Advice on buying new bank of batteries for current off-grid instalation.

Hi Wayne. Thanks for your reply.
System is 24V (Victron inverter 24V / 2000Va- something between 1600~1400 Watts). Cheapest batteries around are 12V, but I was thinking to put 4 ( two series of two in parallel) at least, as Cariboocoot suggests above.
My main concern is I definetly need around 6 Kwh/day and was not sure how many 12 V -C100 250Ah- batteries would need to reach such amount of power. I am not sure paralleling AGM batteries is a good choice.
The charge controller admits 2000W @ 24V and 4000V @ 48V.
One solution I thought was to sell the inverter and buy a 3000Va/48V to increase number of panels till 4000W.
But all equipment is fairly new (still on guarantee as old owner gave me the invoice. Besides he claims it has never been set up).
Researching the local market, a 3000Va /48 V rounds more than 2000 €, so my other thought was to stick with the 24V inverter and buy 2 more panels and batteries as bigger as my current system could deal with, plus maybe another 2000Va/24V inverter to parallel (850 €).


Thanks Cariboocoot for calculations. Figures are clear now.
As told to Wayne, I think I will spend money on 2 more panels, 4 batteries and a new inverter to put in parallel. Seems easier and surely cheaper than a new inverter plus 8 batteries. Also, I will take both you advise and try to reduce my consume.

By the way, if 25% Dod = 2.7Kwh, 50% should be 5.4 Kwh, pretty close to what I'd need. My understanding is AGM technology allows 50% Dod safely.

Following your other advise, I think I could get a cheap Victron charger 50A / 24 V if that makes sense for the C20_480Ah batteries.

Would you please advice on a generator ( to charge batteries only, not for taking up on consume)? Not brand but Watts needed.

Thanks in advance.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    In a situation where the array is already present you have to look at how much battery capacity it can support. Right now you have (6 * 255) 1530 Watts of PV on an MPPT controller and a 24 Volt system. The peak current would probably be (1530 * 0.77 / 24) 49 Amps, so by the 10% rule-of-thumb you could have a 490 Amp hour battery bank, perhaps a tad larger in sunny Spain.

    Your battery choices given are 12 Volt and 225 Amp hours @ the 20 hour rate (according to Trojan). Four of these would give you 450 Amp hours @ 24 Volts, which works well for the charging available.

    On the discharge side 25% of the capacity would be 2.7 kW hours DC, far short of your desired 6kW hours AC. Even 50% DOD would not be that much. As Wayne said you need to get that consumption number down first.

    Failing that you will need to increase the size of both the battery bank and array. To support 6kW hours AC on a 24 Volt system we'd normally be looking at 1175 Amp hours and 3678 Watts of PV. In short, we'd be looking at a 48 Volt system for that much power.

    You'd be surprised how much you can do with 450 Amp hours @ 24 Volts, though.

    You really should have an AC charger and generator too, just in case.
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Hi Wayne. Thanks for your reply.
    System is 24V (Victron inverter 24V / 2000Va- something between 1600~1400 Watts). Cheapest batteries around are 12V, but I was thinking to put 4 ( two series of two in parallel) at least, as Cariboocoot suggests above.
    My main concern is I definetly need around 6 Kwh/day and was not sure how many 12 V -C100 250Ah- batteries would need to reach such amount of power. I am not sure paralleling AGM batteries is a good choice.
    The charge controller admits 2000W @ 24V and 4000V @ 48V.
    One solution I thought was to sell the inverter and buy a 3000Va/48V to increase number of panels till 4000W.
    But all equipment is fairly new (still on guarantee as old owner gave me the invoice. Besides he claims it has never been set up).
    Researching the local market, a 3000Va /48 V inverter/charger rounds more than 2000 €, so my other thought was to stick with the 24V inverter and buy 2 more panels and batteries as bigger as my current system could deal with, plus maybe another 2000Va/24V inverter to parallel (850 €).


    Thanks Cariboocoot for calculations. Figures are clear now.
    As told to Wayne, I think I will spend money on 2 more panels, 4 batteries and a new inverter to put in parallel. Seems easier and surely cheaper than a new inverter plus 8 batteries. Also, I will take both you advise and try to reduce my consume.

