Second opinion on Victron inverter plan

[Mantissa]

Hi all, apologies if this is in the wrong spot, first post on this forum.

I've got my plan mostly finalized for our school bus conversion's solar:



Yes, I know this is an absurdly oversized plan for any normal skoolie. Not particularly looking for verification - though feel free if you'd like, I think I got most of it right.

I'm debating between the following setups to provide 240V split phase:

• Parallel 120V 5kW Quattros (pictured above)
• A single 120V 10kW Quattro paired with a 100A Autotransformer
• A single 240V/60hz 10kW Quattro paired with a 100A Autotransformer

I'm not really sure which route to go. In my mind, the autotransformer route is the better one - load balancing is automatic and free, the 10kw inverter is more efficient and has a lower zero-load draw than two 5kws. System is easier to expand in the future to larger loads by adding another 10kw instead of two more 5kws. All systems cost about the same. My main concern is any additional losses from the autotransformer; Victron doesn't list an efficiency rating anywhere, and I've read that simple toroidal transformers like this sit between 98-99% efficiency. Hence listing both a 120V source with step-up and a 240V source with step-down; being an American, most loads will be 120V, with only our AC unit and welding machine being 240V.

Thoughts?

 

[timselectric]

I differ to our resident Victron Guru.
@sunshine_eggo

 

[740GLE]

WTF are you planning on running a welding shop out of this bus?

Also double check your wiring schematic of those PV arrays, you show 4S but I think you mean 2S2P. Also the VOC of those panels are way too close for a 100v, you’d need 150v at a minimum. Or is that image of a 100/30 just a stick image?

 

[Mantissa]

WTF are you planning on running a welding shop out of this bus?

Also double check your wiring schematic of those PV arrays, you show 4S but I think you mean 2S2P. Also the VOC of those panels are way too close for a 100v, you’d need 150v at a minimum. Or is that image of a 100/30 just a stick image?

You caught me, I was too lazy to switch the picture with a 150/60. I originally calculated the amperages wrong, I was taking the input amperage of the PV, not the output amperage to the DC bus. I'm pretty sure two 150/60s will do the job.

And, no, I'm not even a professional welder lol. I think the accurate phrase would be "well educated idiot". We're going full electric for this bus; AC, inductive stovetop, oven/microwave, hot water heater, only propane for emergencies or outdoor cooking really. I also tend to dabble in stuff I probably shouldn't with an off-grid setup, and I really don't want to blow an inverter. Welding, 3d printing, homelabs, lots of woodworking and metalworking, the kind of stuff you wouldn't want to point at a crummy little AIMS inverter.

 

[sunshine_eggo]

Hi all, apologies if this is in the wrong spot, first post on this forum.

I'm debating between the following setups to provide 240V split phase:

• Parallel 120V 5kW Quattros (pictured above)
• A single 120V 10kW Quattro paired with a 100A Autotransformer
• A single 240V/60hz 10kW Quattro paired with a 100A Autotransformer

Something that's not readily apparent is that an AT has a single leg limit of 28A, i.e., you can't have more than 28A on N. That's only 3360W imbalance between L1 and L2.

The Quattros > 5kW can't be paralleled except for 3 phase power.

I'm not really sure which route to go. In my mind, the autotransformer route is the better one - load balancing is automatic and free

Up to the 3360W leg imbalance. Beyond that, and you're screwed.

, the 10kw inverter is more efficient and has a lower zero-load draw than two 5kws.

My 2 5kW are about 54W total - around 27W each.

System is easier to expand in the future to larger loads by adding another 10kw instead of two more 5kws.

Nope. Per above, the 10kW can't be paralleled.

All systems cost about the same. My main concern is any additional losses from the autotransformer; Victron doesn't list an efficiency rating anywhere, and I've read that simple toroidal transformers like this sit between 98-99% efficiency.

Yep.

Hence listing both a 120V source with step-up and a 240V source with step-down; being an American, most loads will be 120V, with only our AC unit and welding machine being 240V.

Thoughts?

Plenty - mentioned above.

More:

You can't put 4S 24V panels on a 150V controller. You need to go with a 250V controller. "24V" panels tend to be around 45-50Voc. 2S2P as referenced by @740GLE would work fine.

