24V Panels for 12V System and Battery Bank

aleman83
aleman83 Registered Users Posts: 24
Hello everybody,

I've been looking around for 2 new solar panels for my house. I was looking for something in the over 200W range, like two 235W or something similar. I already have a big 12V Battery, and also some other 12V equipment like inverters, fridge and lighting which I would really like to keep. Now I've realized that pretty much all the big panels are 24V or even higher. What would be the most reasonable solution for this? Of course it makes a lot of sense for them to run a higher voltage as it helps a lot in wire size, but I don't want to get into setting up a whole new system.

I've already heard people talk about mppt controllers like the Tristar MPPT 60A being able to work with higher input voltages and being able to charge 12V batteries. Would there be another solution for my problem, or would a MPPT controller the only way to go? I already tried to understand the specs from the Tristar, but could not completely follow. Do I understand correct that I can have voltages up to 150V to charge a 12V battery but only with a total wattage from the panels of 800W?

My last question would then be: My friend has run his solar panels over 3 charge controllers to the same battery because he has added on to his system over the time. He is very picky with his system, and it has worked well for him that way. So would it theoretically or rather practically be possible for me to go with the Tristar 45A for my two 235W 24V Panels in series (to cut down wire size) charging my 12V battery, and if I need to add on to my system in the future, buy another Tristar 45A and two more 235W 24V Panels charging the same battery with the two setups?

Question over questions. It all seems really simple at times, but then it gets me again ;). If it would be possible to use individual charging setups like my friend uses that would be really great as I wouldn't have to predetermine the total size of my future system as it might grow ;). And also it would give me some security in case something brakes on one system I could still keep the other part running while I fix the other one.

I'm really looking forward to your responses! Thank you for your help!

Aleman83

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 24V Panels for 12V System and Battery Bank

    Welcome to the forum Aleman... I will take a try at your questions
    aleman83 wrote: »
    I've been looking around for 2 new solar panels for my house. I was looking for something in the over 200W range, like two 235W or something similar.

    Larger solar panels tend to be cheaper on a $$$/Watt basis... However, shipping and insurance can kill you for small orders. So, confirm the cost delivered to your door step before placing the order. If shipping ends up being excessive, you can get ~135 watt panels that are less expensive to ship (smaller physical size).
    I already have a big 12V Battery, and also some other 12V equipment like inverters, fridge and lighting which I would really like to keep.

    I always like to go back and ask more details about the loads... In general, around 1,200 watts is a "natural" maximum for a 12 volt battery system... Basically, wiring for ~100 amps is not too expensive. Wiring for 200+ amps is expensive (lots of copper), plus supporting fuses/breakers/etc. are not cheap either.

    And what size 12 volt battery do you have (amp*hour rating)? We also like to check the size of the battery vs the available charging current.

    Many times, trying to "save/reuse" equipment while trying to "up-size" the PV/Battery system is not a good ideal. Better to go with a 24 or 48 volt system (less wiring/support equipment costs/plus solar charge controllers are rated for output current--a 24 volt battery bank will support 2x the solar array wattage and a 48 volt battery bank will support 4x the array wattage vs a 12 volt bank==The old Power=Voltage*Current... 2x or 4x the voltage, 2-4 times the power).
    Now I've realized that pretty much all the big panels are 24V or even higher. What would be the most reasonable solution for this? Of course it makes a lot of sense for them to run a higher voltage as it helps a lot in wire size, but I don't want to get into setting up a whole new system.

    Other than using a MPPT type charge controller (which is not cheap), you do not have any practical options. If you use a PWM controller--For example a ~35 volt panel charging a 12 volt battery bank, you will lose ~1/2 the power of the solar panel/array (solar panels are constant current devices... If the panel outputs 8 amps, it will be ~8 amps at 12 volts/17.5 volts/35 volts --- So the Power=V*I hits again).

    MPPT charge controllers are typically "buck mode" switching power supplies which "down convert" from solar array's high voltage/low current to low voltage/high current needed by the battery bank (a little bit like a DC version of an AC Variac Transformer).