    By the way, if 25% Dod = 2.7Kwh, 50% should be 5.4 Kwh, pretty close to what I'd need. My understanding is AGM technology allows 50% Dod safely.

    Following your other advise, I think I could get a cheap Victron charger 50A / 24 V if that makes sense for the C20_480Ah batteries.

    Would you please advice on a generator ( to charge batteries only, not for taking up on consume)? Not brand but Watts needed.

    Thanks in advance.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    :confused: I think I just stepped into the twilight zone there. :p

    DC Watt hours aren't the same as AC Watt hours due to the lack of compensation for inverter consumption and conversion efficiency. So 2.7kW hours DC tends to be only 2.2kW hours AC.

    You don't want to parallel a lot of batteries no matter what the type. If you need more power either go up in Voltage or up in cell capacity rather than parallel lots of batteries. A bit about system Voltages & capacity: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    You can got to 50% DOD with AGM or FLA, but deeper cycling of either shortens their lifespan. 50% DOD on 450 Amp hours works out to about 4.5 kW hours AC, but could be better depending on how much power is used during the day when batteries are charged and panels can still produce. The more power that has to be stored in and taken from the batteries the lower the over-all efficiency.

    It would be a good idea to maximize the array if possible. I wish you hadn't edited the first post because now I've lost what the charge controller capacity is. 60 Amp? At that size the most array you could get would be about 1870 Watts. Eight of the 225 Watt panels would put you at 2040 Watts and you'd be wasting about one panel. This is another area where going up in system Voltage helps, because a controller can usually handle its maximum Amps at any system Voltage: double the Voltage * same Amps = double the power.

    How large a generator you need is going to depend on how large the battery bank ends up being and how big a charger you get to recharge it with. The gen must be able to supply the full charging power and handle the AC loads at the same time. This is where you start thinking about inverters with built-in chargers; on a system this size it is well worth the investment.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Some information from the first post (edited):
    6 x 255 W panels (STC: Vmpp 30,5V, Impp 8,40 A, Voc 37,5 V, Isc 8,83 A)
    1 Victron Mppt 150/70 charge controller
    1 Victron inverter (no charger) Compact 24V/2000Va
    No batteries.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Well a 70 Amp MPPT controller gives a little more room. That will take 2040 Watts on 24 Volts.

    You could go three parallel strings of those 225 Amp hour batteries even though paralleling is not the best idea. Then you'd have 675 Amp hours, 65 Amps peak to charge with, and about 6.8 kW hours AC at 50% DOD. It would just meet the specs for consumption, and with good light/daytime use even better.
  • PhotowhitPhotowhit Solar Expert Posts: 5,655 ✭✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    What are your heavy loads that you need 6+kwh?

    Could you switch cooking to gas, buy a more efficient fridge, Switch from a tower computer to a Laptop? They make some pretty fast laptops now-a-days. If you have a server they even make some pretty efficient servers now.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    My apologies Cariboocoot and B.B. I must have been messing up with posts unwittingly . Lack of skills on computers.

    My understanding is if I keep 24V system:
    8 panels (4 series of two) on the 150/70 controller
    Up to 675 Ah on batteries. Better if I put 6V or 2V cell batteries in series than blocks of 12V in parallel.
    Around 65A charger powered by a generator of xxxxx watts.

    Right, sounds affordable. Just one question regarding inverters-chargers.
    If I get for example a Victron Multiplus Compact 2000Va /50/16 (same as the one already in the house but with charger), could it be possible to connect the output of this inverter in the AC in of the inverter to fool the charge and make it think it is a generator or a grid connection even?
    Maybe I'm just crazy but, like majority I guess, don't like that much generators.

    Thanks
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Not a problem Mel... We all started at the same point of knowledge.

    Normally, only edit your post(s) if there are mistakes or you want to make some quick clarifications to a new post.

    Otherwise, just make a new post with your replies, answers, further questions. Computer storage is cheap these days--And doing this makes it much easier for people to follow the threads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.
    My apologies Cariboocoot and B.B. I must have been messing up with posts unwittingly . Lack of skills on computers.

