Questions for Schneider Experts

Horsefly

Wow Bill! That is absolutely awesome!  On topics 1 and 2, your answers are very reassuring. I think I can do all of that.  On topic three I'll need to think through how best to approach. Sounds like at the very least I need to bump up the current 250A breaker I have between the batteries and the inverter. 

Where you have me fainting is your nominal recommendation of 4kW of PV array and 800AH @ 24V for the battery bank. I'm way under both of those right now.

The basis for my design was a 3700W max instantaneous power (not counting the surge startup, but with a ~2kW cushion for appliances that we almost never use), and a 2100Wh daily energy consumption. I know those two numbers don't align well in a typical off-grid install. In our cabin, the only thing that really draws any power at all is the well pump, but even in the busiest times (3-4 people showering every day, etc.) it only runs maybe 5-7 times for 3 minutes each (filling our pressure tank from empty). Anyway, I'm pretty confident that the 3700W and 2100Wh per day are pretty generous.

A third constraint is that we are not trying to build a solar power system that can power the cabin autonomously for multiple days. We have a perfectly good generator, and that has been doing us fine for the 40 years the cabin has been there. My goal was to size the PV array and batteries for essentially a day's worth of PV energy each nominal sunny day. I.e., collect and store each day enough to supply the 2100Wh.  One of the reasons I liked the Schneider SW4024 is that it looks like I can set a battery voltage (hopefully representing about 50% DOD) and have it just turn off the inverter mode. Everyone in the family knows when there is no electricity and you need some, you go start the generator. That part will be easy.

Finally, I've got agreement from my partners in this (my brother and sister) that we want to be lean now, but accept that we may want to up the PV Array and battery bank later.

Without showing you all my math (I have lots of it), I came up with an end-to-end efficiency derating of my 2100Wh (from PV through Inverter) of 50%, and right around 6 peak sun hours per day (for the 8 months of the year the cabin is used). 

Based on all this I calculated that I needed a nameplate rated PV array of 700W or more, and a 24V battery bank of a bit over 200AH. The parts list I'm currently identifying for potential purchase gives me actually about 885W of PV and 250AH of battery.  My rate of charge will be a slight bit under 10%, but I'm hoping that is OK.  

That's quite a bit less than the 4kW PV array and 800AH of battery you describe. Reading your post (rereading it several times, actually) I'm feeling less confident in my design, but as I pour back over the 25 pages I've written up I can't find what I may be missing.  I know I haven't thought through too of what happens when the pump turns on (a surge, but probably for less than a second), and while it is running at about 1500W for 3 minutes each time. Even though the math says my 250AH battery bank should be OK and stay above 50% DoD, maybe I'm missing something.

Sorry for yet another long post. I really appreciate your absolutely great response. I fell flat on my back reading your numbers, so now I need to get my feet back under me!

Steve

BB.

I am out right now... 4,000 pv array to an 800 ah @ 24 volt battery back was typical maximum recommended, not nominal.

For a 200ah @ 24 volt battery back, a 1,000 watt maximum array and inverter is what I would make as a starting point.

If you are going for around 4kw surge, then a 400 ah @ 24 volt would be the minimum.

If you go with agm or LiFePO4 battery bank, they do support much higher surge current vs flooded cell.

Based on what you asked for, and this is a starting/learning experiment, I would suggest 4x 6 volt @ 200 ah golf cart flooded cell times 2 parallel strings (8x batteries total).

A capable system that should meet your needs for the moment. You can add a 3rd parallel string if needed for surge support/storage) for not too much money.

In 2-3 seasons, recycle the golf cart bank and install the ideal bank after your experience.

-Bill

Horsefly

Thanks for your time Bill.  We were currently going with AGM batteries (probably 4x1 6V or 2x2 12V).  I've made the case to go with cheaper 6V golf cart batteries to start, and I keep losing that argument. No one wants to own maintaining the batteries.

