Critique my system

fiddlerkellyfiddlerkelly Registered Users Posts: 19
This is my first post so I thought to let you know what I have and how set up . that way if you see anything I can improve on or change it would greatly be appreciated.

I have a 12 volt bank comprised of 18 -6 volt golf cart batteries with 232 amp hours at 20 amp hour rating. They are connected in series and charged by 5 - 50 watt panels. I live in eastern Canada ,Newfoundland, to be specific so peak sun hours in summer are approximately 5-6 hours. I have a basic charge controller x 2 with 3 panels connected to one and 2 panels to the other due to amount of amperage controllers can handle. I use on average 200 watts an hour =1.6 amps for 9 hours a day in winter that is what I based by battery bank size on .

My batteries which are connected in series to make 12 batteries are all then connected in parallel so I have 9 12 volt batteries. My 120 volt power invertor is pure sine wave and is a Canadian tire product. My charge controllers are EPIP20 -H series controllers.

I plan on changing charge controllers to a midnite solar 150 and am planning on adding another 500 watts of panels . I have my batteries all connected with heavy welding cable .

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Critique my system
    I have a 12 volt bank comprised of 18 -6 volt golf cart batteries with 232 amp hours at 20 amp hour rating. They are connected in series and charged by 5 - 50 watt panels. I live in eastern Canada ,Newfoundland, to be specific so peak sun hours in summer are approximately 5-6 hours. I have a basic charge controller x 2 with 3 panels connected to one and 2 panels to the other due to amount of amperage controllers can handle.

    Welcome to the forum.

    OMG! you have 9 parallel batteries! Big problem. read: http://forum.solar-electric.com/showthread.php?14674

    OMG! you don't have anywhere enough panels to charge such a large battery bank. Your battery bank is 9 X 232 = 2088 amphours. You need a minimum of 100 amps of DC charging current to charge them... That's about 1500 watts of solar panels.
    I use on average 200 watts an hour =1.6 amps for 9 hours a day in winter that is what I based by battery bank size on .

    You don't really mean 'watts an hour'. You mean that you consume 200 watts for 9 hours = 1800 wattHours = 1.8 kilowatthours.

    That is important info... you can use it to design a balanced system. If you consume 1.8 kwh per day, then you should have a battery bank that can store about 8 kwh. That would be about 3 parallel strings of 2 golf cart batteries per string. And in my opinion that is not a well designed system because 3 parallel strings is too much.

    My advice is switch to 24 volt and use a single string of four L16 batteries or 2 parallel strings of golf cart batteries (8 batteries total).

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: Critique my system

    IMHO,

    - Panels are undersized for loads, and extremely unersized for batteries.

    - Batteries are oversized for loads. However, it all depends on usage pattern and on the generator use.

    - 12V is a very bad choice. 48V would work the best.
  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: Critique my system

    Welcome to the forum Kelly.

    Regarding your system... Your 200 watt loads at 120 VAC for 9 hours per day. That is pretty reasonable for an off grid system. But, your battery bank being so large--Are you trying to have many days of 'no sun' power without having to use a genset (unattended operation, or just don't want to use a genset)?

    For a "balanced" system, I would normally be suggesting a 2 day "no sun" operation and 50% maximum discharge (for longer battery life). But, at the end of 2 days, you would either have to fire up a genset and/or cut power usage until the sun returns.

    When you make an oversized battery bank, for proper charging, you need a larger solar array (and genset) to keep the batteries happy.

    In the "olden days", people would add lots of batteries for "underpowered" solar systems (when solar panels were $10 per watt). And the batteries frequently failed early and were replaced (when batteries were cheaper).

    Also, large banks of lots of paralleled battery strings require lots of ongoing maintenance. Checking cells (water and specific gravity), checking/cleaning wiring/connections. And issues when a cell fails (open/shorted) and having that kill a string or an entire bank--But difficult to find (I suggest getting an inexpensive DC Current Clamp meter/DMM to measure charging/discharging current per string, and cell/battery voltages, as well as logging specific gravity for all cells once a month or so). And there is the "proper" method for cabling to ensure batteries are sharing current.