    My understanding is if I keep 24V system:
    8 panels (4 series of two) on the 150/70 controller
    Up to 675 Ah on batteries. Better if I put 6V or 2V cell batteries in series than blocks of 12V in parallel.
    Around 65A charger powered by a generator of xxxxx watts.

    So far, so good. :D
    Right, sounds affordable. Just one question regarding inverters-chargers.
    If I get for example a Victron Multiplus Compact 2000Va /50/16 (same as the one already in the house but with charger), could it be possible to connect the output of this inverter in the AC in of the inverter to fool the charge and make it think it is a generator or a grid connection even?
    Maybe I'm just crazy but, like majority I guess, don't like that much generators.

    Thanks

    You're talking about AC coupling the output of the existing inverter to the AC input of a new inverter. There is absolutely no advantage in doing this. The first inverter needs batteries to run off of and so does the second. In essence you would have the panels charging the first set, then lose power converting the DC to AC in the first inverter, then lose more power converting the AC to DC in the second inverter to charge the second battery set. Those two conversions simply lose you power.

    The idea of having the generator is so the batteries can be recharged when the sun won't do it, not all the time. BTW you'd be looking at around a 2kW generator for this, possibly more depending on what the concurrent load needs are.

    Really if you're going to go for an inverter-charger now is the time to move up to 48 Volts. Smaller Amp hour but higher Voltage battery bank, greater array accommodation on the controller's 70 Amp maximum, and inverter with built-in charger. Much simpler and more efficient.
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Where were you Cariboocoot during my childhood?? If I'd ever had a teacher who had explained things as easy and clear as you I would have completed my career!!

    I have to make numbers now considering sticking with what I already have vs moving to 48V.
    You know not always we do what we wish, but what we can (in this case, afford).
    Meanwhile, thanks to you all for your time and great advise.

    Pd: Photowhit, 6 Kwh comes as average from my last year electricity bill in another residence.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    6 kWH per day is a good number for a grid connected home... In many countries, the average is around 600-1,000 kWH per month or 20-33 kWH per day.

    Just to give you something to aim at--3.3 kWH per day (100 kWH per month) is a good target. Done right with lots of conservation (and using propane/other fuel sources for heating/cooking), can almost be a "near normal" electrical existence (full size refrigerator/freezer, LED/CFL lighting throughout the home, TV, laptop computer, well pump, clothes washing machine, etc.).

    It is a small enough system that most people can afford, relatively easy for Do It Yourself install and maintenance, etc. Still not cheap, but not jaw dropping expensive either.

    In the US, look at off grid power costing around ~$1-$2+ per kWH, or roughly 10x the cost of utility power. Some folks, with lots of shopping around and doing their own installs/proper power management can get down towards $0.50 per kWH.

    But use the $1-$2 per kWH... That means a $300-$600 per month power bill even at 100 kWH per month equivalent. So--Knowing that off grid power is so dear--What would you do conservation wise, to save that money.

    Gensets (with price of fuel in the US), are similar co$t$ per kWH power costs too off grid solar. So, avoiding a genset is not always a good idea.

    Roughly, most relatively sunny countries get >~4 hours of sun (equivalent) per day for 9+ months of the year.

    In deep winter, you may get ~2 hours of sun per day, and during a week of "bad winter weather", you may get almost no solar power generation power at all (I have seen days that are less than 5% of normal power generation with my GT system). So, generators can keep the solar array to a "reasonable size", and during bad weather, your choice is to fire up the genset a few hours a day for ~10-30 days during winter--Or simply going without.

    And, if you don't take care of your (very expensive) battery bank--Over discharging or letting the bank set at less than ~75% state of charge for days/weeks/month at a time, can take years off the battery bank life.

    Not to say that you need a 10-20kWatt genset... A good quality 2kW-6kW genset (with fuel of choice) can be a very nice friend during winter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,655 ✭✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.
    ... 6 Kwh comes as average from my last year electricity bill in another residence.

    It's good to have a number to start with, Did to air condition the pervious place? Will things change in the new place, cook with gas rather than electric...

    I use 6kwh+ during the summer when air conditioning, but our heat comes with sun... so my system can handle it.