If I bump my batteries to 400AH to handle the surge, I figure my current 3 panel PV array won'd suffice. That is, if I double the AH I probably can't go - even as a start - with my 3 panel array. as my rate of charge would only be about 5.5%, by my calculations.  

This all makes my head hurt, but it's certainly better to have it hurt now than after we've spent $$ on the wrong stuff.

Thanks again. I'll climb back into my hole and start noodling it all over again. 

new2PV

Regarding your panels, how cold will it get in the winter? I am a bit worried they might have too much VOC cold? If Ihave a chance I will run the numbers

Horsefly

new2PV said:

Regarding your panels, how cold will it get in the winter? I am a bit worried they might have too much VOC cold? If Ihave a chance I will run the numbers

The SW295 has a Voc of 40V, and a voltage temperature coefficient of -0.30%/deg C.  I know the Schneider works fine up to 140V and goes into over-voltage fault between 140V and 150V, meaning it won't operate but won't be damaged. I did the math for three series panels, and found that the cold Voc will be 140V at -30.6 deg C (-23.1 deg F), but won't get up to 150V until -58.3 deg C (-72.9 deg F).  The record low in Telluride (nearest city) is -34 deg F, and it is almost always colder in Telluride than at our cabin. Anyway, I'd love for you to check my numbers, but if they are right there is a small chance the charge controller may shut down briefly early in the morning, but virtually no chance it would be damanged by >150V. 

Steve

mike95490

I simply use a 4 port wireless router for the power network,  it does not connect to the outside world, the SSID only connects to 3 pieces of gear, Midnight, morningstar, combox.  The wireless, I listen to with laptop or android phone

new2PV

I ran the numbers, you might get the mppt -60 do into derating mode, above 120 volts in vmp, but it just close at 118.  VOC is possible to 142 volts.  This was done the on the 295s you might be better off with the 285's.

Horsefly

new2PV said:

I ran the numbers, you might get the mppt -60 do into derating mode, above 120 volts in vmp, but it just close at 118.  VOC is possible to 142 volts.  This was done the on the 295s you might be better off with the 285's.

Thanks @new2PV. I didn't use the MidNite tool, I just used the equations to solve for the minimum temperature. I can't tell for sure, but it sounds like we got the same result. With the SW295 the MPPT-60 may go into overvoltage fault (I think that is what you mean by dertating mode?) if the temp actually gets to -34.6 deg F, but my real concern is keeping the max voltage below the 150V limit on the device, where it can be damaged. I think what I found is that the temperature simply can't get that low (-72.9 deg F). I think I'm safe with the SW295.

Thank you for running the numbers. I feel better!

new2PV

Over voltage is not the same as de-rating mode.  The manual doesn't say what this mode acutally does but it just says voltage de- rating mode, maybe Bill or Mike  can elaborate.

Horsefly

new2PV said:Over voltage is not the same as de-rating mode.  The manual doesn't say what this mode actually does but it just says voltage de- rating mode, maybe Bill or Mike  can elaborate.

I was just going off what the Schneider docs published. The installation manual (page A-2) says that the maximum operating voltage was 140V, and that the maximum open circuit voltage was 150V. Similarly the newer datasheet (sent to me by the Schneider engineer) says that the maximum operating voltage is 140V, and that the maximum open circuit voltage from the PV array is 150V.  I know I've read it somewhere (but can't find it now) that between 140V and 150V the CC just goes into over-voltage mode and ignores the input, not doing any charge conversion.

No matter how this is all interpreted, my only worry is that there is not a case where the Voc will exceed what the charge controller can handle (150V), and right now I feel pretty certain that the situation where it would be over 150V is extreme enough that the world may be coming to an end, at which point I don't care.... ;-)  Does that make sense?

Horsefly

new2PV said:

Over voltage is not the same as de-rating mode.  The manual doesn't say what this mode acutally does but it just says voltage de- rating mode, maybe Bill or Mike  can elaborate.

