Two Conext 6848s / Lithium Bank Sizing

faccnator

Hey all-

Love this forum. I’ve been reading the boards daily for 10 years. 

I want to ask this question a little backwards.

I have two stacked Conext XW+ 6848 inverters and a 2kw inverter generator.

What size Lithium bank do I need?

I purposely left some variables non-fixed but I’ll be happy to provide that clarity.

Thanks so much

Faccnator

BB.

Really need to know your loads (Watt*Hours per day, any seasonal variations, etc.).

Also, what are your battery needs... With Lead Acid, I suggest that 2 days of storage to 50% planned discharge (i.e., 4x daily loads) is a good place to start...

When you look at Lithium (LiFePO4 or similar), they have better surge capability (i.e., FLA batteries, 100 AH @ 48 volts per 1,000 Watt inverter capability or 2x6.8kW = 1,360 AH @ 48 volt FLA batteries for your setup--Assuming you want to use near 13.6 kWatts and have surge currents up to 2x 13.6 kWatt).

With FLA batteries--They take time to charge and have limited charging current acceptance (especially during Absorb charge cycle)--And 4x daily load for full time off grid is a good place to start.

With Lithium, they charge much faster. Basically just bulk->float, little to no absorb charge time--Saves 2-6 hours of "on charge" during daylight.

So--That gets back to what is the charging range you are looking at (for FLA, 50%-100%; for Lithium around 20%-90% range of charge), and how many days of no-sun do you want (1,2,3 etc.).

Lead acid batteries being "relatively" cheap (compared to Lithium)--Making a larger bank (2 days/50% discharge) gives 2-3 days of no-sun, and enough capacity for surge current, and keeping up with daily usage in good sun (less genset usage).

With Lithium, you could can run with 1 day storage and 70% capacity usage--And recover just using solar the next day (large array, relativity few hours per day "on charge"). And always crank up the genset the next day if needed...

Just to get some numbers... Using FLA as starting point:

• 1,360 AH * 48 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max planned discharge = 13,872 WH per day
• 1,360 AH * 59 volts * 1/0.77 panel+controller deratings * 0.10 nominal rate of charge = 10,421 Watt array "nominal" (10% rate of charge)

Guessing 
http://www.solarelectricityhandbook.com/solar-irradiance.html

Orlando Florida
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 62° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.29	4.76	5.36	5.79	5.75	4.99
Jul	Aug	Sep	Oct	Nov	Dec
4.96	4.87	4.74	4.89	4.55	4.16

The "break even" array for December would be:

• 13,872 WH per day * 1/0.52 off grid AC system eff FLA * 1/4.16 hours of sun per day (Dec) = 6,412 Watt array Dec Break Even

In "real life", you have "base loads" (loads that need to run every day, such as lights, refrigerator, work computer, etc.), and you have "optional loads" (washing machine, A/C, irrigation, etc.) that you can run on sunny days and/or with a genset. The base loads, you should count on ~50% to ~65% of your predicted daily harvest for running base loads...

Anyway, 10,421 Watt array seems to be a good starting point... So, how much charging current can your Li Ion bank absorb... Such an array will produce a maximum average current (cool/clear days, not many hours a year) of:

• 10,421 Watt array * 0.77 panel+controller derating * 1/45 volts nominal minimum bank voltage = 178 Amps

So, with the above array, the Li Ion bank should take a minimum of 178 Amps charging current (check battery specifications).

My personal bug-a-boo with Lithium is if they ever catch fire, such a fire is very difficult to put out, and can release some highly toxic chemicals (Hydrofluoric acid). And I would highly suggest that a Lithium battery bank be installed in its own outbuilding--Away from people, home, possessions.

The above is just points for starting the discussion... I have made a great many guesses and SWAGS (scientific wild ass guesses) to get some back of the envelope math working.

Once you know your daily loads--It is pretty much just pressing buttons on the calculator to work out the sizes of everything. The bigger unknowns are your "safety factors"--I.e., days without sun, what if genset fails, what about a week of stormy weather, etc.).