    These days, solar panels are ~$1-$2 (or less) per watt (for larger solar panels), and batteries are quite expensive. The proper way to go is to increase the array size so that the batteries are recharged every 1-2 days, and for longer stretches of bad weather, use a small genset to support the loads and battery bank until the sun returns.

    Have you measured/logged your temperature corrected specific gravity for each cell? Batteries tend to sulfate if they are not fully recharged (>90% SOC) every few days (to every couple weeks) and/or if they set below ~<75% State of Charge for weeks/months on end.

    A bank with lots of paralleled batteries can work--And many people do it successfully. But the extra maintenance/monitoring, and safety issues (each parallel string should have its own fuse/breaker to limit the damaging effects of a short circuit)--That most people are tending to fewer batteries, larger AH per cell batteries, and larger solar arrays and accept the use of a genset during bad weather/poor sun/heavy loads (such as running a shop, repairing the home, etc.).

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://www.batteryfaq.org/
    http://batteryuniversity.com/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fiddlerkellyfiddlerkelly Registered Users Posts: 19
    Re: Critique my system

    First ,thank you everyone who responded .

    Bill ,

    When first deciding how much I would need I wanted the genset, I assume you are referring to a generator with this term, to be strickly as back up in case something went wrong or if we had an excessively long time without sun. Up here in Newfoundland week plus with out sunlight is common especially in early summer or late fall. I currently get to the cabin for a few days every couple of weeks but plan on when I retire spending all the fall up here and once winter comes spending all my time up here. So the intention was to build it getting a couple of weeks use with no generator .

    The charge controller I have now is PWM . I pal on getting the other type MTTP. With the size of panels I have now will it be enough to equalize the batteries I have . Bill I have thought about going to a 24 bank set up but that will mean either getting rid of 2 batteries or adding 2. From reading I had done when first getting ready for this project I was left with the understanding that you should only discharge your system by 20%. So as a result that is how I done the math for what I would need . As much as I only usually have about 200 watts burning I used the total wattage in the cabin for my number kind of giving me room for error.

    So if you say my bank is too big for the panels how about I change to 24 volt get rid of 2- 6 volt batteries and split the bank so I am only using half at a time that way as one half is being used the other half is charging . Plus add another 500 watts of panels .


    Bill if I buy more panel do they have to be the same as the ones I have now . When I say the same I mean brand , type ,wattage everything?


    How do I add pics to this . I have tried twice with no success,

    Thanks again
  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: Critique my system

    I live in relatively sunny California--We have quite a few folks from the great white north (including North Guy), so they can probably give you some better advise/details for your situation.

    I don't know how difficult it is to get fuel in your area, or how cold it gets (problems with diesel gelling, propane vaporizing, so gasoline is probably you best bet for a smaller genset).

    To be honest, I am not sure how lead acid batteries would behave if you subjected them to a two week discharge only--But my guess is that they will not like it (sulfation). If using a genset (generator set) is possible for you--Then it is probably the more cost effective solution (packing fuel/noise/maintenance as part of the deal).

    As always, starting with measuring loads is a great starting point. A Kill-a-Watt type meter is a good start. For a large battery bank, a Battery Monitor (Victron is another good brand) is very helpful to understand what is happening. An inexpensive DC Current Clamp DMM is handy to check current flow into each string (charging/discharging) too (high/low current flow, any string that is drawing too much/too little needs to be looked at).

    Normally, we suggest a 1-3 day energy storage and 50% maximum discharge for battery bank sizing. With 2 days + 50% maximum discharge working well.

    Just running the numbers with what you know today--Starting with 1.8 kWH per day, 2 days of storage, 50% max discharge:
    • 1,800 WH * 1/0.85 inverter eff * 1/12 volt battery bank * 2 days of no sun * 1/0.50 max discharge = 706 AH @ 12 volt battery bank
    • 706 AH / 220 AH "golf cart" battery = 3.2 parallel battery strings.
    • 3 strings * 220 AH = 660 AH @ 12 volt "golf cart" battery bank (6 batteries)
    • 4 strings * 220 AH = 880 AH @ 12 volt "golf cart" battery bank (8 batteries)

    From a simple sizing issue--That is about as large of 12 volt bank as you would want. 10% rate of charge (5-13% rate of charge for a typical lead acid battery bank). Around 80 amps is about the largest commonly available MPPT charge controller.

