220 Watts From Battery & 300 Watts From Inverter. Is this a 136% Efficient Mutiplus II?

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bipedalprimate asked Jan 28, '24

220 Watts From Battery & 300 Watts From Inverter. Is this a 136% Efficient Mutiplus II?

DC Watts are measure by a SmartShunt and displayed in VictronConnect, VRM & Venus Console.

AC Watts are shown as Load in VRM & Venus Console.

There are no other DC or AC sources active. Multiplus II AC In is disconnected by circuit breaker.

There are two Multiplus II 40/3000/32 in parallel.

DC Amps has been observed via Clamp Meter and corresponds to 220 Watts. (4-4.5 Amps at 52-53 Volts).

DC Watts are also via a BMS & correspond to 220 Watts allowing for the inaccurate current sensor on the BMS.

Trying to understand - any ideas?

The weight of evidence suggests that the Multiplus II measurements are not correct.

Placing a load on and connecting AC In improves the balance:

When the large load disappears, the balance is retained.

The discrepancy outlined at the start of this post can be induced by simply turning the AC In circuit breakers off.

Does the Multiplus II need to have AC In active to accurately record loads? Or is there is something else going? A wiring issue? But what kind of wiring issue? Seeking some clarity or guidance.

Multiplus-II SmartShunt

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Answer 1 · 2024-01-28T10:16:48Z

nickdb answered · Jan 28 at 10:16 AM

It is fairly well known that at low power levels the inverter is not very accurate, so with idle loads I would not expect precision.

There are various related posts on the forum.

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bipedalprimate commented · Jan 28 at 10:30 AM

What doesn't make sense to me is why it only happens when there is no AC In present.

Tried searching for "Multiplus current measurement" but couldn't find something similar on the forum. Do you have an example post?

Is 200-300 watts an idle load? It is enough to power fridge, lights, computers, etc. Under 50 watts I would consider an idle load. Anyway, pedantics :-)

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Answer 2 · 2024-01-28T12:08:52Z

JohnC answered · Jan 28 at 12:08 PM

@BipedalPrimate

Try enabling the 'Has DC System' in the GX. This will add a 'DC Power' tile to the Dashboard and show the amount that the system itself determines as 'missing' or 'surplus'. There's a widget for it too. It will vary a lot, as the data isn't all sampled at the same time - this is an issue when you show us a single point in time, as it's virtually never representative of an average scenario.

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bipedalprimate commented · Jan 28 at 09:45 PM

I was conscious of trying to capture a representative snapshot - the first snapshot above was consistent for well over 6 hours.

I am just trying to understand why it only happens when AC In is disconnected.

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JohnC ♦ bipedalprimate commented · Jan 29 at 04:57 AM

@BipedalPrimate

The DC tile thing is only offered as a suggestion to help you find/understand your issue.

I often wonder what's happening on my own system (but I'm not consumed by it). You see some things that you think can't be right, but eventually realize that what's been done is logical and good 'best practice'. A small example is this (from your own pic)..Battery symbol, so DC, but doesn't multiply out. Hmm. Maybe with DVCC in effect, the V and A is coming from a shunt? DC Watts isn't measured by Multi directly and is an estimate (that's been documented somewhere, but don't ask me to find it). On my system when nothing else happening at night that W closely matches ACOut, so I suspect that's where it originates. So this tile may not actually represent any thing coming directly from the actual DC port. But it's still relevant. 'Best Effort'..

That W on mine is about 30-40W lower than the shunt's (typical standby/inverter loss) over a 200W load, but the DC Power tile shows 0 W, so I *think* there's been an allowance made to calc that.

In yours, somewhere in there is possibly an allowance made for inversion losses. Like Finding Wally.. where is he put?

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bipedalprimate JohnC ♦ commented · Jan 30 at 01:05 AM

@JohnC

Thank you for that.

I am quite happy with the measurements shown when AC In is present. (Not needing to Find Wally in these circumstances even though he is in there somewhere :-) )

When AC In is disconnected things go weird.

Do you have any thoughts on why this might be?

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JohnC ♦ bipedalprimate commented · Jan 30 at 07:49 AM

@BipedalPrimate

That particular pic suggests a faulty shunt reading. Try a Zero Current Calibration on it.

Unless the bms is providing those main battery figures??

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bipedalprimate JohnC ♦ commented · Feb 22 at 03:04 AM

Thanks @JohnC

I had already validated the SmartShunt current readings as being correct & had done a Zero Current Calibration yet the discrepancy persists when AC In is disconnected.

The BMS does not provide accurate current readings so am using the SmartShunt as Battery Monitor.

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