It takes special settings to be able to charge from 120V on a split phase system. In the Victron System Configurator, you will need to disable "switch as group" on the AC input you wish to charge 120V from:



This may also be possible with VictronConnect, but I don't use it.

You don't want to use the VE.CAN for the battery. You want to use BMS-CAN.

If you get VE.Can capable MPPT (not sure if they're available as 150/60), they can be daisy chained to the Cerbo VE.Can port and will automatically communicate/coordinate charging. This is more robust than the coordination you get with a VE.Smart network and can handle up to 25 MPPT.

150A mega will pop. 10000/51.2V/.85 = 230A Fuse should be 1.25X that and Class-T IF you regard the individual battery breakers insufficient.

Lynx fuses and wires to the inverters need to handle more current. Here's the wiring bit from the manual:



200Ah of batteries is pretty lean for a dual 5kW system. Being limited to 100A each, they can't actually supply the needed current to get max power. Recommend 4X batteries.

"main AC cutoff" on the Victron (+) wires doesn't feel right. Note that if either inverter is shut off or disconnected, both go down. If one is taken offline, the other must be reconfigured to operate as a solo 120V unit.

If you stick with VE.Direct cables, don't forget to order them separately.

You'll need a MK3-USB adapter.

 

[timselectric]

And BOOM, goes the dynamite. lol

 

[Mantissa]

@sunshine_eggo you're amazing, glad I posted.

• 2x5kw it is! That's a pretty big downside of the AT.
• The main AC cutoff switch was intended and diagrammed as double pole, though I am realizing that I should probably put two single pole switches next to one another in the event that one of them blows and I need to reconfigure the remaining inverter to run as a single unit. Redundancy and all that. What seems wrong about it? Is it just unnecessary?
• Thanks for pointing out the panels, I read about Voc and whatnot after I made this diagram, totally missed that error. For the price we're at I think I'll just pick up one of the SmartSolar RS 450 | 100 units, it'll be cheaper than having two 250 | 60s, and I can still have separate strings. I'm gonna set these panels up to tilt, so they may be at different angles... Depending on how I do the racks...

One last question for you if you happen to read back on this- 4 batteries seems like a lot of storage for a 4kw system. Am I wrong there or could I get away with 3?

 

[sunshine_eggo]

@sunshine_eggo you're amazing, glad I posted.

• 2x5kw it is! That's a pretty big downside of the AT.

Yep. I debated that myself. @the_colorist is a Victron pro, and he discouraged me from it for good reasons - some of which I have likely parroted here.

• The main AC cutoff switch was intended and diagrammed as double pole, though I am realizing that I should probably put two single pole switches next to one another in the event that one of them blows and I need to reconfigure the remaining inverter to run as a single unit. Redundancy and all that. What seems wrong about it? Is it just unnecessary?

AC cutoff on DC lines?

• Thanks for pointing out the panels, I read about Voc and whatnot after I made this diagram, totally missed that error. For the price we're at I think I'll just pick up one of the SmartSolar RS 450 | 100 units, it'll be cheaper than having two 250 | 60s, and I can still have separate strings. I'm gonna set these panels up to tilt, so they may be at different angles... Depending on how I do the racks...

I think you should go 2S2P and stick with 2X 150/60. People get misplaced hard-ons for high series string voltage. It's actually less efficient as you increase PV voltage. The individual strings can still be at different orientations with no or negligible impact, i.e., in the 2S2P array, each set of 2S panels could be at different orientations, and you would not experience a significant performance penalty. 2X 2S2P arrays, each on their own MPPT, gives more tilt flexibility than 2 4S strings on the 450/100.

One last question for you if you happen to read back on this- 4 batteries seems like a lot of storage for a 4kw system. Am I wrong there or could I get away with 3?

You have 4kW PV and 10kW output, so you tell me?

With 4kW solar and a typical 5 hours of solar per day, that's 20kWh of daily harvest. That implies that's your daily need. If you need 20kWh of PV harvest/day, would you not need 20kWh of battery to get you through the day assuming gloomy skies?

Have you confirmed with an energy audit that 10kWh of battery... enough to run the inverters at full power for... 1 hour... is enough? You'll consume about 12% of your battery capacity in idle draw alone. And again, with the 100A discharge limit of the batteries, you're not getting more than 200A * 51.2V * .85 = 8,704W max out of the inverters ASSUMING they truly share 50% of the current, and that's usually not the case. They'll be close, but you shouldn't count on more than 180A of current. Three is the minimum in order to ensure you can deliver sufficient current to the inverters for max power output.

