question

cajanus avatar image
cajanus asked

Bulk never ends, sits at 85%, keeps charging from grid overnight

Hi,
What is supposed to trigger the end of Bulk charging?
- Voltage reaching Absorption voltage?
- Low charging current?
- SoC?
- Some combination of the above?

I have a single-phase MultiPlus Compact 24/2000/50 which is AC-coupled to a 2kW Fronius MPPT. Batteries are 300Ah NiFe (nickel-iron).
The MultiPlus is the only device connected to the batteries. There are no other chargers or loads or BMSes.

Six months ago, the MP used to finish Bulk charging and occasionally go into Absorption as scheduled. All good.

But now it stays in Bulk forever (32V, 6A, SoC=85%), continuing charging all night from the grid.
Absorption voltage = 32V
Float voltage = 28.5V
SOC when bulk is finished = 85%
Assistant = Self-Consumption Hub-2 V3 (0182)

Could this be a bug introduced when I last upgraded all Victron software?

If I reload the Assistant, on the first day, it "Disconnects From The Grid At Night" at sundown and starts inverting as expected. From day two onwards it just stays in Bulk, charging overnight.

Thanks,
Mark



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1 Answer
Mike Dorsett avatar image
Mike Dorsett answered ·

Try looking at the multi's battery settings - in particular, "stop after excessive bulk".

Bulk normally terminates on reaching both the absorption voltage and the charge current dropping to a low level - normally about 2 -5 % of Ah capacity set.

Also - test the charge current into the battery as the charge ends - if the current is not dropping, then there may be a battery issue with one cell not reaching full voltage.

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cajanus avatar image cajanus commented ·

Thanks Mike.
How many times have I seen "stop after excessive bulk" and not read it? I have now set it. So far no change in charging behaviour, but it has been cloudy for the past few days.
I wonder what it considers to be "excessive"?


Bulk normally terminates on reaching both the absorption voltage and the charge current dropping to a low level - normally about 2 -5 % of Ah capacity set.

2% of 300Ah = 6A. While sitting on 85%, it sometimes drops to 5A but continues charging.


Also - test the charge current into the battery as the charge ends - if the current is not dropping, then there may be a battery issue with one cell not reaching full voltage.

All 20 cells sitting on 1.59V. I don't think sitting on 6A is a battery fault, just NiFe chemistry. Probably just wasting power making hydrogen.


Sunnier today: it has been sitting on 85%, charging 5A or 6A @ 32V for 3 hours. Why does the MultiPlus stop counting SoC? SoC should be 90% by now.



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JohnC avatar image JohnC ♦ cajanus commented ·
@cajanus

That 1.59V/cell you've measured is ever so close, but not quite 32.0V. Try lowering it a little so that the Multi can actually register reaching Absorb. Then it can use whatever algorithm you've selected to determine how long Absorb continues for. I'd be inclined to use a Fixed time for your unusual chemistry.

Afaik the Multi doesn't use Current to terminate Absorb, that's a great function though in the Victron dc mppts. But you can use it to determine what you set as a fixed Abs time. No point making hydrogen.

The Multi's SOC is a fairly coarse estimate, the 85% (default?) setting is how it syncs itself. But it needs to see Abs V to do that, and only then will increment SOC to a higher level..

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cajanus avatar image cajanus JohnC ♦ commented ·

Thanks so much John, I think you've cracked the problem for me (via Mike's suggestion to measure cell voltages): VAbs was too high.

Yesterday I set VAbs to 31.8V and today Multiplus was again Bulking away at 32V. SoC had reached 83%.
I dropped VAbs to 31V.
Instantly, Multiplus went into Absorption mode and SoC jumped to 85%. It has since climbed to 86%. I'll increase VAbs back up to find the right setting.

The Multiplus charger seems to charge at 0.4V above VAbs up to a max of 32V. Is that normal?

Yes I'm using Fixed charging mode.

Setting VAbs=32V was the battery supplier's recommendation. I'll let them know.

Thanks again. That makes life much easier.

Mark

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JohnC avatar image JohnC ♦ cajanus commented ·
Good work @cajanus

That 0.4V difference you mention might be typical of lead batteries using Temp Compensation, but I've no idea what you're doing there with your NiFe's. May even be the reason your 32V Abs isn't reached? (because the setpoint is now actually 32.4 with TC, but it won't show that anywhere).

The 85% SOC thing is a default setting for Pb's, and you could also look at changing that to suit your different chemistry.


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colin-foster avatar image colin-foster cajanus commented ·
Recently reset Abs from 31.5 to 32 and got the same problem you have had. Did not realise this had caused problem till reading you post. Have now set it back to 31.6 and all back to normal. I have Abs SOC set to 97% fixed curve and 5 hour absorbtion. The 5 hours is not usually complete now we are moving in to winter but am adding more panels to catch winter sun. Wales UK.
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