    You can find Vmp~35 volt solar arrays that would work efficiently with PWM controllers and a 24 volt battery bank... It will save some costs (as long as the wiring distance from the array to the charge controller/battery bank shed is "short".
    I've already heard people talk about mppt controllers like the Tristar MPPT 60A being able to work with higher input voltages and being able to charge 12V batteries. Would there be another solution for my problem, or would a MPPT controller the only way to go? I already tried to understand the specs from the Tristar, but could not completely follow. Do I understand correct that I can have voltages up to 150V to charge a 12V battery but only with a total wattage from the panels of 800W?

    MPPT charge controllers are actually pretty neat pieces of equipment. They not only "match" the Vmp*Imp to Vbatt*Ibatt in your system (down converting), the MPPT charge controller can control their output current. For a "60 amp" MPPT controller, it will safely limit its output to 60 amps--No matter what the available array input power is (within input specs of the controller).

    For the maximum array, I use this equation to give you a "rough" cost effective maximum array for a battery bank:
    • 60 amp controller * 14.5 volt battery charging * 1/0.77 panel+controller derating = 1,130 Watt "max cost effective" solar array

    Basically, the MPPT controller will operate near its 60 amp maximum output on hot days with "typically dusty panels". The controller will hit "maximum output current" on cool/cold days around noon time... So, you will lose some power, but not a large amount (at least not worth buying two 45 or 60 amp charge controllers).
    My last question would then be: My friend has run his solar panels over 3 charge controllers to the same battery because he has added on to his system over the time. He is very picky with his system, and it has worked well for him that way.

    Yep, it does happen... People need to add more solar panels. Or, another issue, as you have seen Solar Panel Mfgs are going out of business left and right--and even for vendors that stay in business, they change models every few years--So you cannot get the older panels (at least not for a good price). So, you would need to have separate MPPT charge controllers if the Vmp/Imp between old and new panels cannot be matched (correctly--there is some wiggle room for some configurations).
    So would it theoretically or rather practically be possible for me to go with the Tristar 45A for my two 235W 24V Panels in series (to cut down wire size) charging my 12V battery, and if I need to add on to my system in the future, buy another Tristar 45A and two more 235W 24V Panels charging the same battery with the two setups?

    For a 45 amp MPPT controller:
    • 45 amp controller * 14.5 volt battery charging * 1/0.77 panel+controller derating = 847 Watt "max cost effective" solar array

    So--Assuming you can get matching panels later, you could build out to ~850 watt cost effective array with one 45 amp MPPT controller (note, I am carrying out decimal places so you can reproduce my math--there is nothing magical about 847 watts vs 850 watts in this discussion--typically if you are within 10%--That is virtually "the same" in solar math).
    Question over questions. It all seems really simple at times, but then it gets me again ;). If it would be possible to use individual charging setups like my friend uses that would be really great as I wouldn't have to predetermine the total size of my future system as it might grow ;). And also it would give me some security in case something breaks on one system I could still keep the other part running while I fix the other one.

    Back to loads--Peak Watts, average watts, average watts*hours of operation per day. How much sun do you get where it will be installed. What is your need (summer cabin, 3 season, 4 season, backup genset, grid power and solar emergency backup, etc.)....

    It is very easy to get in the "every expanding" system mode--But that can get expensive and frustrating as you start hitting limits and figuring out you really did not save a lot of money overall between your "starter" system and your "ideal/final" setup.

    Solar PV systems do not "grow" well. I would suggest discussing/measuring your loads (Amp*hours or Watt*hours per day) and doing a couple "paper designs" before you start laying out cash. You might find better solutions once you have done the comparisons/tradeoffs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 24V Panels for 12V System and Battery Bank

    welcome to the forum.

    yes, many pvs are higher in their vmp (voltage max power) and losses would occur using a standard pwm controller with those pvs. they would still work, but the voltage difference between the pv and battery would be multiplied by the current being passed and that's a major loss of power. you could find some high power 12v nominal pvs out there and this would negate the need for a higher costing mppt controller and this would also limit you in your pv choices. 12v nominal pvs usually have 36 cells and vmp ratings between 17v and 18.5v.

    note- try to keep at least a 5% minimum charge to the batteries and preferably around 10% or a bit more.