    Might look at your big energy users now, an old fridge might use 2+kwhs a day, while new ones can creep under 1kwh. Most heating things should be handled with gas, water heater and cooking, house heating with gas or wood...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Hi Photowhit,

    The place is an old 300 m2 house built in 1942 into 3000 square meters of vegetable garden and fruit trees surrounded by a river in the middle of nowhere. Closest neighbour 1 mile, little village 15 miles, petrol station ?? miles. In the other hand, there is a huge variety of animals: eagles, mountain goats.... a paradise.

    I intend to live in while refurbishing the house, so plenty of carpenter and constructor tools: concrete mixer, compressor, grinders, chainsaw, keyhole saw, drills, ....to name a few.

    I've found that existing house appliances are all inefficient: water heater 1600w, 2 water pumps 800w each, 2 fridges 130 and 230w, 1 freezer 90 w and so on.
    Former owner did run everything on a generator, but he took it with him when he left.
    If I buy better appliances I won't have money for photovoltaic stuff. So far I've decided to get rid of both fridges and make a chest-fridge out of the freezer. Try to find the way to send from the controller solar surplus to the water heater and run only a pump for a few minutes to fill up a deposit on the roof.

    Right now, fire place, gas cooker, no mobile phones, no TV (not signal even). Just a radio and a tablet + satellite modem/router... an a lot of candles.
    I plan to work and live on a 3.3 Kwh range, as moderators suggest it's possible. Let's try, why not.
    Thanks
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Another thing to look at is solar heating/hot water. Making solar thermal panels can be a good Do It Yourself type project (solar electric panels, almost never a good DIY project).

    With solar hot water, there can be a lot of plumbing (piping, circulating pumps, insulated tanks, etc.) involved--Not easy if you are not already good with plumbing.

    Solar Shed and other Solar Thermal Links

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Hi BB,
    I've spent last two days looking at links you passed me. Very, very interesting indeed. I'll bear in mind. In the other hand, my initial idea was to supply surplus from charger to current water heater, but as I couldn't find for the last fortnight a way to do it from victron charger, I put an advertisement and amazingly it's been sold in 2 days. Not same luck with 24V inverter. So here my question. I've got money enough as to buy 1 Midnite Lite 150 or 2 Midnite kids.

    Each kid should support 3 of my current panels in series at 24V or 6 panels at 48V.
    A Lite 150 gives 34 A more at 24V and a little less at 48V, but I should sell the 24V inverter

    Which one would you guys choose?
    Lite will keep me at 24V if I don't sell the inverter.
    2 kids allow me to set 2 systems: 1 kid at 48V with current 6 panels and another kid at 24V with current inverter and 4 new panels.
    Lite is managed by a Pc ( I'm very bad at) and kids bring a display.

    Don't know. Any advise by experience?

    Many thanks
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    If you want to use excess power from your solar array, The Midnite Classic has a "waste not, want not" output. You can program the output to drive a relay (or electronic relay) to heat water, run a water pump, etc. when excess power is available.

    However, my two cents--I would suggest seriously considering the DIY type solar water heating. In general, solar electric power is too expensive to "waste" on heating water.

    My own suggestion is to keep it simple. One larger solar power system vs two smaller systems. And one Classic is generally more capable/less costly (per amp rating) than a pair of smaller units.

    Many people do start with a smaller system when they are building/doing major construction--And keep the smaller system for a Mother-in-Law/Guest building. And build a second larger system to run the main home.

    In general, it is very difficult (i.e., expensive) to expand a solar power system by much more than 2x. As you see, the battery bank voltage changes (new AC inverter for higher voltage) and you usually need a completely new set of batteries for a higher voltage/larger AH bank (don't really want to mix old and new batteries--You end up with replacing part of the bank 2x more often, etc.). And sometimes it can be difficult to find new solar panels (at a good price) that will mix+match with the existing array (different Vmp/Imp ratings can be difficult to match--Available panels can change drastically over just a couple of years, and the older panels are either not available, or 2x the price of the current market price).

    OK--If you pick 3.3 kWH per day--Here is what a system may look like. Assuming you have a minimum of 4 hours of sun per day (average) for 9+ months a year (if you want to look at the http://solarelectricityhandbook.com/solar-irradiance.html website for a city near your new home--That will give you more accurate numbers).

