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ben-stein asked

Multiplus II battery consumption watts wrong, but amps and volts correct

I'm trying to track down a behavior where the DC power consumption shoots up when a large load is placed on the inverter. In this case, the load is a 15,000 BTU RV air conditioner. It's wreaking havoc with the math:

1682363113881.png

The MPII is showing 12.83 volts and consumption of 140.7 amps of current. 12.83*140.7 = 1,805.2 watts. But, as you can see in the screenshot. The Multiplus is reporting 1,542 watts. That happens to be exactly the wattage of the AC load, but seems to ignore efficiency overhead (which is larger than I would have expected, but...). As a result, the mismatch seems to be being lumped into DC Power. That makes sense since we know that's a calculated value of the "leftover" power. As a result, DC power is being over reported by 250-300 watts when there's a large load on the inverter.

An additional oddity, the overall math doesn't match up either. If I look at power sources, solar and battery, they total 2,163 watts. If we then look at AC loads or battery power from the MPII plus DC Power we get 1525+340 = 1,865. So, where did the other 300 watts go? I know there's a discrepancy between the inverter's watt draw reported and actual, but that should already be accounted for in the inflated DC Power number.

I'd love to figure out what's happening. I'd also love for the system not to be losing 600 watts in efficiency overhead, but that might be wishful thinking.


-Ben

Multiplus-IIVRM
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nickv answered ·

Hi Ben. The air conditioner is an inductive load, so on first impressions, I guess you've got some power being used as "reactive power". This might help explain more detail https://www.victronenergy.com/media/pg/The_Wiring_Unlimited_book/en/ac-wiring.html#UUID-58ef59a4-8ce3-16c1-2cfe-01bc3a723908

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ben-stein avatar image ben-stein commented ·

Thanks Nick, I appreciate your response. I think you're right, some of the losses are likely the result of inductive power. But, that doesn't explain a few things. The first and most prominent is the apparent calculation error in the battery stats reported for the MPII. In DC, V*A=W, so battery voltage times battery amps, should be watts, but it's not. That discrepancy seems to be driving others. Or, maybe I'm being thick and missing something obvious.


Ben

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nickv avatar image nickv ben-stein commented ·

I agree, there are some strange calculations going on there. Even the sum of PV doesn't seem right - should it be 691 or 725? It's probably W received from the panels totalling 725 but W sent to the batteries after efficiency losses coming to 691. The MPII Battery section of UI doesn't seem to match the Discharging box. It could be a BMS voltage reading vs the internal MPII DC bus voltage reading.

Despite that, I think you could be seeing a 5 - 10% efficiency loss inverting to AC so maybe as much as 150W. The power factor expected at residential houses is 0.8 but if your air conditioner is the majority of your consumption then the power factor could be worse. Even at 0.8 you could be inverting an extra 380W to provide the VA to the AC.

I've spent many hours being equally puzzled by the miss-matching values reported by my system so I sympathize. I even bought two ET112 AC meters to try to sort it out but there are still niggling discrepancies. I've stopped looking for reasons and put them all under "system inefficiencies" so I can have some closure. I probably won't be of much more help so good luck with your energy hunt!

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