    as to putting 150v voc to a cc (controller), this is not usually a good idea. the bigger the difference between the input and output voltages on the cc the lower the efficiency of the cc. some do need the higher voltages to overcome high wire losses, but it's best to have the thicker copper wires to begin with if possible. also, one needs to watch that they don't damage the cc going higher in voltage because some circumstances will cause the voltages to increase beyond the norm such as cold temps. the cc wattage capacity is gauged by the output of the cc. if it's a 45a cc at 12v this is 45a x 12v = 540w. if that same cc is outputting 24v at 45a then the output capacity is double or 1080w. go with a higher current rating on a cc and it can look like 12v x 60a = 720w and so on. to be clear, yes, a higher battery voltage will give you a higher potential to handle more in pv. also, some cc manufacturers will site their max input wattages and it may differ from what i said. you can go with what the manufacturer says, but what i said is a good general rule of thumb.

    one can use separate controllers if need be, but why not just get a 60a model and be done with it and more cheaply than with 2 45a controllers? 4 235w pvs could push beyond the max current output on one 45a model cc, but it would be fine with a 60a cc.

    i should mention that many of these so called 24v pvs are not true nominal 24v pvs as they lack sufficient voltage to actually properly charge a 24v battery. these are usually 60 celled pvs with vmps in the area of 28v to 30v or so. a true 24v pv will need 72 cells to push the voltage high enough to be useful in charging the 24v batteries.

    bill,
    looks like i took twice as long to post half as much as you.
  • aleman83
    aleman83 Registered Users Posts: 24
    Re: 24V Panels for 12V System and Battery Bank

    Hello BB.
    Thank you for your help. I still have some more questions coming up now. What did you mean with "In general, around 1,200 watts is a "natural" maximum for a 12 volt battery system..." ? Would the maximum load that I can take from a 12V Battery 1,200W?

    The battery I am talking about I don't actually have yet. It is a 1000Ah Forklift battery that I will get from a friend.

    I was planing on adding on to my initial ~500W over time to reach eventually between 2000W or more. which I should be able to do with two Tristar 60A according to your calculation. I am a bit surprised by the Amps staying the same when having a higher voltage input. Does this mean that if I connect two 24V Panels in series to charge a 12V Battery I'm really only getting half the power of one?

    What I'm planing on running with the system in a small house with way under average power usage compared to the US. I live on an island on the 16th parallel in the Caribbean, and have 80% sunny days around the year with temperatures around 12-30C all year around.

    Looking forward for more info.

    Thank you so much for your help,

    aleman83
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 24V Panels for 12V System and Battery Bank
    aleman83 wrote: »
    Hello BB.
    Thank you for your help. I still have some more questions coming up now. What did you mean with "In general, around 1,200 watts is a "natural" maximum for a 12 volt battery system..." ? Would the maximum load that I can take from a 12V Battery 1,200W?

    It gets back to sizing loads and wire sizes... For example, most people use AC inverters for their larger loads (a lot easier to send 120/230 VAC a 100 feet / 30 meters vs 12 vdc). A typical 1,200 watt inverter design would be:
    • 1,200 Watts * 1/0.85 efficiency * 1/10.5 volts batt cutoff * 1.25 NEC derating (for wire/fuses) = 168 amp rated fuse+wiring
    • 1,200 Watts * 1/0.85 efficiency * 1/10.5 volts batt cutoff * 1.25 NEC derating (for wire/fuses) = 134 amp running current

    So, wiring size, you have several variables... Both the absolute current and the voltage drop... In the US, we would use the NEC (national electric code) and that would be around a 1/O copper cable minimum or if we use the Boating version, that could be a 2 AWG cable.

    Then you also need to worry about the length of the cable... You would assume the battery runs down to ~11.5 volts (under load) and 10.5 volt-off for the inverter--Or only 1 volt drop maximum. Inverters can source about 2x the rated power for a second or so--And you would want to have less than 1 volt drop at ~2x134a=268 Amps.

    Using a generic voltage drop calculator, with 4 foot one way wiring, 268 amps and a maximum of 1 volt drop, we would use:
    • 4', 268 amps -> 6 awg with 1.0 volt drop

    If your cables are longer, then more voltage drop, and you have to use heavier wire.
    The battery I am talking about I don't actually have yet. It is a 1000Ah Forklift battery that I will get from a friend.