Another thing....

Please read the installation portion of the manual. It has some critical info. Your 10awg output wires are woefully inadequate. The inverter can output 42A, BUT the unit has a transfer switch of 100A, which it can pass through when fed by external AC. Additionally, PowerAssist can provide another 42A on top of the 100A, so the Quattro, fed by a 100A power supply can output as much as 142A.

Granted, you are limited to 50A input on a typical RV shore power connection, so that drops it to 92A total. You'll need overcurrent protection between the Quattro and 50A source, and you'll need a dedicated breaker for your AC out lines. It looks like you have one, but you need to revisit your wiring and overcurrent protection. You don't have to size for the 142 or 92A output, but you must have the right wire and overcurrent protection for your system.

 

[nebster]

You've gotten excellent advice so far. A few additional comments/opinions:

• You can parallel multiple ATs to increase capacity. Generally two Victrons is the most someone needs in USA RV land. You can also buy or commission a custom AT for your desired power.
• Depending on the topology you elect, you may be able able to overload your neutral. Be sure to provide OCP, as suggested.
• The ATs are about 98% at load. I think they are a little less at low load. They are almost 100% efficient with no current flowing at the center tap, of course, because they are effectively not in circuit at that point.
• Think carefully about heat and derate. In a vehicle, depending on where you put your equipment, you may see markedly degraded performance, perhaps when you most want it (warm days, on asphalt, after coming off a drive and everything is wicked hot). Inverter capacity, charging current, and AT center tap current will all fall off substantially as temps rise. Victron does not specify much derate data, but it's aggressive in my experience.
• Related to that, Victron uses fans, of course. Not always great ones, and with suboptimal PWM on some of the more recent builds. If quiet is important, you need to position the equipment carefully, or oversize more so it runs well inside its thermal envelope, or add active cooling, or be okay reducing loads. You may need to do some of these even if you don't care about screaming fans.
• Your batteries seem really small relative to your implied loads, as others have mentioned. Keeping rates well sub-1C helps with heat, lifespan, etc.
• The second-generation Victron inverters are about twice as efficient at no-load. But they are taller (and trickier to place in many vehicles, as a result).
• I'm not sure you've done a shore current minima analysis, but if you're planning on tapping smaller shore supplies sometimes, you'll need to look at the data for the Quattros and make sure your config can do what you want.

My instinct is that you are mis-sizing your system somewhat, but you've clearly put a lot of thought into it. And since you're spending big, I'd guess you can spend again if you need to re-optimize. Good luck.

 

[l00semarble]

The Quattros > 5kW can't be paralleled except for 3 phase power.

I did not know this. The spec sheets imply that they can be paralleled.

Where is is stated they can't be paralleled. Any known reason why?

They can be put in split phase though, right?

 

[sunshine_eggo]

I did not know this. The spec sheets imply that they can be paralleled.

This debate has raged on and off for years. In retrospect, I may have it confused with the Multiplus/Multiplus II or 230V units. My statements may not have been accurate for the 120V models.

https://www.victronenergy.com/upload/documents/Datasheet-MultiPlus-II-inverter-charger-EN.pdf

Virtually unlimited power thanks to parallel (not for the 8k, 10k and 15k models) and three phase operation Up to 6 Multis can operate in parallel to achieve higher power output. Six 48/5000/70 units, for example, will provide 25 kW / 30 kVA output power with 420 Amps charging capacity. In addition to parallel connection, three units of the same model can be configured for three phase output. But that’s not all: up to 6 sets of three units can be parallel connected for a 75 kW / 90 kVA inverter and more than 1200 Amps charging capacity.

Where is is stated they can't be paralleled. Any known reason why?

They can be put in split phase though, right?

For sure.

@Mantissa

My statements about parallelling 120V Quattros may have been wrong. Would confirm with a Victron dealer.

 

[HighTechLab]

The Quattros > 5kW can't be paralleled except for 3 phase power.

I think this is possibly referring to the 230v model Multiplus-2. I just helped a client who installed a 40kva 120/240v system with 4 120v quattro 10k's. No issues at all. I've also done the same config with Quattro 5k 120v units as well

This may also be possible with VictronConnect, but I don't use it.

