question

Rasmus Mikkelsen avatar image
Rasmus Mikkelsen asked

Mppt goes to absorption before set voltage

Hi all, I'm struggeling to figure out this problem.

As you can see my batteries are not near full, my batt voltage is 55v, should hit absorption at 56.7v using 18650 lithium cells.

But system went from bulk to absorption already at 55v, that means the mppt is holding back on charge current.. If I turn on a 2kw heater you can see the PV power goes way up, and goes down when I turn it off.

Here's a video I took from last weekend, trying to illustrate it. I would want all the current going into my batteries, I only charge to 4.0v / cell currently at 3.9v so should just dump all it can at the moment. It's quite weird to me, bms charge target is 100a atm.

https://youtube.com/shorts/Z2UW2XZulUI?feature=share

If I set absorption and float higher in the app, it kinda ignorer this, I just tried to set it to 58.8v for both abs. And float and reboot mppt, same goes directly into abs. Mode. And tapers down current

I hope some of you can help me out with this strange problem :-)

Have a great day

20221002-105333.jpg



MPPT Controllers
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5 Answers
Rasmus Mikkelsen avatar image
Rasmus Mikkelsen answered ·

I see my picture didn't go through

It is possible i have a setting somewhere i have missed, im trying to learn :-)
screenshot-20221002-105321.jpg20221002-105501.jpg


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Alexandra avatar image
Alexandra answered ·

@Rasmus Mikkelsen

The main difference bakeetween bulk and absorption is the current drawn by the battery.

Do you have a shunt as well?

If the internal resistance is going up then the amps going to the bank goes down.

I dont see a tail current set? Is there a reason for that? This thread discusses more on its effect.

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Rasmus Mikkelsen avatar image
Rasmus Mikkelsen answered ·

Hi Alexandra thanks :-) I have the batrium shunt, and currently 400ah of batteries. Uhm tail current, I was under the impression that wasnt for lithium hmm, will have a read

I guess my problem currently is down to mppt going into absorption at 55v, while it's set to 56.7v.

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Alexandra avatar image Alexandra ♦ commented ·

Tail current switches it into float at the end. But for it to go to float is has to reach the charged voltage. So not an immediate thing right now.

Have you checked what voltage is shown/detected at the mppt terminals?

How balanced are the cells?


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Rasmus Mikkelsen avatar image Rasmus Mikkelsen Alexandra ♦ commented ·

Tail current makes sense now thanks.

Cells are within 0.02-0.03v, voltage at mppt is 53.13v atm, mppt showing 53.1 and my touch screen reporting 53.1 aswell. Actually battery voltage is around 53.2 for some reason. Yea I find it strange that it's going in absorption before my set voltage, can't quite figure that one out.

This is the inverter settings, if that has anything to do with it, I'm not sure

I thought maybe it had something to do with the setting: State of charge when bulk finished, changed that from 95% to 99% because 55v/56.7v = 97% that would make sense to me to stop absorption then(I was within the last 5%). But now at 99% its more like 56.2v when bulk finishes. Still not behaving as I would think it should.


The system has been running for little over a week offgrid now, the time for them to approved the system to grid is upwards of 12weeks now due to everyone wanting solar because of the energy problem here in EU. Atleast I can test it out and charge my car :-)

I kinda wanted it to reach full battery to see if my bms behaves as it should, and start balancing and so on. But havent been able to so far because of some setting, or me not fully understanding it propperly.

Sorry for the screen pictures


Edit: what would happen if I change the charge curve setting from fixed to adaptive within VE configure 3

20221002-162005.jpg

20221002-161852.jpg


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Alexandra avatar image Alexandra ♦ Rasmus Mikkelsen commented ·

@Rasmus Mikkelsen

Unless you are charging with the victron inverter the settings there wont change much either.

However a good test may be to try charge from AC and see if the battery will take more.

Just a suggestion, reset up the mppt but do it this way instead. On your mppt go to the battery presets - use the lithium preset. Then use expert mode to tweak the voltages from there. Save the new setup profile for your battery pack.

see of the behavior changes.

whith your battery pack do you have a 15s or 16s setup?

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Rasmus Mikkelsen avatar image Rasmus Mikkelsen Alexandra ♦ commented ·
Good evening, thanks I will try that tomorrow. Yea I figured it was only when inverter charging.

Just to be sure, the turn knob on the bottom side of the mppt is only there if no app control is used, right? I mean, the settings in the mppt from app will overrule that turn wheel / knob thing. I think it's at 2 from factory. I will try a new profile tomorrow.

Battery is 14s @ 3.7v nom. Can be charged to 4.2v

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pwfarnell avatar image pwfarnell Rasmus Mikkelsen commented ·

SOC and voltage do not work like that, you can not divide voltages to give SOC. See the attached typical charging curve for lithium cells, you need ti find the one for the cells you are using. The last few % of charge really kicks up the voltage.

charge.jpg

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Rasmus Mikkelsen avatar image
Rasmus Mikkelsen answered ·

Hi all, I am also wondering why my victron connect app says that mppt is externally controlled. Is this because of my bms (Batrium)? It's wired via can to Cerbo GX

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pwfarnell avatar image pwfarnell commented ·
Yes, this explains why your MPPT settings are being ignored, the BMS is controlling the charging and you need to make your changes in that device. The BMS should also control the Multiplus. See my comment on SOC vs Voltage above, do you have the correct specs for your cells, do you know the correct settings because you appear to have some confusion on the basics.
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Rasmus Mikkelsen avatar image Rasmus Mikkelsen pwfarnell commented ·

I know the correct specs of my batteries. I don't see my self as an total newb, but only had the system running for little over a week, and trying to correct a few things :-)

The bms software is also new to me, and not very familiar or experience with it yet, will come with time. I however believe I have all the correct settings in the bms. Will double check when home after work.

I'm wondering if there's a bug in the bms software, because one of the charge settings is set to 56.00v and it won't charge higher than 55.00v atm.

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Rasmus Mikkelsen avatar image
Rasmus Mikkelsen answered ·

Just to follow up on the problem.

I stumbled upon this topic. What solved my issue was, go into settings and DVCC menu, change that voltage setting there.

I would assume this has solved it for me, just waiting for some sun to see, fingers crossed.

Thanks all for helping

https://community.victronenergy.com/questions/12525/mppt-networked-operation-charge-voltage-to-low.html

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