DC Solar Trailers: 2 Trailers

[FarmerChas]

Hello all. I'm new. I'm trying to help my brother who purchased two of the DC Solar Trailers. One has a generator, the other does not. Both seem to work ok. His goal is to have emergency backup power, but to also get a little bit of something day-to-day.

We've got one trailer hooked up to provide power to a subpanel in the home that was wired to accept a backup generator. Standard switch over from line to generator for essential circuits. This seems to be working.

My question is, has anyone ever tried to connect two trailers together to double the output? It looks like I could do a 4 unit, 2 phase set up with 3 of the Sunny Islands set as slaves. I'm not sure how well that is going to work with two separate battery banks, one on each trailer. I think the master unit is the only one that would look at state of charge. Any ideas on how to most easily merge the batteries, panels, etc, into a single parallel unit?

Thanks

 

[GVSolar]

@Hedges

 

[Hedges]

The @ sign only shows up under bell icon for notifications, where I rarely notice it.
Not in my email, which only gets postings to threads I'm following.

Yes, 4 SI can be configured 2phase4, AC outputs wired 2p2s. All need to share same battery, so batteries in parallel (so should be similar condition.) I hear batteries with the trailers are all in fairly bad condition? If after charge and settle with everything shut off, they end of similar voltage, you could try paralleling.

I think output of each trailer could parallel downstream at sub-panel "generator" breaker. Where is Neutral/Ground bond? Should only be one place in system. Need of course to have RJ-45 cable from Slave 1 to what used to be Master on 2nd trailer; its inverters are now Slave 2 an Slave 3.

The multiple arrays could all be aimed at sun during time of peak use. Or multiple orientations to flatten production. But I think included arrays may be small compared to desired charge rate of batteries and absorption/equalization requirements. Generator should provide for equalization.

I've also thought about cascading one SI system after another. Various issues and opportunities. Not that much increase in available current, compared to paralleling.

 

[FarmerChas]

The @ sign only shows up under bell icon for notifications, where I rarely notice it.
...

I've also thought about cascading one SI system after another. Various issues and opportunities. Not that much increase in available current, compared to paralleling.

Thank you Hedges.

So far batteries don't look terrible. But we do need to check the specific gravity. I'll post a schematic later. But pretty simple so far, single trailer.

Grid power into L2 (where the generator normally goes on the trailer, back side of the box with the SIs in it), then out to emergency gen switchable subpanel from L1 (side plug on the box). So one trailer, two SIs, in 2phase2 configuration just as the trailer was built.

We've got the ethernet cable between the two trailers so we can do 2phase4. My thought was to do the 2nd trailer in full parallel with the grid power into L2 (same cable length as to first trailer), then out of L1 to the subpanel using same cable length. That gives me a single, common ground/neutral point at the junction box, before going into the subpanel.

I was wondering if connecting the batteries in parallel using some beefy cables would be ok. There is a second massive Anderson PowerPole connector (SB350 type???) on each trailer. If I connect those together, will the two Midnight chargers work ok with each set of panels connected to one charger? Or is something more complicated needed?

Now, a more complicated question:

Do I want to adjust any of the battery voltage cut in and cut out levels so as make good use of the solar, but without wearing out the batteries too fast? Ideally, I'd like to capture as much solar as I can (not a lot off 20 panels), but maximize battery life for when they are really needed. I'd like to run off the grid before stressing the batteries too much, but have some way to get all the power possible out of the batteries if the grid goes down. In the event of grid failure, the plan is to unplug the the gird from L2, plug the generator back in, but in parallel to both trailers, and run it as needed to keep supplement what the solar won't provide.

Thoughts? Suggestions? Warnings for the new guy? Thank you again.

 

[Hedges]

There is a second massive Anderson PowerPole connector (SB350 type???) on each trailer. If I connect those together, will the two Midnight chargers work ok with each set of panels connected to one charger? Or is something more complicated needed?

I think if you connect those two together, the batteries will be in series and shorted, rather than in parallel. Is there a fuse, by the way? Check voltages/polarity of terminals. Maybe make an extension cable with Anderson connectors doing polarity swap if needed.

I think the two cables are for two batteries; do you have only one per trailer?

The two Midnights should work together fine. The battery shunt may not - sketch out where two batteries, two shunts, two midnights, two sets of 2x SI connect. Only the Master's shunt will be used. Everything from second trailer has to be connected such that it lands on same side of Master's shunt as the equipment on Master's trailer does.

If you have higher current passing through SI relay (not just higher current sourced from parallel SI), you'll find out whether they are balanced or not from the four SI's displays.

I think Master will qualify AC input to connect. Slave(s) will connect if they see in-phase power. Having grid to one pair and generator to other won't be good; need grid and generator to feed through a transfer switch and into master (optionally also slaves.) A signal should tell SI when generator is connects (maybe that doesn't matter if you only tell it there is a generator, so it doesn't backfeed; it would think grid was generator.) Without GT PV, won't backfeed anyway unless "Sell" is enabled and Midnight drives higher voltage than SI wants.

I agree with saving battery for when you need it. I think FLA costs $0.25/kWh of cycle life. Cycled 15%, perhaps battery would last 20 years. Deep cycling more like 6 years (assuming it started out new.)

I don't know how to program SI for PV utilization with grid connected. Maybe if grid charge set to something minimal like 10A or so, that would let PV do most charging and grid kicks in when needed. Let me know what you figure out.