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alaskannoob avatar image
alaskannoob asked

What controls charge going into batteries in DVCC?

I have 8 x Pylontech US5000 in my Victron system set up with DVCC and my MPPT is a SmartSolar 450/200. I was under the impression that the batteries told the MPPT how much charge they wanted. But I've noticed that when the batteries charge up to about 90%, the current going into them gets severely limited despite a) there being plenty of PV available and b) the CCL set by the batteries is much higher.

Just now I recorded the batteries throttled at 90% where only about 7 amps was going into them at 90% SOC, and the MPPT was showing only 707W coming in. So I turned on a space heater to confirm, and instantly the MPPT ramped up to 2000W to cover the load.

So does the MPPT lower the charge current independently of what the batteries are asking for? That seems to be the case.

DVCC
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mondeoman avatar image
mondeoman answered ·

52.4 V is the maximum voltage that Pylontech batteries are charged to in Victron systems. And that is just fine, even if the batteries are requesting 53.2V.

Up to 52.4V, the charger works in constant current mode. It just pushes amps into the battery up to a maximum limit (the lowest between battery requested CCL, DVCC charge current limit entered by the user or the max. charge current configured in the MPPT charger or the illumination on the panels).

When the voltage reaches 52.4V, the charger enters the constant voltage phase. It maintains this constant voltage at its output and that obviously means fewer and fewer amps can be pushed into the battery. In the "constant voltage" phase, it may appear that the current is throttled by the charger.

This is the "CC/CV" charging algorithm that is standard procedure for charging Lithium batteries.

Now if you put an additional load, the voltage has a tendency to drop below 52.4V and the charger begins to push amps to raise and hold the voltage to 52.4 V.

PS: You can't tell anything about the cell balance at cell voltages below approximately 3.45V. You could have an imbalance of 10% state of charge and the cells would have very, very close voltages.

cc-cv.png


cc-cv.png (31.1 KiB)
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nickdb avatar image nickdb ♦♦ commented ·
You are right. I keep forgetting this pylon quirk.

So SOC is inaccurate.

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alaskannoob avatar image alaskannoob commented ·

In my video you see the battery voltage is 52.32 which would mean it falls under the constant current model in your explanation.


Which you state means it pushes amps into the battery up to the lowest of:

a) CCL (which is 160A in this case)

b) DVCC charge current limit entered by the user

c) or the max. charge current configured in the MPPT charger or the illumination on the panels

Without going in and checking any settings for b) and c) above, I assume those are good given that my battery routinely charges with the MPPT pushing 140-150 amps into it. If b) or c) above was the issue, I would never see that kind of current from the MPPT going into the batteries, correct?

I went into the DVCC settings and confirmed I am not limiting the current there.

dvcc.jpg

I also went into the MPPT settings and see the charge current is not limited (it's 200A because that is the max this MPPT can put out).

mppt.jpg

So I'm at a loss why the MPPT was only charging with less than 10A when the battery requested CCL was 160A.

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dvcc.jpg (56.0 KiB)
Alex Pescaru avatar image Alex Pescaru alaskannoob commented ·
The difference between 52.32V and 52.40V is the drop on the cables.

For sure at the MPPT output, the voltage is limited on 52.40V (set by the system), but those 80mV are the drop on the cables, so at the batteries are only 52.32V.

For sure you are on the constant voltage stage as mondeoman explained.


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alaskannoob avatar image alaskannoob Alex Pescaru commented ·

That makes sense. It certainly behaves that way.

Thanks to everybody and especially @MondeoMan for explaining this to me.

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kevgermany avatar image
kevgermany answered ·

The batteries do it. They override the settings you enter.

In VRM advanced try plotting SOC and CCL in a custom widget. You'll see they coincide.

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alaskannoob avatar image alaskannoob commented ·
But if they do it, why do the batteries set a CCL of 160A while the MPPT only provides 7A (and yet tons more amps are available via PV)?


Or are the batteries setting the limits in some way other than the CCL they send to the Victron system?

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nickdb avatar image nickdb ♦♦ alaskannoob commented ·

Have you limited CVL in DVCC at all? This is a common cause when people fiddle with values in DVCC and set it below what the BMS wants.

Check what the battery is asking for, voltage wise, and what the MPPT and BMS are recording.

If you set a CVL limit, remove it.


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alaskannoob avatar image alaskannoob nickdb ♦♦ commented ·
I don't think I have. If I had, then this limit would always be there right and so I'd never see 7KW going into the batteries as I see on a good PV day when SOC is below 90%. If I had set a CVL limit I'd be complaining about my batteries never charging right?
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Alexandra avatar image
Alexandra answered ·

@AlaskanNoob

Ok so it ramped up, did the charging also increase? They can to some degree control what they are receiving as individuals. So may not actually need more charge? So there are some things to verify....

What voltage is the bank at and how does that compare with SOC. Some Pylons do get SOC drift BTW. Did they take more charge when you turned on the load?

{Removed incorrect statement about CCL...}

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alaskannoob avatar image alaskannoob commented ·

No, the charge remained the same I believe.


Here is what I saw so you can see for yourself:

https://youtu.be/GsvqUdCTcXE?si=oJDcKbiNlRtw-2er

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Duivert NL avatar image
Duivert NL answered ·

looks completly normal to me, batterys are almost full and bms then decrease charge current

mppt's can deliver more power but there is nothing else to feed, if your allowed to feed back to grid then the surplus would go there

my grid connected ESS with pylontech's do exactly the same, charge current gets lower the fuller the batterys are only in my case mppt's will feed back surplus in to grid instead of throtteling down

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alaskannoob avatar image alaskannoob commented ·

I agree it looks normal, I'd expect the power to ramp down as the batteries get full. But if the BMS is decreasing the charge current, I thought I would see this via the CCL dropping in the system. I thought the batteries controlled what was fed to them by issuing CCL. So I figured I'd see that number ramping down in the system, not staying at 160A while only 7A is fed into the batteries.

I'm just trying to understand how it works a bit better. Obviously something else is limiting the charge to the batteries given that CCL isn't the driving force.

Maybe it's the voltage limit that is changing that I'm not considering.

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Related Resources

DVCC chapter in GX manual