I apologize @new2PV, as I see what you are saying, but I'm still not too worried about it. The table you are talking about I believe is table 2-1 (page 2-2) which says that between 120VDC and 140VDC the charge controller will reduce the current output to protect the unit from voltage spikes on the input. That's OK. I don't see that having a dramatic negative impact on what we are doing, since in almost all normal operating conditions the voltage will be below 120VDC, which is the Voc at STC (not likely in the real world).  

BB.

Is there a long distance from the array to the battery bank/shed? In general, your Vmp-array~95 volts is OK... For a 24 volt battery bank, the charge controller will run a bit cooler/more efficient with 2x panel in series for Vmp-array~62.5 volts or so.

Anyway, back to your needs. An off grid system needs a little TLC every month (checking connections, wiping battery tops, taking a clamp meter and measuring current while charging/under load between strings, volt meter checking each battery (or each cell) to make sure they are reasonably balanced. Checking cells for specific gravity and water levels are a pain--But it does give you a lot of information about how well your battery bank is doing. AGM batteries cannot check specific gravity and do you leaving guessing sometimes about what is going on.

The reason we try for a standard/balanced system (2 days of stored energy, 50% maximum discharge, 10% to 13% or so solar charging current) is because thinks "work out better"--Both for the battery bank (hours of sun is a limitation for lead acid--A deeply cycled battery bank may take 6-8+ hours of sun to recharge, and during non-summer time, hours of sun may be limited. A bit larger AH battery bank does not discharge as deeply (normally) and is quicker to recharge. Also--When you have multiple people "managing" a system, 10%+ rate of charge reduces the amount of "baby sitting" of the battery bank (how full is it, is there enough sun tomorrow, etc.). A 5% rate of charge requires hand holding (i.e., no loads during the morning to get the bank charging quickly--Once full in the early afternoon, you can start turning on loads, etc.).

Say 400 AH @ 24 volt battery bank, 5% to 13% rate of charge:

• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 753 Watt minimum (weekend/seasonal system)
  
• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,506 Watt nominal (full time off grid)
  
• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,958 Watt array "cost effective maximum"

And energy from battery bank assuming 2 days storage, 50% max discharge:

• 400 AH * 24 volts * 0.85 AC inverter eff * 1/2 days * 0.50 max discharge = 2,040 WH per day (battery stored energy)

Hours of sun per day using PV Watts to estimate Telluride Co:
http://pvwatts.nrel.gov/pvwatts.php

Month	Solar Radiation 38 degree tilt fixed array from horizontal ( kWh / m2 / day )
January	4.52
February	5.22
March	5.78
April	6.31
May	6.62
June	6.93
July	6.72
August	6.82
September	6.58
October	5.79
November	4.78
December	4.05
Annual	5.84

Toss the bottom four months--Gives you 5.22 hours of sun for a February "break even month" (lots of sun there on average).

• 753 Watt array * 0.52 off grid system eff * 5.22 hours of sun (Feb) = 2,044 Watt*Hours per day (ave Feb)
  
• 1,506 Watt array * 0.52 off grid system eff * 5.22 hours of sun (Feb) = 4,088 Watt*hours per day
• 1,958 Watt array * 0.52 off grid system eff * 5.22 hours of sun (Feb) = 5,314 Watt*Hours per day
  

So--You are not that far off of your planned output... I always suggest going back to understanding your loads and looking at conservation.

In your case--The battery bank+5% solar array does pretty closely match to your 2,100 WH per day plan... Your well pump does not really use that much total energy, but it does put a strain on surge current capabilities.

If you did not want a 2 day storage (just 1 day and 50% max discharge)--You could look at pumping alternatives. There are in well pumps that have virtually no surge current requirement (and operate around 900 Watts)--So that could dramatically reduce your bank size (at least in terms of surge current)--But in well "solar friendly" pumps are not cheap (~$1,000 - $2,000+ or so). Another alternative would be to put in a cistern that would hold a couple days of water and use the genset to fill the tank when needed. And use a simple DC RV  (12 and 24 volt models available) demand pump (a few hundred dollars) to pressurize the cabin water. Either would dramatically reduce your pumping power requirements.