    Charging current wise, for a 1,000 AH @ 12 volt battery bank, we would suggest 5% to 13% rate of charge with solar panels (if solar only). And, in fact, with forklift batteries, probably closer to 10% to 13% rate of charge. So:
    • 1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 942 Watt Array "minimum"
    • 1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,883 Watt Array "nominal"
    • 1,000 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,448 Watt Array "cost effective maximum"

    So, for a forklift type battery (which is very nice, but slightly less efficient/more self discharge), you should be looking at a ~1,883 watt array (if this is your primary source of charging).

    MPPT charge controller size would be:
    • 1,883 watts * 0.77 panel+controller derating * 1/14.5 volts charging = 100 amps

    The larger charge controllers are around 80-90 amps maximum (Outback and Midnite).

    So, depending on your choice for a solar array, you might be looking at 2x 60 amp MPPT charge controllers. (again, this is starting point for discussion).
    I was planing on adding on to my initial ~500W over time to reach eventually between 2000W or more. which I should be able to do with two Tristar 60A according to your calculation. I am a bit surprised by the Amps staying the same when having a higher voltage input. Does this mean that if I connect two 24V Panels in series to charge a 12V Battery I'm really only getting half the power of one?

    If you use a "PWM" controller (these are basically "on/off" switches), the efficiency with a Vmp~35 volt panel would be 1/2 of the array when charging a 12 volt battery bank.

    With a MPPT charge controller, you will not have any issues (basically around 95% charging efficiency of the MPPT controller, and ~82% derating for nominally hot solar panels).
    What I'm planing on running with the system in a small house with way under average power usage compared to the US. I live on an island on the 16th parallel in the Caribbean, and have 80% sunny days around the year with temperatures around 12-30C all year around.

    Such a system would be good for around (again using rules of thumb) for a 1,000 AH @ 12 volt battery bank (1-3 days no sun, 50% maximum discharge)--Assuming 2 days of no sun, 50% max discharge (for long battery life) and 85% efficient inverter:
    • 1,000 AH * 12 volts * 1/2 days no sun * 0.50 max discharge * 0.85 inverter eff = 2,550 Watt*Hours per day

    Sizing an inverter... Basically, around C/8 for maximum continuous discharge, and C/2.5 maximum surge current from battery bank (and inverters can usually source about 2x rated wattage for surge):
    • 1,000 AH * 12 volts * 1/8 max continuous current * 0.85 eff inverter = 1,275 watt inverter
    • 1,000 AH * 12 volts * 1/2.5 max continuous current * 0.85 eff inverter = 4,080 watt inverter surge
    • 1,000 AH * 12 volts * 1/2.5 max continuous current * 0.85 eff inverter * 1/2 inverter surge rating = 2,040 watt inverter type max rating

    So, for your proposed battery bank, I would be suggesting a total of ~1,300 to 2,000 watts worth of inverter...

    You can also mix/match inverters... There is a really nice 12 VDC 300 watt TSW inverter (and has a standby and remote on/off if you need it) you can use to power your smaller 24x7 loads, and you can get a second/larger inverter for your other loads.

    However, if you are going to power your fridge, you probably should be looking at a single 1,500 to 2,000 watt inverter anyway (I suggest TSW--but they are not cheap).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: 24V Panels for 12V System and Battery Bank

    Aleman, welcome to the forum.
    I'm coming in late, but I just have to reinforce some of the info that the others have given. This system SCREAMS to be 24 or 48 volts. I understand that you already have some investment in 12 volts, but you are in danger of painting yourself into a corner. The more you invest in in 12 volts, the more painful it will be to get out of that corner.

    Sometimes it makes sense to have two systems (redundancy can be good). Keep a small 12 volt system to run your fridge or whatever, and start building a 24 volt system for your general house wiring.

    BTW, what 12 volt fridge do you have? Many of the ultra efficient 12 volt fridges use the Danfoss compressor which will self configure to run on 12 or 24 volts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: 24V Panels for 12V System and Battery Bank
    aleman83 wrote: »
    .... The battery I am talking about I don't actually have yet. It is a 1000Ah Forklift battery that I will get from a friend. ...

    Just a note, forklift batteries are not always the most efficient battery, they bubble a bit more because of the plate chemistry. I'd plan
    on loosing another 5% of efficiency with it.