    Personally, I like to use larger AH cells and get fewer strings. I like to suggest 1-3 parallel strings maximum. I think 1 string is better, but some folks like a pair of strings--They can disconnect one string if there is a cell failure and limp along on the other batteries until a replacement can be installed.

    For example, here is a Trojan L16P 6 Volt, 420 AH Deep Cycle Battery -- You could use two strings in parallel for 12 volts @ 840 AH or one string in series at 24 volts and 420 AH (still 4 batteries, but 1/2 the current for charging/loads/inverter at 24 volts).

    If your loads remain low--A smaller AC inverter can be nice. The MorningStar 300 Watt 12 VDC PSW inverter is really nice for a 12 volt system (very efficient, search mode, remote on/off connection).

    If your loads are going to be >~1,200 watts (electric refrigerator, well pump, etc.), then you should be looking at a 24 volt system anyway (1,200 watts is >100 amps at 12 VDC--Very heavy cables and sensitive to voltage drop issues).

    I will do the 420 AH at 24 volt system--But 12 or 24 volts does not change the math (430 AH @ 24 volts vs 840 AH @ 12 volts is the same amount of stored energy in kWatt*Hours)--So the array and generator sizing answers remains the same.
    • 420 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 791 Watt array minimum
    • 420 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,582 Watt array nominal
    • 420 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,056 Watt array "cost effective maximum"

    And, how much sun do you get vs generator run-time and fuel costs... Using PV Watts for Gander NF, fixed array tilted to latitude (aprox 49 degrees from horizontal--Note, you may wish to mount arrays vertical high off ground in winter if you have lots of snow--help keep array clear of snow and get reflections from snow field in front), you should see (long term average "useful" noon-time equivalent hours of sun per day by month):
    Month    Solar Radiation (kWh/m2/day)
    1      2.62     
    2      3.44     
    3      3.99     
    4      4.01     
    5      4.21     
    6      4.29     
    7      4.46     
    8      4.13     
    9      3.68     
    10      2.99     
    11      1.62     
    12      1.71     
    Year      3.43      
    
    No sun is no sun... If we toss the bottom three months and use 2.99 hours of sun in October as your "break even" generator month:
    • 1,800 WH * 1/0.52 system efficiency * 1/2.99 hours of sun = 1,158 Watt Array minimum (based on loads and sun)

    So, these assumptions would suggest a 1,158 to 2,056 Watt array would be a good fit for your area/power usage. With the 10% rate of charge 1,582 Watt array being a healthy value of charging.

    And, if you use the 10% array, you could support "darker days":
    • 1,800 WH daily AC loads * 1/0.52 system efficiency * 1/1,582 Watt array = 2.2 hours of sun "break even" day (10% charge)
    • 1,800 WH daily AC loads * 1/0.52 system efficiency * 1/2,056 Watt array = 1.68 hours of sun "break even" day (13% charge)

    With a larger battery bank, you need a larger generator too... 5-13% rates of charge are more efficient--But with a genset, some folks will run upwards of 20-25% maximum rate of charge.

    There are lots of different chargers out there (and if you go with a 24 volt or 48 volt inverters--You can get inverter-chargers which can give you better efficiency and more options/configurations. So, assuming worst case "nominal" battery charger efficiencies and running at genset at 80% of maximum rated load:
    • 420 AH * 29 volts charging * 1/0.80 charger efficiency * 1/0.65 Power Factor * 1/0.80 maximum gen loading * 0.05 rate of charge = 1,171 VA minimum
    • 420 AH * 29 volts charging * 1/0.80 charger efficiency * 1/0.65 Power Factor * 1/0.80 maximum gen loading * 0.10 rate of charge = 2,342 VA nominal
    • 420 AH * 29 volts charging * 1/0.80 charger efficiency * 1/0.65 Power Factor * 1/0.80 maximum gen loading * 0.13 rate of charge = 30.45 VA nice size unit
    • 420 AH * 29 volts charging * 1/0.80 charger efficiency * 1/0.65 Power Factor * 1/0.80 maximum gen loading * 0.20 rate of charge = 4,685 VA not too much larger+monitor battery bank for temperature
    • 420 AH * 29 volts charging * 1/0.80 charger efficiency * 1/0.65 Power Factor * 1/0.80 maximum gen loading * 0.25 rate of charge = 5,856 VA really should not go larger

    I suggest you want to run the genset at ~50% or more loading for best fuel efficiency (and for diesels, they need 40-60% minimum loading to avoid coking/wet stack/cylinder glazing). It is very easy to run a genset at 25% or less loading, and spending 2-3x as much for fuel.