AGMs are, in many ways, a better lead acid battery. Their down side is they are not cheap, and a more sensitive to overcharging/over voltaging. And most rechargeable batteries do very poorly with under charging/over discharging--Something that is very common with people new to off grid power (people leave loads on, miss-programmed charge controller, guest/kids come by and turn everything and then go outside, etc.).

A bit easier to maintain battery bank would be to use larger AH cells (i.e., instead of 2x parallel strings of 200 amp batteries and 24 cells to check, 1x 400 AH batteries and just 12 cells to check). Just 4x of these:

https://www.solar-electric.com/trl16vo225ah.html

• 6 Volts
• 420 Amp-hours at the 20 hour rate
• Dimensions: 11.625"L x 7.125"W x 16.75"H
• Weight: 113 pounds
• BCI Group Size: L16

And to give you an idea of what a (very good brand) AGM will cost that is similar to the above flooded cell:

https://www.solar-electric.com/concorde-sunxtender-pvx-4050ht.html

There are a lot of variables out there... And you have to design/build the system to be supported by your end users (you may take great care of the system, somebody else may not). Things that can help--Simple meter that somebody can look at. Battery monitors are nice, but accuracy can drift (you have to make sure all is running correctly with the battery bank/charging system):

Trimetric TM-2025-RV Battery Monitor System

And there are voltage based monitors that may be "good enough" for folks to understand how much power they are using/have available before needing to start the genset:

MidNite Solar MNBCM Battery Capacity Meter

Not saying these are the only answers/choices--Just some places to start your research. I keep falling back on design rules of thumbs because they get us "quick and dirty answers" and (usually) give us very workable/capable systems. And let you quickly estimate the costs/capabilities of the system.

But, energy usage is a highly personal set of choices--And any of the above can be adjusted to better suit your needs--The old engineering saying--Fast, Good, or Cheap, pick any two really applies to off grid power. Cost, maintenance, amount of energy it can produce--All based on your needs/choices.

-Bill

Horsefly

Bill - 

Wow - you are incredible. If I could just plug your brain in to my laptop so I could tap it now and then....

"Fast, good, cheap - pick any two" - Boy that takes me back to my engineering days!

I think I'm going to need some time to digest all you've given me here, and my wife tells me we are leaving or New Orleans for a week. That probably means my solar data digestion will be on hold.  A couple of thoughts until then:

First, I got my data from the NREL Red Book for Colorado. There wasn't any data there for Telluride, so I used Grand Junction. That data by month isn't too far off from what you had. However, I did two things: First, we don't use the cabin Nov, Dec, Jan, and Feb, so I just left those months out. Second, since we don't use winter months I favored a Latitude-15 deg tilt, which gets an average for the remaining months of well over 6.0 peak hours. So I went with 6.0. That logic may not be right, but that's how I got 6.0 hours vs. your 5.2 hours. 

As for conservation, there's not much more to squeeze out of that turnip. We are converting all the existing lights (all 15 or so of them) to LED. Other things are cell phone chargers and laptop chargers, which you need when you need. All that is left are the microwave oven (which gets used perhaps 20 minutes per year), and the well pump, which gets used when it is needed. Replacing our current water system with something more efficient (maybe a low voltage, low volume well pump into a gravity feed tank to the house) will cost more than the solar will cost. Probably a non-starter for now. So we are stuck with the 2100Wh per day, although we likely will normally use 50-70% of that.

A 900W well pump would be something to consider, if we knew it would still keep up with our water needs.  I may need to find that thing. My wife and daughters are willing to "rough it" at the cabin because there are lights and enough hot water for a shower. If either of those are suspect I won't get to go to the cabin any more so I won't care about how all this shakes out!  