    And the morningstar controllers are always shohwn with the optional meter installed. My next controller will be a Midnight Classic Lite
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • aleman83
    aleman83 Registered Users Posts: 24
    Re: 24V Panels for 12V System and Battery Bank

    Thank you so much everybody for your amazing help!

    Bill, your information has been absolutely amazing. I can follow your numbers, but I could never have come up with them on my own. So basically I can still do what I'm planing on, but should definitely use a bigger array size to start with.

    Looking at your numbers I understand that for my size battery it would be right to start with lets say 3X Kyocera KD315GX-LPB 315 Watt with the Morningstar TriStar 60 Amp MPPT to meat my minimum array size, and then further down the road I could get another 3X 315W Panels with another 60A cc on the same battery to meet my optimum array size?

    I guess I could connect the panels in series to cut down on wire size as I have a 50' run from my panels to the battery, right? Or should I still try to keep the voltage as close to my 12V as possible?

    I already have a 400W and a 1500W Inverter that I will use, which I understand is the perfect size.

    @vtmaps: My problem is that I live in a 3d world country with very limited supplies in those areas, and 24V appliances and inverters are not available. If I stay on my 12V system I can easily replace something if it brakes while on 24V I would have a lot of problems getting it here.

    Again thank you so much for your amazing help!

    aleman83
  • aleman83
    aleman83 Registered Users Posts: 24
    Re: 24V Panels for 12V System and Battery Bank

    One more thing,.... If I can connect the 3 Kyocera KD315GX-LPB 315 Watt in series, what would be the calculation to figure out my wire size on a 50' run from my panels to the cc and battery? and otherwise if not in series what would be the calculation there?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 24V Panels for 12V System and Battery Bank

    You are very welcome Aleman--We are still missing some information. How many "hours of noon-time equivalent" sun do you have where you live?

    Many islands have significant marine layer effects which can kill average production--and/or cause multiple days of less than optimum production (why a backup genset is still needed for most people).

    For example, say your usage is an average of 2,550 Watt*Hours per day (note, you should only plan on using ~66 to 75% of your "average" production per day--some days will be better, and some days will be worse). Assuming ~4.5 hours of "average sun" for at least 9 months of the year:
    • 2,550 Watt*Hours per day load * 1/0.52 system efficiency * 1/4.5 hours of sun per day = 1,089 Watt Solar array minimum

    Since you are looking at a 1,800 watt or so array, you will be able to easily meet the 2.55 kWH per day estimated loads (based on battery bank size):
    • 1,800 watts * 0.52 system eff * 4.5 hours of sun = 5,022 WH per day

    Notice, that we are sizing the array both on Battery bank capacity and amount of power per day... If you have a smaller bank (1 day of power) and don't use much electricty, you can go with a smaller array. If you have a bank with 3 days of backup (and 50% maximum discharge), the bank is very large and you need a good size array for proper charging (letting a battery bank sit partially discharged for days/weeks/months causes them to sulfate and fail in months/year or so--instead of the 15+ years you would hope for from a good quality forklift battery.

    Towards that end, I would suggest a hydrometer, accurate volt meter or DC Current Clamp Meter (Sears has an "OK' one at a good price), and a Battery Monitor (Victron is another good brand). Batteries are easy to kill with improper loading/charging. A Battery Monitor makes it much easier to keep an eye on your battery bank state of charge (especially if you have a spouse, kids, and/or house guests).

    Also a Kill-a-Watt meter (or equivalent) for measuring your AC loads (there are 230 VAC versions if that is your line voltage)

    Also, if you do things like run loads during the day (washing, vacuuming, etc.)--That will allow you to run directly from the array and skip the ~80% battery efficiency (another 25% more "free" power).

    Please note, I am being fairly conservative here in my recommendations. This allows you some growth in power usage and for system/battery aging and dusty panels, etc... Plus I do not know your location/needs/locally available devices--So I would rather have you happy with a "conservative" design vs something that is limping along a few years from now.

    Regarding array voltage... If you have some distance to cover--Yes, keep the voltage of the array high (typically, around 100 volts Vmp maximum for a 150 VDC max input controller--The vendor panels have calculators that you can use to verify operating voltages. Also, because you are on a Caribbean island, you most likely don't have sub zero weather ever--so Voc-cold will not rise very much).

