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haig avatar image
haig asked

Total Multiplus out put (AC and DC) limited to BMS charge limit

Victron Multiplus limiting total output to SOK 48V 100ah server rack battery (16 cell Pylontech BMS) charge limit.


System is off grid with 3kw of Solar and 20Kva Generator for backup. Cerbo running v3.00-22 also tested with v2.93


I have 3x Multiplus-ii 5000 with a set of SOK 48v 100ah batteries commanding a charge rate of 95a (well below the 210a max available from the Multis when charging via generator) that's great, I'm happy with the 95A however when I place a 4kw AC load on the charge rate of the battery goes down to about 1100w or 20a. Still giving a total output of the commanded 95a.


The generator is only running at 25% capacity so it's not a genny issue.


The way I understand is that the Multi s should be providing the 95a in DC to charge plus an additional 4kw to the AC load?


Now, if I remove the load the batteries continue to charge at the low 20a rate. Turn off the Gen and restart full 95a to the batteries again until a load is applied.


Current limit by switch is off on the Cerbo and each Multiplus is set to max charge rate of 70a.


Any ideas why total max output (AC and DC) is limited by BMS charge limit?

Multiplus-IIcerbo gxBMSoffgrid
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Alexandra avatar image Alexandra ♦ commented ·
@Haigand your input current limit on the inverter?

Do you have weak ac checked?


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haig avatar image haig commented ·

Sorry @Alexandra input current limit set to 70a on each of the 3x Multiplus-ii 5000/70

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4 Answers
Rod avatar image
Rod answered ·

1…

Please confirm if your 3 Multi’s are in parallel, or 3 phase ( Delta? Wye?).

2…

Doublecheck in your manual if the maximum output IS 5 KVA each; 15 KVA all together including both loads:

- AC load, 4 KVA

- Battery Recharging load. (20Ax57.6V ≈ 1.15KVA)

- Total 5.15KVA, equivalent to only one multi

3…

70A x 57.6V ≈ 4KVA limit per multi

Three paralleled multi’s limit should be 12 KVA


4. Check why are the other 2 multi’s NOT contributing with their share. Is it possible that the 70 A limitation is imposed over the 3 multi es all together ? It should be on each


5…

Seems that your 20 KVA genset fits the 3KVA from solar x 6 hours a day ( 21KVAH)

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Kevin Windrem avatar image
Kevin Windrem answered ·

Are you maybe using the Multi's internal battery monitor to determine battery current (and SOC)? If so, with your DC loads the Multi has no way to determine that some of it's DC current out is going to loads.

If your battery bank has an internal battery monitor, you may need to manually select this in Cerbo. If your battery does not have a battery monitor, you probably need to install one external to the Multi so it knows how much current is flowing in/out of the battery. You'll need a Victron BMV or SmartShunt so it will talk to Cerbo.

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haig avatar image
haig answered ·

Thanks @Rod the Multis are in 3 phase Wye. As for the generator it's a full 20Kva 3 phase 380v diesel unit.


Thanks for your suggestions. I'll investigate

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haig avatar image
haig answered ·

Thanks @Kevin Windrem I haven't checked but think you are right re batt monitor. Right now as per the Pylontech DVCC guidance I have multi bat monitor selected on and using the Pylontech battery shunt as the shunt (there is a Lynx Shunt in the system that was disconnected after reading @Guy Stewart (Victron Community Manager) advice about guy with two watches not knowing the time).


I'm sure you are correct about the Multi not knowing where the output is going. I'll take a look and report back. Thanks again

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