It's not in VictronConnect as of today.


I also recommend a third battery for this config of inverters because any time you cross into the surge ability of the inverters the BMS is very likely to trip, aside from the fact that running near full load would empty the batteries in a bit over an hour.

Generally solar wattage (4000w) x 5 gets you a good approximation of battery bank size, so 20kWh - obviously a proper energy audit is the best way to size the batteries, but it's rare to see a properly sized solar array (based on loads) that doesn't have at least this much battery attached to it.

 

[Mantissa]

Thanks everyone for pointing out the flaws in the plan so far, I really should've swallowed my pride and just asked for a review up-front.
Y'all have been incredibly helpful, I definitely have more legwork to do. Glad the Quattro vs. Multiplus-II thing was sorted out, I was confused because I know CurrentConnected sells a 20kW kit with 2x Quattro 10kWs.

I will definitely be reading manuals before pulling the trigger on this system, we're a few months out on our skoolie build before we'll be ready to install everything, and solar will be one of the last things I'll be installing in order to maximize battery/panel lifetime.

Some bits of reasoning on a bit of the oddities in this plan:

1. I've been told by other folks in the skoolie community that, unless you're on a tight budget, to just panel the entire roof while you're up there and building racking anyways. This makes sense to me; I'd rather have more PV capacity that I don't use, or which I use for dump loads/daytime loads like heating water, water filtration, etc.
2. These panels will be flat-mount; again, more advice from the skoolie community. If we really find that 4kW is somehow not enough for our needs, then it's easier to install additional "slide-out" panels that stack while in transit than to deal with the complexities and maintenance hassles of a tilting system. As a result panels generally work less efficiently than a south-facing or sun-tracking system. Using Global Horizontal Illumination from https://globalsolaratlas.info, panel surface area and efficiency, we're calculating closer to 15kWh a day in places we'll be liiving. Our daily usage, both day and night, is looking closer to 10 kWh total. Most of our usage is during the day, and we have ample surplus, hence the reason the system is a bit downsized both in terms of inverter and battery capacity.

I think the ultimate conclusion I've ended up coming to is to purchase an initial system of 4kW panels, 5kW inverter (forgoing 240V appliances for now), and 10-15kWh of batteries. I figure if we need a beefier system in the future, I can add an additional inverter in parallel, more server rack batteries and possibly more panels.

If anyone is curious, we have picked up a Kill-A-Watt and audited some of our usage, and budgeted for the items that we don't have yet: https://docs.google.com/spreadsheets/d/1GtpSb29WCoBD8fetnZGk6HVc6zDERJNre4MvcTh7lPE/edit?usp=sharing

 

[sunshine_eggo]

2. If possible leave 4" clearance between the roof and panels for cooling.

 

[justinm001]

I have a similar system as you planned, 2 SOK 48v, quattro5000 and will have around 4000w solar.

I think I get quite a bit more solar with my array angled over flat. I have ACs in the middle and angling them seemed better but overall height is a huge concern, mines under 13'5" at least I think.

You really need to increase battery capacity or at least prepare to add additional batteries. Just today on a cloudy day we ran 1 AC for 1.5 hours while driving and was at 55% when parked with just 2 48v batteries.... and that was with my 12V system disabled and it having its own 10kw bank for lights. Before I had 10k 12v only and maxed out my mppt at 1400w and it still was barely enough for day trips.

I'm curious on why 2 MPPTs instead of like a 150/100 with vecan? Wiring is easier and vecan makes things simple.

I don't see a shunt so I'm assuming you're relying on SOK interface for SoC. I haven't had time to setup that part but documentation seems lacking. Tried for a bit yesterday and didn't work.

Have you considered a 48V alternator? I'm thinking on adding a 48v 100A Belmar one, but I'm mainly a day/weekend tripper so it's nice to know it's charging and whenever I stop I'm 100% charged. Was super nice when on 12v with a 270a alt. Plus I didn't see a backup power option like a genny. A rainy weekend where you're stuck in the skoolie using a bunch of power isn't fun when it runs out. An alternator could help recharge as needed. My bus has a rabbit (high idle) button which jumps the rpms up when idling for this reason. My rig came with a massive genny though.