    Since you will be using genset+batteries a lot during the dark months of winter--There is a more efficient way of running the genset... Discharge the batteries down to ~50% state of charge and recharge them back to 80% the next morning. That will keep the loading on the generator high (don't get cheap generators, battery charging is a very difficult load for them compared to what most people use generators for). Cycle 50-80% for 6 days a week and recharge >90% State of Charge once a week. The batteries cycling will prevent sulfation (some battery vendors say 90% charge once every 2-4 weeks) and be the most efficient use of fuel.

    With some inverter-chargers--You can also run your AC loads at the same time and share the generator power between charging and running your loads (inverter charger will limit AC load to a maximum programmed AC current). This can save fuel too--You don't have an extra conversion step (AC to DC to battery to AC) so your system "wastes" less fuel.

    Anyway, I will stop here--I have typed a lot and made lots of assumptions. It is very possible that you would be best with a "small system" as long as you are there only part time--And then when you move there full time, look at a new system design (it is very difficult to "cost effectively" grow an off grid power system). Keep the old system for backup or sell it to a neighbor.

    Questions, corrections, disagree with my assumptions? Energy usage is a highly personal set of choices. What you need/want is what matters (no right or wrong answers--just lots of shades of grey). Not my guesses at your needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: Critique my system

    Adding pictures--If you can, keep them smaller. The forum software will down convert automatically. On my system, when I click "insert image" icon, I can browse and attach a photo. What is not obvious (at least on my Firefox browser), below the browse/file name stuff is an "upload files" text--You have to click on that to actually upload the file.

    Here is a 900 kByte photo from a decade ago I took:

    The James Johnston House: The White House of Half Moon Bay


    Attachment not found.

    Down converted to a 48kByte file automatically.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fiddlerkellyfiddlerkelly Registered Users Posts: 19
    Re: Critique my system

    Bill ,

    Here are the pics . I had them too large. Some questions also and anyone else feel free to post a comment.

    You said my bank ( 18 -6 volt batteries set up at 12 volts and then ran parallel )was too large for the amount of panels ( 250 watts total ) I am currently using and that I need to discharge my batteries to approximately 50 % for better life out of my batteries. So I have been thinking about this and at first was going to 1/2 my bank but that would have ended with one bank being 2 -6 volt batteries bigger. So I thought why not divide into 3 giving me 6 -6 volt batteries in each bank .

    Now with 232 amp hour rating at 20 amps I would have a total of 696 amp hours a bank and using 50 % would allow me to use 348 amps before needing a recharge . So would my 250 watts of panels that I have now suffice in recharging that size bank . Even if I use 5 amps an hour 0r 40 amps a day seems like I would be good for 7-8 days before having to recharge, so then while it is recharging I can change over to one of the 2 other banks. Any of this make sense. As I said before I built this bank initially on the understanding not to discharge the system anymore than 20 % that is how it ended up with 18 battereis . Sorry don't mean to ramble on by the way .

    What is the name of the piece of equipment that I can use to recharge and equalize my batteries using my generator. I have by the way a Honda EU 2000 will that suffice.

    Also the temp in cabin when I arrive in the winter is sometimes -10 to -20 celius so the battery bank size of 6 would allow for some loss there till they warm up ,right ?

    Thanks for advice,

    ChrisAttachment not found.Attachment not found.Attachment not found.
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: Critique my system
    So I have been thinking about this and at first was going to 1/2 my bank but that would have ended with one bank being 2 -6 volt batteries bigger. So I thought why not divide into 3 giving me 6 -6 volt batteries in each bank.

    I don't think it's a good idea to make two banks. This will be very hard to handle and will not really improve anything. Money on batteries are already spent.