    So, for your wiring... Assume 240 watt panel with Vmp~30 volts (keep numbers simple). That would be 3xVmp=~90 volts for Vmp-array (perfectly OK).
    • 240 Watt panel / 30 Volts Vmp = 8.0 Amp Imp

    A three panel array in series would be 90 volts Vmp with 8 amp Imp and 50 foot run and 1% to 3% recommended voltage drop. Using Voltage drop calculator:
    • 90 volts * 1% = 0.9 volt drop => 50 feet, 10 awg wire -> 0.96 volt drop
    • 90 volts * 3% = 2.7 volt drop => 50 feet, 14 awg wire -> 2.4 volt drop

    If you did the same thing with 30 volt Vmp (three panels in parallel) and 24 amp Imp, the 3% drop would be:
    • 30 volts and 0.9 volt drop => 50 feet, 4 AWG -> 0.6 volt drop

    Much more expensive copper wire for the run.

    There is a bit of a down side with many MPPT charge controllers... They tend to run most efficiently with Vmp roughly 2x battery charging voltage (12 volt battery would be around 30 volts Vmp-array). So, you may lose a couple percent in overall efficiency with a controller having Vmp-array of ~90 volts and a ~14.5 volt charging battery (read the manuals, many include efficiency charts for different Vmp-array vs battery bank voltages).

    I probably would not recommend running at 30 volts Vmp-array and 4+ awg cable--Also, placing three or more solar panels in parallel, you should add fuses/breakers (one breaker/fuse per string). This helps limit short circuit current in one shorted panel when driven by 2 or more parallel connected strings (look for the series protection fuse rating in your solar panel specifications).

    Also check the price of shipping... Breaking your order into 2 or more lots may kill you on shipping/packing/insurance charges.

    Otherwise, yes, placing two or more charge controllers in parallel ("home run" the wire from each controller back to your battery bus connections... Do not "daisy chain" from battery to Controller A to Controller B--The controllers tend to be electrically noisy and can confuse each other if sharing the same wire--i.e., changing current and voltage drops makes for "unstable" battery voltage readings between controllers).

    The MorningStar TS MPPT controllers are very nice... However, if you add panels to your array--You might look at the Midnite or Outback solar charge controllers, they are rated at 80 amps or more.

    Some more night time reading:

    Deep Cycle Battery FAQ
    www.batteryfaq.org
    All About Charge Controllers
    Read this page about power tracking controllers
    All About Inverters
    Choosing an inverter for water pumping

    Do you have lightning strikes in your area?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • aleman83
    aleman83 Registered Users Posts: 24
    Re: 24V Panels for 12V System and Battery Bank

    Hello Bill,

    Yes I do have a chance of lightning strikes, and as of the noon-time for my area I don't know. As far as I know there are no calculations for this area. I live on an island 60 miles of the north coast of Honduras.

    The solar panel I am looking at shows Vmp = 39.8 Volts DC on the specs. Would that still be ok connected in series for the Tristar 60? Should be fine right?
    I will definitely look into the battery monitors and kill-a-watt meter. Also I'm not all sure yet where I need brakers from the panel to the battery and what type to use if I use the three Kyocera KD315GX-LPB 315 Watt Multicrystalline Solar Module. Also lighting arrestors I don't know about.

    I do have a 2KW backup generator.

    Are you also part of the sales team at Wind&Sun? I would really like to discuss what I would like to order from you then if that is so. :)

    Aleman83
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: 24V Panels for 12V System and Battery Bank
    aleman83 wrote: »
    One more thing,.... If I can connect the 3 Kyocera KD315GX-LPB 315 Watt in series, what would be the calculation to figure out my wire size on a 50' run from my panels to the cc and battery? and otherwise if not in series what would be the calculation there?

    I see that Bill has already explained the problem of putting those panels in series.... it is much less efficient to convert 100 volts to 12 volts than to convert 100 volts to 24 or 48 volts. What he didn't tell you is that the lost efficiency translates into heat in the controller. The major cause of short lifespan in electronics is heat.

    That is therefore another reason to have a 24 or 48 volt system. If you stick with 12 volts I suggest that you put your panels in parallel and spend a small fortune on heavy copper cable between panels and controller. Your controller will last longer.