Although I would have liked to go with cheaper golf cart batteries, I was willing to give in to the push for AGM. Not just because of the maintenance free aspect, but because I thought (I can't find where I got this) that AGM batteries can deal with the higher surge that we are talking about for the well pump. Is that true? Otherwise, I may try again to make the pitch for more AH, and not to be afraid of FLA batteries.

As for the battery monitor: I can stay within the Schneider family with the charge controller and inverter and get their Xanbus battery monitor. If I were to go with my original 4 battery 200+ AH bank (don't give up - I'm still reading what you wrote), that battery monitor looks like it could effectively monitor all four batteries at once. I'm kinda assuming that the Schneider monitor will better integrate with the charge controller and inverter, although I don't even know what that would mean.  Maybe good?

I'm hoping that the low voltage cutoff off of the inverter will allow me to do what you've described with the battery capacity meter (if I understand correctly). It sounds like I can program in a battery voltage into the inverter settings, below which it shuts down. I want to make it so when the battery bank gets close to 50% DOD (or maybe lower for AGM?), I just shut off the inverter. Then I don't have to worry about people not knowing they are going too far. 

I admit that at this point I don't know what I don't know. I can't be afraid of what I will be able to do and someone else in the family won't know what to do, because I don't have a clue what to do myself. The good news is that the adults in our family are all pretty technical, so I think everyone is trainable.

You have been an incredible resource. Thank you so much for the time you've spent to educate me. I don't think I yet know what I need to, but I can see the horizon!

Steve

new2PV

You can program the mppt-60 using the aux contact to trigger a low battery alarm of volts over time. If you want to do the hot water thing you could use a dc element and control it with the mppt-60 also, but it only has one aux contact in it , so only one function at at time, unless you have more than one mppt 60:)

BB.

The PV Watts used Grand Junction as the weather source--I am not sure, but it may adjust for latitude and such for Telluride (or not?).

If you adjusted -15 degrees, yes, that will give you more summer power (I don't like the new PV Watts, I cannot just cut and paste the settings I used).

2 kWH per day is not bad... A "cabin" may be 500-1,000 WH per day. A "home" (refrigerator, well pump, washing machine, TV, laptop, etc.) works out well at 3.3 kWH per day (for a heavily conserving family).

For flooded cell batteries, C/2.5 is about the maximum "recommended" surge from the battery. For AGM, C*1 (discharge battery in 1 hour) to C*4 (discharge in 15 minutes) full running current is possible (for high end AGM batteries). UPS systems use AGM and GEL because of their high discharge current capabilities. True UPS batteries are designed for float service and usually do not last very long for deep cycling (50% or deeper) application. The Concorde Sun Extender are designed for 80% discharge cycle--But I would suggest 50% maximum discharge (during normal use) so that you can get enough hours of sun to recharge.

I have am not off grid, and not in the solar business--Perhaps some other folks with Schneider (and other Battery Monitor) experience can help you with those answers. The Midnite is a "simple" meter that you can give people instructions (like start genset at 50% State of charge and turn off at 80% SOC, call you if it drops below 50% SOC and does not recover with genset and/or full sun, etc.).

Regarding water pumps, here is a Nice German $2k set of pumps--You will need to match your well conditions to the correct model:

https://www.solar-electric.com/residential/solar-water-pumping/submersible-and-well-pumps.html?manufacturer=Grundfos&nav_pump_voltage_range=90+-+240+Volts+AC

I don't know the Schneider ability to use the Battery Monitor to shut down the system. Doing it by pure voltage (say 22 volts for >10 seconds, 24 volts > 30 minutes or something like that can work... But lead acid battery voltages are dependent on load, temperature, state of charge, age, etc... "Picking a single voltage setting" like 23 volts is not usually that accurate (but better than the default ~21 volts most 24 volt inverters use--that is there to protect the inverter, not the battery bank).

You are very welcome--We are all volunteers here and trying to help people with their power needs. All we ask is you share your experiences (good and bad) so that we can all learn.