    Using our rules of thumbs to save hand waving time (please ask questions, the RoTs will give you a good back of the envelope calculation to see if it makes sense for you--We can always talk details if your needs differ). Assumes that you charge during the day and use most of your power at night./during bad weather (we are designing a "balanced" system between battery bank, solar array, AC inverter, and your loads). Don't get stuck in the details of what product to buy just yet.

    Assuming 3.3 kWH per day, 2 days of backup power (no sun), and 50% maximum discharge (for longer battery life), the battery bank will look like:
    • 3,300 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 maximum discharge * 1/24 volt battery bank = 647 AH @ 24 volt battery bank
    Note that this can also be a 324 AH @ 48 volt battery bank. The cost of batteries is roughly the same as the amount of stored power is the same (power=Voltage*current--Double the voltage and 1/2 the current is the same power). In general, about a 600-800 AH battery bank is about the largest that is "easy" to cable out for off grid solar. So a 24 volt system would be more expensive/difficult to expand later if you started with a 647 AH battery bank. Will talk more about that later.

    The above battery bank would support around a recommended maximum AC inverter size of ~3.3 kWatt -- Although, I would suggest you keep the inverter smaller. Costs less, and usually wastes a bit less power. You could go possibly go down as far as 1.5 kWatt and still run your home (full size refrigerator, small well pump being the typical minimum AC inverter limits).

    Next, sizing the battery bank. The rules of thumbs that work well are ~5% to 13% rate of charge. 5% can work for a weekend/seasonal cabin--But is small for a full time off grid home. People do make it work, but it requires a bunch of active energy management on your part to make sure you don't kill the battery bank.

    Normally, for a full time home, 10%-13%+ makes for an easier to manage/more capable solar power system. Good for your batteries, and keeps generator run time to a minimum.

    For the solar array calculations, we will do two types. One based on the size of the battery bank. The second based on your loads and hours of sun per day. You need to satisfy both conditions to have a happy off grid power system. First based on battery bank capacity (note, 24 or 48 volt system does not really matter here, because the 24 volt system has 2x the AH rating of the proposed 48 volt battery bank--store energy is still the same for either):
    • 647 AH * 29 volts charging * 1/0.077 panel+controller derating * 0.05 rate of charge = 1,218 Watt array minimum
    • 647 AH * 29 volts charging * 1/0.077 panel+controller derating * 0.10 rate of charge = 2,437 Watt array nominal
    • 647 AH * 29 volts charging * 1/0.077 panel+controller derating * 0.13 rate of charge = 3,168 Watt array "cost effective" maximum
    By the way, the solar math is not that accurate... Anything within ~10% of the above numbers is pretty much the same. I do this to avoid rounding error and you can follow where I use the numbers next. Your battery bank will not be exactly 647 AH--You would re-calculate some of these as you finalize the hardware.

    And now based on your daily load and 4 hours of sun minimum (long term average) per day. And this is full sun from at least 9 in the morning to after 3 in the afternoon--4 hours of noon time equivalent sun harvested during the entire day. Any shading will kill solar panel output--Can lose 50% or more of array performance with just the shadow of vent pipe on the roof on one panel.
    • 3,300 WH per day * 1/0.52 end to end system efficiency * 1/4 hours of sun per day = 1,587 Watt solar array minimum
    So, I would be suggesting a minimum of 2,437 Watt array (to meet minimum 10%) rate of charge. And assuming that February is the "break even" month for ~4 hours of sun per day, you could get:
    • 2,437 Watt array * 4 hours sun * 0.52 system eff = 5,069 Watt*Hours per day minimum (average) for 9 months of the year
    You would use ~2.4kWH over night (and during bad weather), and during sunny weather, you would have another 1.6 kWH (or more if you use 5-6 hours per day for summer) to run your tools/home appliances during the day.

    Your choice if you keep the 24 volt inverter or look for a 48 volt inverter to meet your needs. Smaller 48 volt inverters are hard to find--But your system could probably support a larger 48 volt Inverter OK (towards the 3.3 kWatt range maximum).