Your 2kWatt genset is probably "too small" as a backup genset for an FLA battery bank (5% to 10% minimum rate of charge needed). But for Lithium Ion which do not have a "minimum rate of charge" specification... You are just looking at time on charge:

• Say 1,000 Watts charging current (run genset at less than 1,600 Watts to keep from overloading):
• 1,000 Watts * 1/45 Volts charging * 0.90 charger eff = 18 Amps @ 48 volt bank charging

1,000 Watts charging for a ~13,872 WH per day load => over 14 hours "on generator charge" per nominal day's usage.

It works--But you may want a genset that is capable of running your daily loads (A/C, pumping, +household loads).... But this is just one of the "fudge factors" you need to think about (running a 2kWatt Honda inverter generator for 5 days a year, and ~1,600 Watts to run a fridge/lights/laptop computer does work)--Hurricane season--Ain't that bad--And a 13 kWatt genset would probably be way overkill).

-Bill

faccnator

Bill thanks so much for that detailed reply. I had hoped you would comment. I’m going to digest what you’ve written with a pencil and paper and then ask more questions.

Thanks again

faccnator

Some more information 

my panels are ground mount. I have almost an unlimited amount of room to add panels if needed

At any given time loads are approximately 800w to 6500w with occasional loading to about 12kw. The larger loads don’t run for longer than 30min-1hr on any given day. 

If I need to run anything larger like my shop equipment then I just fire up one of the larger generators. I’m a general contractor and have access to several generators that are really too large for what I need on a daily basis but I have them if I need them.

the batteries I am using now are too small and too old. A bank of 12v outback AGMs.

I built a battery room on the new carport. Poured concrete with a cast in place roof about 200ft from the house so I figured I’d put the lithium batteries in there. 

I *think* I want a bank that can run my typical loads described above for 6 hours and that can handle the short duration larger loads with any associated surge that may come with them. All the while knowing I can fire up a larger louder fuel sucking generator if I need to

 

Dave Angelini

Most people who think they need 2 XWP's really do not. It is a matter of choosing the correct offgrid appliances and loads. Not a bad idea to have 2 with a spare. A master slave combo is just extra work that should not be used for an offgrid home, in my opinion!. 

faccnator

Thanks Dave I wouldn’t be against that. I already have the two... and an Conext sw.. and a xantrex aw... and tons of other power gear... 

Dave Angelini

Welcome !  Equipment that is not connected is harder to damage with lightning than equipment that is connected.
  Use SPD's at all critical points in the system. AC in, AC out, DC buss, and DC HV in.
The AC in SPD is usually not needed unless you use alot of generators and the chance that a genset, in the right failure mode could be faster than XW is at disqualifying.

faccnator

Dave do you have one that you like or recommend? (An SPD)

jtdiesel65

fyi , you can get some insight to battery bank size from this manual. Looks like basically 12kwh per xw. But I think the devil is probably in the details due to variable battery max discharge rates. That will have to match expected loads. You don't want to exceed max discharge rate. You really want to have a fairly low discharge rate for battery longevity.

https://solar.schneider-electric.com/wp-content/uploads/2020/08/Conext-XW-PRO-Li-ion-Battery-Solution-Guide990-6359_rev-.pdf

The midnite solar spds seem to be the standard these days.

BB.

Midnite SPDs seem to be a very good choice... The older (?) Delta and such are just a simple spark gap filled with sand. Midnite use Metal Oxide Variable Resistors (much more accurate set points):

https://www.solar-electric.com/search/?q=midnite+spd (not cheap)

This is Midnite's website for one of the models... You can review their documents and video demonstrations:

http://www.midnitesolar.com/productPhoto.php?product_ID=283&productCatName=Surge Protection Devices&productCat_ID=23&sortOrder=1&act=p

Midnite has a nice video of testing their SPDs vs Delta:

https://www.youtube.com/watch?v=vkeFYuHxJoU

Dave certainly has a lot of experience with off grid power installs (vs me--No direct experience)... In decades past, our (now retired) forum founder from NAWS--His experience over the years was the output stages of AC inverters were the most likely to be killed with nearby lighting strikes.

It is possible that the newer hardware from Schneider and others is better built/protected... but given that 120/240 VAC circuits are usually all over the home, and frequently to outbuildings/wells/etc., i.e., a big antenna, I would not not try to save money on avoiding SPDs on AC output for inverter(s) in lighting prone areas.