-Bill

new2PV

The battery monitor will only measure the amount of amp hours taken out and put back in the battery and the voltage, maybe more accurately than the inverter.  Also it will allow you to use the AGS (auto gen start ) to start and stop the gen based on battery SOC. It does a few other small things logs battery cycle count, avg discharge, etc. 

Dave Angelini

The battery monitor gets most data from the inverter and so the data can never be any better. The shunt allows precise data for the battery for the Peukert function and it's error. Its real value to me is in large applications you can add a new bank/and new monitor and use the old one with both batteries being charged independently based on end amps for each bank. The same for multiple BMS.
 More on that next year.

Kind of a waste for an offgrid home unless you have AGMs. Really nice in really large apps or microgrids allowing them to add as they grow!

Horsefly

Dave Angelini said:

The battery monitor gets most data from the inverter and so the data can never be any better. The shunt allows precise data for the battery for the Peukert function and it's error. Its real value to me is in large applications you can add a new bank/and new monitor and use the old one with both batteries being charged independently based on end amps for each bank. The same for multiple BMS.
 More on that next year.

Kind of a waste for an offgrid home unless you have AGMs. Really nice in really large apps or microgrids allowing them to add as they grow!

Hey Dave - I'm not sure I get exactly what you mean. You may need to dumb it down for me. 

Just to make sure, I'm talking about this battery monitor: https://www.solar-electric.com/schneider-electric-conext-battery-monitor.html

It seems like via the pre-scarler, it gets lots of data directly from the batteries, (four monitor points) and from the shunt. Are you sure it gets most of its data from the inverter?  Regarding your comment about "unless you have AGMs" - Since it appears I will have AGMs, what is the increased benefit?  My design has 4 12V AGMs in a 2x2 series / parallel bank, and since the battery monitor has the two mid-point battery voltage sensors, it seemed like I would be able to effectively monitor all four batteries, not just the overall battery bank. If at some point I do expand my battery bank, it sounds like you think the monitor will be especially useful? Is that right? Why?

Sorry if I'm being dense.

Steve

Horsefly

new2PV said:

The battery monitor will only measure the amount of amp hours taken out and put back in the battery and the voltage, maybe more accurately than the inverter.  Also it will allow you to use the AGS (auto gen start ) to start and stop the gen based on battery SOC. It does a few other small things logs battery cycle count, avg discharge, etc. 

Thanks @new2PV. Our generator is pretty old, and I'm pretty sure it won't work with the AGS. It's got a manual choke, which is probably a no-go for any auto start.  Besides, in some ways, I think it may be good to force people to remember how to stop and start the generator.  

I see from your sig that you use the 6.8kW Schneider Inverter. Do you use it off-grid, with a genset? 

Dave Angelini

Most all battery monitors get voltage data, AC load data from the system from schneider and Outback and the current thru the shunt from the battery.

AGM's have no way to check (specific gravity)  that they are full other than the end amp spec the battery manufacture specs. The Concord AGM's I get have the end amps and the Peuket function cal you enter into the monitor. 

My comment to new2PV was that the battery monitor really is no better than the SCP battery tank gage (which is voltage based only) on the SCP or in the combox. They both have error that an SG measurment does not have. 

The battery monitor depends on the peukert function and I would guess dozens of people on this forum alone in 10+ years have said my battery monitor says I am at 50% and the system hit the low voltage disconnect.. Maybe more. Any make of battery monitor calculation of how full the bank is can get increasing error as the number of days from full charge increases. Also from battery aging as the end amp spec is not as accurate and from the user.

 An AGM bank usuually is in need of replacement every 5 years for most folks. The battery monitor makes it easier for AGM
if you do not understand what the SCP tank gage is telling you. I can teach (most) of my clients to not  need a battery monitor by another method.

 The Battery monitors real value to me is in large applications you can add a new bank/and new monitor and use the old one with both batteries being charged independently based on end amps for each bank. It allows parrallel banks with each bank being charged correctly instead of the bad things that typically happen with multiple banks. The same for multiple BMS in advanced battery technology.

