Adaptive absorption after deep discharge several days ago

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momonu asked Aug 21, '21

Adaptive absorption after deep discharge several days ago

Hi, I do have two AGM batteries with a total of 190Ah, 200W Solar Panels and a SmartSolar 75/15. It happens that my batteries go down to 50% and due to not enough sun the charging back to 100% takes a few days. But then only 50 minutes of adaptive absorption is applied. As far as I understand 4 hours would be preferable after a discharge to 50%. But as there are other days where the batteries only move down to around 80% and are fully charged again the same day, 4 hours absorption may not be the best option for every day? Do I understand correctly? And what is the best setting to use? Should I change the setting once a while to get a full 4 hours absorption?

The image shows a 50 minute absorption time today, after 6 days of bulk loading. The batteries where down to around 50% on the 17th. Thats when I turned off the fridge to get them charged again.

battery charging VRM

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Answer 1 · 2021-08-21T15:01:43Z

JohnC answered · Aug 21, 2021 at 03:01 PM

Hi @momonu

The actual time spent in Absorb isn't really that important. You've been putting charge back in over days, and that all counts too, whether Abs has been reached or not.

What is important is the current the batts can accept when the algorithm decides to finish Abs. That's the 'Tail Current', and you may find that your mppt even has a setting there that's overridden the algorithm. That's not a bad thing, actually a great feature to prevent too much time in Abs.

I don't know your batts, but my setting (floodeds) is equivalent to ~6A of your 190Ah. This is useless with concurrent loads interfering, but with a BMV or Smartshunt can be networked to pass on the actual battery current, so that can then be used for a reliable Abs termination.

What you choose is up to you. Adaptive alone works pretty well, but the Tail function is a step up. Study what's happening in your system and give it a try..

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momonu commented · Aug 21, 2021 at 06:51 PM

Great. Thanks a lot for your explanation @JohnC. I do have a SmartShunt installed. What I do not understand is the mechanism of the tail current. If the controller is using the current reading of the Shunt and there is a load, let's say typically 1-4A, and the charger delivers around 8-10A around noon, how would the 6A tail current be determined? Could you explain this a bit more? And why 6A? I would very much appreciate this. Thanks

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JohnC ♦ momonu commented · Aug 22, 2021 at 01:03 PM

@momonu

A snip from VRM of my mppt today..

It says 'Battery', but really means 'output'. The pv A tapering at Abs V. (43 min Abs). Some loads happening there too, but unseen. At ~12A it drops to Float, but that's not the setpoint..

From a Smartshunt at the batts..

What the batts are getting.. smoother A curve, which I hope you can see starting to level off just before 7A (the mppt Tail setpoint). Then it trips to Float.

The SOC at that 7A I know is ~99.0%. No point then going further in Abs, all I'll be doing is either heating the batts or boiling off water with more A than necessary. The 7A is my choice, and the 6A I offered you is a conversion from my 225Ah of batts (ymmv wih that). I don't care about the last 1% if I run out of sun, and the A in Float might get down to ~1.3A when the shunt is set to sync.

I use a CCGX with DVCC to pass the Smartshunt A across to the mppt. The Tail function just grabs that as a priority to drop from Abs to Float. It should work just the same with the BT networking.

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momonu JohnC ♦ commented · Aug 23, 2021 at 08:27 PM

Thanks a lot! Yes, I understand better now. But I do wonder if 6A do make sense for my setup. On some days like now, I am standing in shade the first half of the day and thus I will have only 3-4 hours with more than 6A current coming from my controller. Wouldn't it then be pure luck that absorption phase starts in those hours? The tail current would often be hit by not enough sun power. Or does it only use that if the battery is taking less than is provided?

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JohnC ♦ momonu commented · Aug 24, 2021 at 07:04 AM

@momonu

The 6A I quoted is just a guide, suits my reactive floodeds, and what you determine for your agms may/will be different. And as you're 'creeping up' on Abs over days, even just reaching Abs then is a challenge.

Just for fun I'll throw in some more snips (I love my VRM, but these from VictronConnect Trends (per-second monitoring)).

Today a cold, sunny morn, then intermittent cloud, this towards the end. A total of 8x 'rebulks' when the sun returns, and (look closely) the A levelling off as it approaches my 7A Tail setting and drops to Float. (The 6.4A at the very end just needs another minute to register).

Overall 2hr 28min in Abs (so far), unusually long for me, but it's counting time that it wasn't actually holding full Abs V. Recall too that I mentioned '~99%' at end of Abs. This snip where I wanna be..

This without a programmed sync for at least a month, but maybe reached 100.0% under it's own steam twice (just) in that time.

Your system is way different to mine, but my point is that you have some excellent algorithms and functions at your disposal to enable the finest of tuning. Study your system, test and refine your settings. Understanding what's going on is the key, and it's great you've asked, rather than just rolling in and saying "this doesn't work".

I wish you well with it. Keep asking as you need..

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momonu JohnC ♦ commented · Aug 25, 2021 at 07:58 AM

@JohnC Thanks a lot, your detailed explanation is very much appreciated. I will play around and see what works out well for me.

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