    I would remove 2 batteries, re-wire the rest for 24, or better 48V, then add some panels, to get some minimum power to charge batteries. This would provide an oversized, but somehow balanced system. Battery use would be small which would make for a long life for these batteries. You could add some loads if you ever need.
  • BB.BB. Super Moderators, Administrators Posts: 30,485 admin
    Re: Critique my system
    You said my bank ( 18 -6 volt batteries set up at 12 volts and then ran parallel )was too large for the amount of panels ( 250 watts total ) I am currently using and that I need to discharge my batteries to approximately 50 % for better life out of my batteries.

    Just to be clear, we usually recommend discharging batteries, at a maximum, to about 50%. You can go deeper, but batteries may not last as long. Going below 20% state of charge risks permanently damaging battery bank and requirement replacement of cells/batteries/entire bank.

    And discharging ~10% to 20% per average cycle is, roughly, the minimum cycling (i.e., cycling/charging deep cycle batteries from 90% to 100% is hard on them too and can reduce their life; use lots of water; etc.).

    Also, to be clear--Batteries are rated at different discharge rates. The 232 Amp*Hour capacity is at a 20 Hour Rate. So, the ability of the battery to supply a steady current (from 100% to 0%) is:

    232 AH / 20 Hour Rate = 11.6 amps (AH for one battery/one series string)
    3 strings * 11.6 amps * 12 volts (average voltage) * 0.85 inverter eff = ~355 Watts average AC load for 20 hours (20 Hour Rate)

    And you would discharge around 20-50% of 20 Hour Rate or ~4-10 hour discharge at ~355 watts average load.
    So I have been thinking about this and at first was going to 1/2 my bank but that would have ended with one bank being 2 -6 volt batteries bigger. So I thought why not divide into 3 giving me 6 -6 volt batteries in each bank.

    Yes, this is probably a good idea (I personally like to recommend a maximum of three parallel strings--Any more than that becomes a bit of a maintenance nightmare). Although, there are people with more parallel strings and are happy with the results.

    Remember that we use a 5% to 13% rule of thumb for charging (you can go to 20-25% or so, but that is usually done with a genset--higher rates of charge are generally not needed with solar and a bit expensive/waste of money).
    Now with 232 amp hour rating at 20 amps [20 hour rate] I would have a total of 696 amp hours a bank and using 50 % would allow me to use 348 amps before needing a recharge . So would my 250 watts of panels that I have now suffice in recharging that size bank.

    In general, your goal should be to recharge the bank >90% several times a week and avoid going below 50% state of charge.

    There is an alternative charging profile. Basically, you cycle between 80% and 50% state of charge daily (or every 2 days or so), and once every week, or every two week, recharge back >90% SOC (one company says you can go as long as 4 weeks between >90% charge).

    As long as you are actively cycling in the 50% to 80% SOC, the batteries should not sulfate. Letting the batteries set below ~75% SOC for days/weeks/months--They will sulfate and fail.

    Size of solar array for a 696 AH 12 volt battery bank:
    • 696 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.02 rate of charge = 262 Watt array to "Float" a battery bank
    • 696 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 655 Watt array minimum
    • 696 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,311 Watt array nominal
    • 696 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,704 Watt array "cost effective maximum"

    With a 250 Watt Array, you are still way under powered for even that size battery bank. At best, this is a bit more than needed to "float" a battery bank in standby (around 1-2% charge rate for float/storage of battery bank).

    And if you want to run 1.8 kWH load per day for ~9 months of the year, you will need a minimum array of:
    • 1,800 WH * 1/0.52 system efficiency * 1/2.99 hours of sun = 1,158 Watt Array minimum (based on loads and sun)

    Assuming only ~2.99 hours of sun per day for October (break even month where you may need a bit of generator).
    Even if I use 5 amps an hour or 40 amps a day seems like I would be good for 7-8 days before having to recharge, so then while it is recharging I can change over to one of the 2 other banks. Any of this make sense.

    First, Amps and Watts are already "rates". Amps is Coulombs per Second, and Watts is Joules per second. So, a "5 amp an hour" rate is just typed 5 amps (like gallons per hour). Or 40 Amp*Hours per day (at XX volts). Amp*Hours and Watt*Hours is an amount (gallons pumped).

    And, no, I don't think that I would discharge for 7-8 days between charging cycles. I believe that will give you early sulfation.