WaterWheel

On the Conext Battery Monitor, the factory preset for the Peukert's exponent (function 2.4) is 1.25 and left that way I found that the "battery percentage remaining" was about 15% off after a few days of rainy weather with my relatively new battery bank.       After playing around with it (function 2.4) over several months I found that 1.12 made the  "battery percentage remaining" meter much more accurate.      As the batteries age I'll need to increase this number a bit. 

The way the leads for the monitor are wired it will give you a total battery bank voltage (only one lead so only one total voltage reading) and can monitor the mid points (4th battery on a 8 battery string) on two strings or,,, monitor two mid-points if you are only running one string such as the 1st and the 4th battery on a 8 battery bank.     It's really designed to allow you to spot uneven charging/battery condition between 2 strings.

One additional thought.     When the meter says full it doesn't mean your SG is at 1.277.       When the meter believes your batteries are 92.5% (about 1.263 SG)  full it then reads FULL, this is listed in the documents.

new2PV

The battery monitor will monitor the midpoints, but I don;t think it does anything with that data, other than just tell you the midpoint voltages.  Horsefly I use an EU7000IS genset in offgrid mode, and it works beautiful. Only problem is that if you want to charge the battery you can't use generator support at the same time, which allows you to use some of the battery and partially load the generator. 

Dave Angelini

WaterWheel said:

On the Conext Battery Monitor, the factory preset for the Peukert's exponent (function 2.4) is 1.25 and left that way I found that the "battery percentage remaining" was about 15% off after a few days of rainy weather with my relatively new battery bank.       After playing around with it (function 2.4) over several months I found that 1.12 made the  "battery percentage remaining" meter much more accurate.      As the batteries age I'll need to increase this number a bit. 

The way the leads for the monitor are wired it will give you a total battery bank voltage (only one lead so only one total voltage reading) and can monitor the mid points (4th battery on a 8 battery string) on two strings or,,, monitor two mid-points if you are only running one string such as the 1st and the 4th battery on a 8 battery bank.     It's really designed to allow you to spot uneven charging/battery condition between 2 strings.

One additional thought.     When the meter says full it doesn't mean your SG is at 1.277.       When the meter believes your batteries are 92.5% (about 1.263 SG)  full it then reads FULL, this is listed in the documents.

You would need (2) shunts to monitor the charging between (2) strings. The voltage of the strings is not really what you need. It is the end amps of the strings and the ability run longer on the string that needs more charging.

WaterWheel

Currently I just use a amp meter occasionally to verify that both strings are charging/discharging equally.      The battery meter only has leads for one amp shunt.

Dave Angelini

And ba da boom, that is why I said I use 2 battery monitors for large muti battery bank systems....

new2PV

I hope those 2 strings are fused between them

Dave Angelini

They do not need fuses in the case of an XW+ with a 250Amp main breaker and 4/0 wiring. In really large systems the banks are from multiple 250 A breaker distribution boxes and there could be a need to take additional steps like a fuse/breaker for over current conditions in multiple battery strings. A really rare type of battery failure might require overcurrent protection between banks of the older technology. Certainly not the new storage which is all internal to the BMS.

The multiple bat mons is really an elegant way for a microgrid to grow with more humans joining the system. The old battery bank capacity can be added to with a new bank without the issues of different age batteries.

It can also help someone who may be reading this with adding new Lithium technology storage and an existing lead acid bank to use them both! Hint

new2PV

I would just use the XW fused combiner box, its nice and not too expensive, and you get comfort of added protection, if a rare event would happen.

WaterWheel

Dave Angelini said:

And ba da boom, that is why I said I use 2 battery monitors for large muti battery bank systems.... 

I agree that a 2nd battery monitor would be nice but at $480 a bit expensive, I'll make due with one and check with the clamp meter occasionally.      While I have the Conext 250 amp breaker in the PDP I went ahead and added a Blue Sea fuse between the battery banks for extra safety.