    The solar charge controllers you are looking at support 12/24/48 volts, but at a fixed (more or less) output rated current. For a 2,437 Watt array, the "maximum typical charging current" on a MPPT charge would be around:
    • 2,437 Watt array * 0.77 panel+controller derating * 1/29 volts charging = 64.7 Amps into a 24 volt battery bank
    The Kids are rated for 30 amp output--So you are slightly above my suggested cutoff point on a 24 volt battery bank. The controllers will still work OK, but you will "lose" ~5 amps of charging current during the middle of cool/sunny days.

    And even with 70-80 amp rated charge controllers, a 24 volt batter bank and a ~3kWatt array max them out for a single charge controller. This is another advantage of a 48 volt battery bank--Same charge controller can manage a ~2x larger solar array (as always, read the charge controller specifications closely, there may be voltage based deratings on current output).

    The drawback of 48 volt systems, 72+ VDC rated fuses and circuit breakers are a bit harder to find and may be more expensive. Also, there are few "native" 48 VDC appliances out there (although, I usually recommend running your loads from 120/230 VAC inverter anyway. The modern appliances are almost as energy efficient as their DC brothers and usually cost much less--Again, look at your needs/system options as a whole, on paper first, before spending any money).

    And there is a genset to charge the battery bank. I would suggest that a 10% to 20% rate of AC charge rate would be nice number. A generator may look like:
    • 647 AH * 0.10 rate of charge * 29 volts charging * 1/0.85 AC charger eff * 1/0.95 Power Factor for AC charger * 1/0.80 genset derating = 2,904 Watt (VA) rated genset minimum
    I am a fan of keeping generators relatively small/low power. Being off grid, you will probably have several. A smaller/fuel efficient one to carry you through bad weather and a larger unit for shop tools/backup power/emergency power (3kWatt small, 6-10kWatt large would be a starting suggestion).

    Well--That is where I would start--None of these are written in stone or handed down from God... Just a starting point in sizing your system.

    Does this meet your needs for loads? What is your location (major city) to make sure solar insolation is correct? Do you have a lot of daytime power needs? Changes between winter and summer loads? Do you have a minimum power need (refrigerator, business equipment, etc.)?

    Your questions/answers?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,655 ✭✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Not sure why you wouldn't go for the full classic with the battery monitor module (Whiz Bang Jr)? it runs about $100 more in the states and comes with the Module, you might check to be sure a unit you're buying comes with one, they just started doing this this year. That way you can setup from the display and have the whiz bang for future use once you setup with a shunt, The Classic's also come with a battery temp sensor, which would be recommended for the Kid,

    Either option doesn't limit you to a 24 or 48 volt system, You do understand that at higher voltage/strings of panel, you can handle 2x the array at 48 volts than a 24 volt system? At 24 v something around 2400 watt array and at 48 volt around 4400 watt array (from my feeble mind)
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • 706jim706jim Solar Expert Posts: 437 ✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    My first post here which will include both comments and questions.
    I have an off grid cottage that I use for 6 months of the year. I presently have 430 watts of PV panels and 12 Dunlop 220Ah golf cart batteries in a series parallel configuration giving me 660 Ah @ 24 volts. I use a Trace C40 charge controller and DR1500 modified sine inverter.

    From what I've read here, it sounds like I don't have enough power going into the battery bank even though I usually reach float voltage by day's end

    but.....

    I am just now needing to replace the GC's which were purchased in July of 1996!

    I'm looking at 8 Trojan L16's; the 370 Ah versions and am curious if anyone else has gone this route.

    And FWIW, those 12 GC's have spent 18 winters in the cold in temperatures that can go as low as -30F.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    You have some good sun hours then?

    I'd say if the GC2's have given you 18 years that is phenomenal!! I'd try the GC2's again.

    I don't doubt they can make it through the winters as long as the PV system is left on. That's whay ours does for several months of winter each year. We also have 12 GC2's but a little more panel capacity. We never draw down very deeply unless we have clouds for 3 days. That is unusual though; the 3 days of clouds, I mean.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • 706jim706jim Solar Expert Posts: 437 ✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    The sun hours are actually pretty good up here and the panels are exposed for part of the winter anyway.

    There isn't really much difference in price between 12 GC's and 8L16's when all factors are considered, so I was just wondering what others here had experienced.

