Voltage Fluctuations with Phoenix 250va and MPPT 75/15

Q&A and troubleshooting
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1

Howdy!

I’m setting up a lab and am using an MPPT 75/15 with a LiFePO4 6Ah battery to power a Phoenix 12V 250VA 120V. Currently I have a Phoenix Contact Quint Power 24V 20A power supply connected to the PV portion of the MPPT to simulate the panels:

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I have a SCADA setup through a Venus GX. I am monitoring the Output Voltage using the Inverter’s 3101 tag (Output Voltage) and Output Power using the Venus GX’s 817 tag (AC Consumption L1).

Voltage seems stable whenever there’s no load. It’ll bounce between 119.9 and 120.1 VAC:

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When plugging in a cellphone power supply rated at 0.8A input, it’ll fluctuate more, but the inverter won’t normally cut off:

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When I plug in a power supply for a laptop or managed switch rated somewhere between 180W and 200W, it will bounce wildly on both power and voltage. Max 135.3V Min 97.9V. This is when pulling less than 70W most of the time, according to AC Consumption L1. It’ll also cut off repeatedly:

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What is this? Have I set up the system incorrectly? I have checked and rechecked the wires and connections and they are stable. Input voltage is steady the entire time.

Could this be a bad inverter?

Thanks!

2

No, i think its more a much too small battery.

Pls compare the datasheets of the battery and the inverter, i think you overestimated the power of an 8Ah battery

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3

Thanks Ludo! I will try a car battery for testing and see if the fluctuation disappears.

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4

Inverter must be connected directly (via fuse) to the battery.

The load terminals of the mppt are not rated to handle an inverter.

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5

The manual for the VE direct inverter gives the min battery size as 30Ah for the 12/250. A lithium battery slightly smaller than this may work. Also note the wire sizes, if your are too small the voltage drop can cause problems.

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They are rather small! Thank you, will reconfigure and take a look at the datasheets for everything to ensure I’m good on ratings.

E: We are now working correctly with the fused direct connection from the battery! There’s some voltage fluctuation still from the size of the battery, but not enough to knock the inverter over. Will be adding a larger battery soon as well. Thank you all!