question

Werner Frank avatar image
Werner Frank asked

MP2 is charging from grid for no reason

In general everything is working fine since 6 month.

But sometimes (feels like once in two weeks) the MP2 starts charging from grid with no reason at a low power rate.

Even if minimum soc is set to 0% (normally I have 10%) it suddenly starts charging when it is below 10%.

But most days it's not charging when it's below 10% and I can reach 5% with no charging or error.

Can anybody explain why this happens?

The battery SOC was 100% 2 days before.

It's a Multiplus 2 5000 with 2x Pylontech US3000C (same happens when it was 1x Pylontech months ago).

mp2-charging.jpg


Multiplus-IIbattery charging
mp2-charging.jpg (32.5 KiB)
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3 Answers
Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

If I see it correctly than your system is in "Sustain Mode":

https://www.victronenergy.com/media/pg/Energy_Storage_System/en/controlling-depth-of-discharge.html#UUID-6b8f380a-3e19-c040-9dad-673577d775d5

In daily usage you shouldn't go below 10%, that reduces the lifetime of your batteries.

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nickdb avatar image
nickdb answered ·

Have you confirmed that your settings are identical to the victron/pylon configuration guide?

Setting ESS to 0 is never going to work well, also, as much as pylon say you can discharge the battery low (and it is never to 0%) they don't commit that it will hold it's voltage under load.

ESS and the inverter are voltage driven, so while you have set a low SOC, you are most likely dropping beneath the set Voltage minimums which is forcing the system into a sustain or slow charge mode.

The pylon requirements want 100A as a minimum for a 5kVA multi, you have 70A with those 2 batteries so are hopelessly undersized. At 35A for 1 battery it would be worse and would likely have ruined the battery.

Driving an underspecced pack down to low SOC on that inverter will only cause these issues.

So either raise your minimum SOC or add another battery.

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Werner Frank avatar image
Werner Frank answered ·

Thank you Matthias for pointing me to Sustain Mode.
I will set SOC to 15% or more and check the voltages when it happens again.

That's a logical and sufficient explanation for that behavior. I did not take account the mode for some reason :(


@nickdb:

Yes, I was going through the Victron pylontech Guide step by step.
I also read about the 4x Pylo recommendation.
I'm no expert, so I'm interested why or in which situations a 5kVA can ruin the batteries? Specifically the Pylontech Battery.

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nickdb avatar image nickdb ♦♦ commented ·

Simple, batteries don't just have to supply power they have to be able to sink it when loads turn off. With a large disparity in sizing you can end up with a very differently shaped battery, because power has to go somewhere, and in many cases that is to the battery.

Too many size for capacity and fail to take discharge current limits into account, and almost always, charge current limits. They fail to consider transients, or even the effect of temperature on operation.

That is before you consider the effect of driving the battery voltage down under load and low SOC will have on its lifespan.

The sizing exists for a reason, and where you want a stable and long-lasting system, it is best to follow it.



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