    I also have a long cable between my panels and controller. I posted some calculations on how to configure (series-parallel) my panels. Look at the numbers and you can get a sense of the heat production: http://forum.solar-electric.com/showthread.php?15907

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: 24V Panels for 12V System and Battery Bank
    aleman83 wrote: »
    Are you also part of the sales team at Wind&Sun? I would really like to discuss what I would like to order from you then if that is so. :)

    All of the moderators here are volunteers and are not employees of Wind-Sun. Wind-Sun does have a competent sales force. Talk to them.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 24V Panels for 12V System and Battery Bank

    Yes, the controller will run hotter--but if you have good ventilation and not very hot, in general (no desert), would not worry about too much. Again, different brands/designs will have difference performance/efficiencies.

    Some vendors put a fan on their controller--So, if the fan runs a lot, you may need to replace it very 3-5 years or so--Still a lot less than the copper. Especially if you end up with two array's worth (that can easily be several thousand dollars worth of copper wiring).

    You can look at PV Watts and see if there are any sites that are near where you live (there are a lot of Honduran locations--but I don't know if their weather is similar or not) and if they have similar weather patterns (I don't know if the land nearby blocks weather or not for you). Anyway, here is Belize Intl airport:
    Month Solar Radiation (kWh/m2/day)
    1 4.40
    2 5.01
    3 5.62
    4 5.69
    5 5.28
    6 4.93
    7 5.20
    8 5.18
    9 5.12
    10 4.83
    11 4.57
    12 4.41
    Year 5.02

    Around 4.5 hours of "full noon time equivalent sun" does not look too far off...

    You can contact NAWS directly (start with a phone call if you can):
    Northern Arizona Wind & Sun
    4091 E Huntington Drive
    Flagstaff, AZ 86004

    Sales & Service: 800-383-0195 | 928-526-8017
    Fax: 928-527-0729

    Our office hours are from 8AM to 4PM, MST Monday through Friday. We are closed for major holidays. If you are in the Flagstaff area, please stop by our store and visit.

    As Vtmaps said--Only the two admins (Windsun and Rick) work at NAWS. The rest of us here are volunteers.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: 24V Panels for 12V System and Battery Bank

    If you are spending the $$ for a MPPT controller, I'd still run 3 panels in series (IF they are below the voltage Voc Limit for the controller)

    If you have a reliable source of ALUMINUM electrical wire, that is much less expensive than copper, I used the "increase 2 wire gauges" factor. So #8 copper would be #6 aluminum.

    I also used breakers rated for aluminum, anti-ox paste, and the right torque for the set screws. That may be hard to find 150VDC breakers, so you may have to use a fuse, and maybe a rotary marine battery switch. you do want a way to cut off the array power somehow.
    Attachment not found.


    Where I had to splice or tap, I used these:
    Electrical Splice Blocks suitable for Alum wire
    Attachment not found.Attachment not found.

    They are not cheap, but the cost savings in copper, makes it work.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • aleman83
    aleman83 Registered Users Posts: 24
    Re: 24V Panels for 12V System and Battery Bank

    Hello Bill,

    I was just reading over all the posts again and I have a few questions. On your first post you calculated the max cost effective solar array for the 60A cc.

    60 amp controller * 14.5 volt battery charging * 1/0.77 panel+controller derating = 1,130 Watt "max cost effective" solar array

    Where does the 1/0.77 derating come from? Is that a standard number? Also I've seen that in the specs of the Tristar 60 it shows a max Wattage of 800 Watts at 12V. Is that something I get around because of the derating? Because I was looking in using 3x 315Watts to meet my minimum array size for the battery.

    The Tristar specs also show a max solar open current voltage of 150V. The specs of the panels show the following:

    Pmp = 315 Watts
    Vmp = 39.8 Volts DC
    Imp = 7.92 Amps DC
    Voc = 49.2 Volts DC
    Isc = 8.5 Amps DC

    So I guess it would be just within the range. Is that correct? Also we never have temperatures drop below 25C on a sunny day, and 18C after several days of complete cloud cover.

    Aleman
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 24V Panels for 12V System and Battery Bank
    aleman83 wrote: »
    Where does the 1/0.77 derating come from? Is that a standard number? Also I've seen that in the specs of the Tristar 60 it shows a max Wattage of 800 Watts at 12V. Is that something I get around because of the derating? Because I was looking in using 3x 315Watts to meet my minimum array size for the battery.

