Newbie from NW Ohio

[Hedges]

BMS is last line of defense against cell over/under charge.
If you get 12V batteries, make sure they are compatible with connecting in series for 48V; some can't take the voltage when they open.
You can get 48V pre-assembled batteries. Or individual LiFePO4 cells and assembl 16s + BMS for 48V. Or 2p16s with one BMS, or 16s2p with 2 BMS. Some BMS control an external relay for higher current.

Charge controller should charge to a voltage low enough that BMS doesn't disconnect for cell over-voltage, but high enough to allow passive balancing. Some BMS use a data bus to communicate with some inverter/charger to hold voltage in a range for balancing.

Lithium may be best utilized deeply cycling every day, so you get your use out of its cycle life before it dies of old age.
Lithium can accept about 0.5C charge rate near ambient 25 degree temperatures, but reduced to about 0.1C or 0.05C as temperature approaches freezing. For off-grid, I think 0.15C is enough to recharge during the day in most conditions.

If a battery is sized for 3 days without sun, it would only use about 15% DoD most nights. With that sort of usage, premium FLA like forklift batteries or Rolls Surette are reported to last 15 to 20 years. They do need watering and equalizing periodically. They tolerate charging below freezing.

I use SMA Sunny Island and Sunny Boy. That is one of the most expensive brands, but liquidation and old stock could be a deal for off-grid (doesn't need latest features required by code for on-grid).
Split-phase 120/240V can be up to 24kW battery inverter and 48kW PV inverters. PV panels 50% greater (e.g. 72kW) could be connected without clipping production if array has two different orientations (SE, SW).
Half that size system might meet your needs.
3-Phase 120/208Y 18kW battery inverter 36 kW PV inverters can also be done.

Some other brands like Victron, Midnight, Schneider could build similar size systems, usually DC coupled but some support AC coupled PV.

There are also SolArk, MPP, others.

The better brand low-frequency inverters will do better starting large motors. Soft-Start kits may help for smaller inverters.

If you keep utility connection, that can be a cheap reliable "backup generator", let you get by with smaller system, smaller battery especially for time without sun.

 

[wildman2]

Thx for info on BMS kinda went over my head but not quite.Lot of info to take in. I’ll get it sooner or later.takes time and I’m not in a huge rush.
Utility back up charging sounds interesting. So for that you’d basically wire up a 240v 50 amp line to a Nema transfer switch then send that to a charge controller, if it’s set up for external or grid or generator hook up.Some of the charge controllers I’ve seen can auto transfer for backup generator/grid can’t remember which ones at the moment but I bookmarked them.

 

[wildman2]

I do want to thank you guys for suggesting conservation of electricity instead of the creation of energy to lower the electric bill. I did not think I could lower it as much as i did in a short amount of time.End of October November last year time frame was 1598 KWH,this year it was 989KWH. Way more manageable to figure out some solar.The electric bill is the lowest it has ever been in over a decade or more.

 

[Hedges]

Thx for info on BMS kinda went over my head but not quite.Lot of info to take in. I’ll get it sooner or later.takes time and I’m not in a huge rush.
Utility back up charging sounds interesting. So for that you’d basically wire up a 240v 50 amp line to a Nema transfer switch then send that to a charge controller, if it’s set up for external or grid or generator hook up.Some of the charge controllers I’ve seen can auto transfer for backup generator/grid can’t remember which ones at the moment but I bookmarked them.

If you set up Sunny Island or various other inverter/chargers with internal transfer switch, you could tell them to run off PV/battery as much as possible, but connect to grid when battery gets to low. That will work as zero-export with a battery. Then you could get by with a battery sized for just one night, recharge from PV when the sun comes up or draw from grid if production is too low.

https://files.sma.de/downloads/SI6048US-DS-en-28.pdf

 

[wildman2]

That’s a complex piece of equipment for sure. Does a lot of stuff. Bookmarked for future reference. Thank you.

 

[Hedges]

They've been around for years, top quality. Hookup is pretty much plug-and-play, except you might need a data interface to Sunny Boy and free software on PC to select off-grid mode; default is on-grid so need to enable feature of variable output wattage depending on frequency.
Probably behind in terms of software for grid-connected peak load shaving, might come in future models, but that's not something you need.

You might find some deals on DC Solar trailers with two of these. They have GNB FLA forklift batteries which would make them a deal if the battery is good, but many were allowed to dry up. Some have diesel generators. Those and many new in the box Sunny Islands flooded the market due to DC Solar bankruptcy auctions. Great bargains 12 to 18 months ago, like 25% to 33% of list price. Recently I've seen them for $3500 on eBay.

Mentioned in the manual but not shown in the data sheet is that if battery drains to the point of shutting down, the AC coupled PV and the grid don't get connected to charge it back up. There is an "emergency charge" method feeding AC into the output. To avoid the issue, I have their recommended "load shed" relay after the Sunny Boy and before the house. At 70% DoD house is disconnected, but GT PV remains connected and it remains up unless 80% DoD reached before there is sun. Other solution is DC coupled PV.

I wired four Sunny Island (two would have been enough but limited to 56A 240V pass-through from grid, and I got a deal on a pallet of them.) I'm using the older model Sunny Boy 5000US. Most Sunny Boy work with it, and any other brand that implements the "frequency-watts" optional portion of UL-1741-SA should too.