Paul Beskeen avatar image
Paul Beskeen asked

Parallel Multiplus-II - what is acceptable level of imbalance between two units?

I have my 2x MP-2 48/5000/70-50 parallel system up and running. I note that there is between a ~4-7% imbalance between the current used by the master vs the second unit. Is this normal/acceptable or cause for concern?

For example using a clamp meter I can measure DC 70.3A (master) and 66A (2nd unit) on battery charging. With AC at 19A (master) and 18A (2nd). Loads bounce around, but on average it seems roughly about a 6% imbalance, always with more on the master.

My setup is documented further here:

So good enough, or need to fix? If I need to tweak are there alternative options to installing unnecessary additional lengths of AC feed cable?

Thanks, Paul.

Multiplus-IImultiplus in parallel
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Phil Gavin avatar image
Phil Gavin answered ·

Ideally, you'd want things spot on, cable lengths precisely equal and equally torqued connections.

In reality, you're probably not going to achieve that precisely.

So I'd say it is safer that the master unit gives the current limiting commands, and it is the master unit that sees the higher current, which is how you have it now.

I think the downside of this is that you will throttle both units for the master unit's measurands and miss out on a few % of available capacity because of uneven heating between the units.

Obviously, at lower loads, there will be less of an absolute difference in the currents.

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Paul Beskeen avatar image Paul Beskeen commented ·

Thanks Phil,

My main concern was that a slightly imbalanced system could cause a deleterious effect on the inverters. From what you are saying this does not seem to be the case, just slightly more loading on the master.

In terms of trying to even things out for the last few percent capacity, perhaps a bit "suck it and see", but I could try shortening the 2nd unit AC cable, and depending on any measurable effect, also try lengthening the master. As the AC cables are relatively short though, the difference in the inverters internal resistance probably dwarfs most things I can do with small AC cable length changes. So perhaps good news that perfectly balanced doesn't seem to be vital!

Cheers, Paul

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