    "Over sized" Lead Acid battery banks are a pain. It is done in some cases such as a Telephone Central Office. But those batteries will be replaced at the end of the "Power Emergency" if done more than a few times (TeCO's batteries are designed to float and only provide limited amount of power until the standby generator starts--And they want to equipment to run to destruction (if needed) to provide emergency phone service as long as possible.

    We don't want to do that with off grid battery systems.
    As I said before I built this bank initially on the understanding not to discharge the system anymore than 20 % that is how it ended up with 18 batteries . Sorry don't mean to ramble on by the way.

    Not a problem--Batteries are imperfect at best. Lead Acid, even after 100 years of battery development, on average, are still the best bang for the dollar out there for most people. But you have to live within their limitations.
    What is the name of the piece of equipment that I can use to recharge and equalize my batteries using my generator. I have by the way a Honda EU 2000 will that suffice.

    Oh boy, finding the "ideal" AC battery charger. There are not many stand alone chargers that are ideal for operating with a genset. Today's (good chargers like Iota) are stuck in the 1970's. Little in the way of controls/adjustments. And not that efficient. However, they are reliable and will work. Iota does have a 10 turn pot in their chargers you can adjust voltage with. And they can use a manual (or automatic) switch to control float vs charging. You have to manual crank up the voltage to get them to do corrective equalization (charging >15 volts for several hours).

    If your battery bank is running correctly, you can usually use your solar charge controller to equalize the bank (outside of the dead of winter).

    And, if you use larger inverters (~several kWatt) and higher battery bank voltages (24/48 volt banks), you can get some pretty nice Inverter-Chargers. These have software adjustable set points and can support other modes too (basically act like a UPS with an external battery bank, and even support large loads with a smaller genset by using the inverter+battery bank to add AC power to a smaller genset).

    For your battery 696 AH battery bank, the Honda eu2000i is a bit on the smallish side. You can run around a 40-45 amp 12 volt AC charger on the Honda:
    • 45 Amp Iota / 696 AH battery bank = 0.065 = 6.5% rate of charge

    It will work OK--But you may be putting a lot of hours on that genset. If you choose 15% rate of charge:
    696 AH * 0.15 rate of charge = 104 AH

    Iota makes a 90 Amp 12 volt charger... The recommended minimum rated genset to run that would be (in my humble opinion) would be:
    • 90 Amps * 14.7 volts charging * 1/0.80 charger eff * 1/0.65 Power Factor * 1/0.80 max genset loading = 3,180 VA rated genset minimum

    You have lots of options here... But you also have to make some choices.

    Large battery banks, as you can see, have high DC current requirements. And for this size battery bank, you should really be looking at a 24 or possibly 48 volt battery bus. If you go with the higher voltage bank, your charging currents are cut by 1/2 (or to 1/4). And you have more options (such as inverter-chargers).

    But this stuff will not be cheap. And the complexity of the system grows. You have some choices you need to make here.
    Also the temp in cabin when I arrive in the winter is sometimes -10 to -20 Celsius so the battery bank size of 6 would allow for some loss there till they warm up ,right ?

    I have a difficult time getting -18C in my freezers--I am not the person to ask about low temperature operation. Yes--they will work (don't discharge them too far, or they can freeze). But you will probably have to run the genset to get the cabin and battery bank up to a reasonable temperature.

    Some folks here run insulated and/or battery banks in a cellar/partially buried. Once the batteries are at a reasonable temperature (>0C) the normal battery cycling will usually keep the bank warm enough. One person (I think) got those 120 VAC battery heaters for car/truck in cold climates and used the genset to help warm up the battery bank that way.

    I hope I am helping here... There are lots of issues. And we try to design a "balanced" system (loads drive battery bank, battery bank drives solar array+backup generator sizing).

    Your "extra batteries" are going to be a millstone for you... If you can find somebody to buy them from you (or you can give them too)--It would be better. Trying to manage 2-3 separate battery banks is going to be a pain--And if you use Bank "A" daily, by the time it wears out/fails, Bank B and C will probably be near end of life too.

    I certainly do not have all of the answers. But "unbalanced" systems tend to be costly and can cause maintenance issues.

    Your thoughts/questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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