Outback Skybox with AC Coupled Enphase Inverters

[RCinFLA]

With disconnect PV power and grid connection, try a near 5 kW load on inverter (like water heater + clothes drier) to see how much battery voltage drops.

5 kW should pull about 120 amps from battery so a 280 AH LFP string should hand it with no problem.

Each 280 AH cell should not drop more than 0.1 vdc so 1.6v max for string, plus any other cable, breaker, or connections loss. Net should be less then 2v drop at inverter DC input between light load and 5 kW load, so not less than 48-49 vdc at inverter terminals.

That would check inverter and battery for excessive battery voltage drop. Should be ready to check voltage at inverter input, before and after BMS near batteries if it does have excessive net voltage drop to find where it is dropping.

Having a cheap IR temp gun to point at connections is a quick way to check for heating due to poor connections.

I would guess bad connection, bad bms, or bad cell. Beyond that it might be a intermittant issue in inverter.

Double check you don't have selling to grid enabled on inverter from batteries during the night.

 

[wtfg]

I put a 86.74 amp and 4422 watt load on inverter
IT read 50.9 volts before BMS and 50.99 at inverter a 0.3 drop
Inverter showed LBV 50.58
It lasted about 4 min
I can't think of a way to simulate a 130 amp surge
Ill try again tomorrow and find more amps to draw, but it seems like with this load its not dropping bad

 

[RCinFLA]

You should have less voltage at inverter unless charging battery.

You should first reference open circuit voltage of battery with at least 3 minutes of no load / minimal load before applying heavy load.

Unloaded battery voltage should be 52.0v to 53.5vdc

Even if the voltage is measured with 87 amps draw on batteries, 50.9v is fairly low, at 3.18v avg per cell. Check all the cell's voltages at this 87 amp load to see if there is an outlier cell or cells with lower voltage than others. Any abnormally low cell voltage may get much worse at higher load current.

Have you balanced your battery pack recently? If your BMS does not start a cell balancing dump until a cell reaches 3.4v and you have not charged total battery to 56v to 57v for some time you may have drifted out of balance.

A hybrid inverter with AC coupling usually likes to keep batteries below 90% state of charge so there is room for battery to absorb PV back surge power. This is in conflict with keeping LFP cells balanced when BMS does not balance below what is essentially a full charge level.

 

[GXMnow]

This morning at 8:08 AM I had a Non Recoverable Fault
Preceding it was a DAB-UVP Battery fault again than a Gate Fault

As RCinFLA asked I reset lowest battery voltage, it was 40.65V
So does this prove its the cables or BMS ?
Thanks

That 40.65 volt reading is very low. That is most likely the BMS went into protect shut off for some reason. A single cell going to low or whatever.

 

[Surferdudemi]

I don't mean to sidetrack this discussion, but I'm in the process of putting my system together and ran across a question related to this. The Enphase IQ7+ inverters are not split phase. I looked at the cable, which is 2 conductor, when drawing up the wiring diagram and thought "huh." Does that cause a potential problem when you AC-couple them in via the subpanel to a Skybox? When you're running on-grid, I guess the utility would just absorb or balance the load. But if you're running on battery, then the inverter has to balance the load somehow by boosting one side of the split phase, right? Is that a potential issue that would give you error codes?

 

[Ampster]

The Enphase IQ7+ inverters are not split phase. I looked at the cable, which is 2 conductor, when drawing up the wiring diagram and thought "huh." Does that cause a potential problem when you AC-couple them in via the subpanel to a Skybox? When you're running on-grid, I guess the utility would just absorb or balance the load. But if you're running on battery, then the inverter has to balance the load somehow by boosting one side of the split phase, right? Is that a potential issue that would give you error codes?

They are not split phase but they work fine with my split phase inverter. The nomiclature is consfusing because if I understand North American power the 240 legs are single phase. The legs only get split by the neutral tap on the transformer from the grid or the neutral from the inverter. If you are referring to the loads on each leg from the micros, there is no balancing needed because without a neutral there can be no imbalance between L1 and L2 from the IQ7s. My hybrid inverter powers a subpanel that has 120 volt circuits and it takes care of handling the imbalance on that sub panel. I distributed the loads on my subpanel so it is fairly closely balanced most of the time. The 6000 Watts of power generated by my solar is balanced and my loads rarely exceed 1000 Watt of 120 volt AC loads. My big loads are all 240 volts.

 

[GXMnow]

With my Schneider inverter, it has the big 120/240 volt output transformer to create the neutral while it is running off grid. It act like an auto transformer. The iQ7 inverters feeding in 240 volts is not a problem at all. Low frequency inverters are basically limited by how much current the neutral tap from the transformer can handle.

On a high frequency inverter, it might become an issue. If you had 3,000 watts of solar coming in from Enphase micros, but only had 2,000 watts of load, all on just one phase, what does it do? 1,500 watts of the load is basically coming straight from the iQ inverters, all the current, but at half the voltage. The unloaded leg sees that 1,500 watts as all back feed and is pushing it to the DC battery bus. And then the loaded leg has to pull 500 more watts from the battery bus to power the 2,000 watts of loads. Obviously there would be some losses, but you still end up with just under 1,000 watts left charging the battery bank. As long as both legs always see less than 2,500 watts, the 5,000 watt total skybox should be able to handle this situation.