-Bill

faccnator

Thanks everyone. The only SPDs I was aware of was the Midnite. I’m not sure why. I assumed maybe Schneider had a unit as I have all Schneider gear.

After reading all of these responses to my question. I have revised my question. 

I’m looking to determine the minimum size lithium battery that will allow a conext 6848 to run at maximum load including the surge current. I have a basic understanding of I was using lead acid but I want to use lithium. 

This won’t get me to my final battery size but it will
allow me to get to the next question that I have not formulated yet. 

jtdiesel65

faccnator said:

I’m looking to determine the minimum size lithium battery that will allow a conext 6848 to run at maximum load including the surge current. I have a basic understanding of I was using lead acid but I want to use lithium. 

That question is likely moot ,because once you size your battery for loads, you will be will beyond the minimum requirements.

Schneider says that 3 LVS 4.0 batteries in parallel is the minimum for one xw pro. These batteries are 51.2v, 4kwh, 65a continuous output, and 90a peak output current. One thing to keep in mind is that none of these batteries operate at 48 volts. The LVS is 51.2 v nominal, but charge settings are typically ~54-57v for lifep04 so that changes the math a little.

Three LVS 4.0 in parallel is 12kwh and 195a continuous output, 270a peak. PDPs have a 250a breaker per inverter.  250a*48v=12kw. An xw6848 is rated at 12kw for 60 sec. Based on the doc, I think it's safe to say they think 12kwh is minimum. They may be assuming that lifepo4 has a 1C max discharge rate.

faccnator said:

At any given time loads are approximately 800w to 6500w with occasional loading to about 12kw. The larger loads don’t run for longer than 30min-1hr on any given day. 

 

I think you are saying here that you have a 12kw load for 30 minutes to 1 hr on a given day.  1 hr of that and your 12kwh battery is at 0 soc. Even if you have 24kwh (12kwh per xw), your soc is 50% at 1 hr of use. That's why I say the question is likely moot.  Once you get into the 36kwh ( 585a continuous discharge,  810a peak) and above for two XW, max discharge rate is no longer a concern.  From what you said you want to run 6500w for 6 hrs. 6500w for 6 hrs is 6hr*6500w=39kw.

faccnator

Thank you for that reply. I was thinking 36kw.

I have XW+ models.

I need XW Pro models to integrate lithium bms don’t I?

KenMorgan

faccnator said:

Thank you for that reply. I was thinking 36kw.

I have XW+ models.

I need XW Pro models to integrate lithium bms don’t I?

Just out of curiosity what sort of load are you running that takes that much power?  A deep well?   I ask because  I am just starting with LiFePo4 batteries and had the same sort of questions.  with LA batteries I only had one day plus IE one day running with about 3/4 of an additional day.  that was with a 600 a/h 48 volt bank.  with the batteries I just bought (19200 watt) I will have a 400 a/h bank at the same voltage, but with the ability to pull a few more amp/hours from the bank.  I really want to add an additional 200a/h to the bank to give me  a solid one day plus a second day without input.  (my average in reality allowed me to run 2+ days with the solar inputting through the day.  not charging per se, but adding and keeping the bank from dropping to low (max discharge was about 30 percent of total capacity with the lead acid setup), so between 150 amp hours and 200 at worst case.  Just curioius as too what cause that kind of draw.  i did all the energy saving measures as I setup the original battery bank.  LED lights, energy efficient fridge and reefer (two biggest draws on a 24 hour schedule).    I also have a split pack that I run in the rainy months to keep the hosue dry, but it is et to only run during production hours. (1000-1400).  

R/

Ken

jtdiesel65

faccnator said:

Thank you for that reply. I was thinking 36kw.

I have XW+ models.

I need XW Pro models to integrate lithium bms don’t I?

I don't know. The integration happens in the gateway and I think it is fairly limited in terms of which manufacturers can integrate.  At least I haven't found much other than a limited number of pre-fab batteries.

IIRC, the gateway is supposed to speak canbus, but I haven't found any examples of diy people trying to get it to work.