    I just stumbled across this site this evening and am simply amazed at how much activity there is with regards to renewable energy.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • SurfpathSurfpath Solar Expert Posts: 439 ✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.
    706jim wrote: »
    I'm looking at 8 Trojan L16's; the 370 Ah versions and am curious if anyone else has gone this route.

    Aside: Welcome to the forum Jim. 18 years on GC's must be a record (3 x what most folks seem to average). Shows you what cold temps can do. Heck, if you get those Trojan you should expect three times their 7 years avg: 21 years!!! Right??8). I will have to check back with you then, I hope my memory will still be with me;)

    I have them (the L16REB-s) & so far for 2 years everything seems fine. I expect in about 3-4 years time we will really know how they perform, because there are several folks on this forum who installed them 3-4 years ago, like Mangas, who has 64 of them.

    To get back to the thread. BB described our current system use to a T: 3.3 kWH per day (100 kWH per month)....with lots of conservation (and using propane/other fuel sources for heating/cooking), can almost be a "near normal" electrical existence (full size refrigerator/freezer, LED/CFL lighting throughout the home, TV, laptop computer, well pump, clothes washing machine, etc.).

    Our 370 amp hour bank [@ 48 volts, I believe. -BB] seems to work well with this "near normal" life. We cycle from 100%/90% to 70%/60% SOC on average.
    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.
  • 706jim706jim Solar Expert Posts: 437 ✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Just noticed that your battery bank must be 48 volts giving you similar capacity as to what I am considering.

    And it sounds like this would work just great particularly when I bump up the array size.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Yes, I believe Surfpath's system is a 370 AH @ 48 volt battery bank.

    I added the clarification to SP's post.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Just for update on the topic: I finally keep the Victron 150/70 charge controller. The guy came back saying it didn't work properly, so I gave him back the money. Nevertheless, I decided to setup the whole system to check out components, so bought proper fuses, generous cable sections and borrowed 2 spare second hand batteries from local dealer under promise to buy him a new set once I decide which ones suit me better.
    Amazingly, controller works smoothly, inverter handles everything and batteries perform well till the point my battery monitor shows a 100% Soc everyday after a few hours in absorption and an average DOD of 50%.
    I obviously run a few appliances only, and tools just one by one at a time.

    So far, my system is made of 1530W of PV, MPPT 150/70, 2000VA/24V inverter, BMV-700 battery monitor ( all these three Victron branded), and unknown-brand 2nd hand pair of 12V C20 2?? Ah AGM batteries. Not that much capacity but still enough for my requirements right now. Learning to live with the essentials, I would say.
    Cheers
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    1530W of power is going to be 60+ Amps into those batteries that are rated at ~200+Ah, that comes to ~ .3C charge rate so no wonder you re floating daily. watch the water usage as you may drive off water quickly at that rate. You may want to consider larger Ah batteries...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    You are right westbranch, I bear that in mind.
    I have guessed capacity by charging/discharging batteries for a few days, controlling in/out Amps on a battery monitor.
    During bulk I limit charge current on the Mppt to 20% of the supposed capacity till gets absorption stage. During flotation - concurring with sunny hours - I run all tools I need for work.
    Also, as they are AGM, can't check water level. I has set low parameters for Abs. 28.4 V and Flo. 27.0 V as at 28.8~29.0 V I could hear "boiling" water inside the batteries.
    Hope this helps a little bit batteries health...
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    What charge parameters does the manufacturer recommend? You might want to start at the mid point rather and adjust from there, plus, I assume the batteries are new and should not need to be at the higher end of the range now, possibly higher as they age..
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Mel.GesanthenMel.Gesanthen Registered Users Posts: 9
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    Can't answer that westbranch, don't know manufacturer. As said in an early post, local distributor lent me the batteries -to find out whether Mppt and inverter wok out- till I buy new ones. They are unbranded -second hand and the guy didn't supply any info on them. Don't think anybody lends batteries if they are new, do you? I'm looking at buying 500-600Ah as suggested by Bill. Meanwhile I just try to keep borrowed batteries in good state. Best regards
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Advice on buying new bank of batteries for current off-grid instalation.

    For an AGM battery 28.4 Volts Absorb would be normal, 28.8 would be high.
    You don't want to hear 'boiling' in an AGM. Whistling from one is even worse.
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