in the upper SOC range will lower overall CEF. There's also a reationship with Peukert, and even Temp, but I still think you'll need to raise CEF into the low 90's. Easily tested, if sync always comes with a large stepup in SOC from the calculated, then it needs raising.

I draw down 15-25% usually, and for comparison these are my summer settings (48V FLA batts)..

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