    Roughly, a solar PV panel from factory STC (standard test conditions) will operate at an average of ~81% of rated power (really ~82% of rated Vmp) when exposed to full sun at "normal" ambient temperatures (maybe 25-35 oC). The actual solar cells can get upwards of 25oC over ambient temperatures from the heat of the sun). And you have another ~95% efficiency due to standard MPPT charge controller efficiency.
    • 0.81 panel derating * 0.95 charge controller eff = 0.77

    You there are also wiring/dirty panel/etc. losses too... Here is a list of typical deratings:

    DC to AC Derate Factor


    Some people/panels/systems may do a little bit better (especially if you have snow on the ground and reflected sunlight from the snow--can get upwards of 100% of rated power)--But that is not standard for most people--Plus, overall, many people closer to the poles are looking at ~2-3 hours of sun per day, so overall, there still is not a lot of available energy (let alone the folks above the arctic circle where they get no sun for part of winter).

    In the end, we try to error a bit on the conservative side so that people get, at least, what is predicted on an average day... Overall, you can have +/- 10% of average power due to weather conditions and minor differences in hardware/dirty panels/mounting (panel that get good cooling air flow run a bit cooler and generate more power, etc.).

    Also, it is not unusual for solar charge controllers to have 5% (or even 10%) error in their displayed values (solar charge controllers tend to overestimate power generated--on average)--It costs money (and even a bit of energy) to have 2% accurate displays (GT inverters tend to be more accurate, somewhere around 2%, since some states paid rebates based on logged power and would require separate meters if GT inverters were not accurate enough).

    Note that MPPT charge controllers are designed to safely/reliably limit their output current to their maximum rating so can take any solar array wattage (within reason) without problems... PWM controllers cannot and should only have the maximum array (as defined in their manuals) connected to prevent damage/reduce risk of overheating of the PWM controller.

    The larger array, the longer an MPPT will be limiting its output... at 77% derating, that does not happen very often for most people. The more array you add, the more time during the middle of the day the controller will be current limiting.

    Does not hurt anything, just "lost" power generating opportunities. With panels so much less expensive these days ($$$/Watt), the "max cost effective array" is a pretty squishy number. And if you go with 75% or even 60% derating (or oversizing of the array) is just a discussion between you and your wallet (for example, large/cheap panels that are "over-sized" for a single controller, vs a smaller panel that cost more $$$/watt, vs getting a $econd controller to manage the extra power).

    There is no specific "right answer" for this question. Just rules of thumb to get people "close" to the optimum configuration.
    The Tristar specs also show a max solar open current voltage of 150V. The specs of the panels show the following:

    Pmp = 315 Watts
    Vmp = 39.8 Volts DC
    Imp = 7.92 Amps DC
    Voc = 49.2 Volts DC
    Isc = 8.5 Amps DC

    So I guess it would be just within the range. Is that correct? Also we never have temperatures drop below 25C on a sunny day, and 18C after several days of complete cloud cover.

    Because Voc is around 50 volts, you cannot put three of them in series as this will give you Voc-stc ~ 150 VDC--Too high for most charge controllers (Midnite Solar does have charge controllers with higher rated input voltage version available--at a reduced maximum current--I am assuming higher voltage switching transistors have higher resistance/lower rated current/die size, so carry less current).

    So, your options with these panels is to either limit yourself to 2 panels in series and either one or two strings in parallel (2 or 4 panel arrays)... You, of course, have the option of 3 panels in parallel (and use lots of copper to run power back to battery shed). Unfortunately, these issues are real and make designing a reliable system a bit more complex/confusing.

    Note that we are looking at Voc-array-cold--So it is, typically, the cold/clear morning lows we are concerned about when calculating Voc-cold for maximum array voltage.

    Most vendors have a calculator for their MPPT charge controllers where you plug in the numbers to see what array configurations are supported. Here is MorningStar's.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • aleman83
    aleman83 Registered Users Posts: 24
    Re: 24V Panels for 12V System and Battery Bank

    Hello BB,

    I've just read through the thread again and found something I didn't quite understand:
    note- try to keep at least a 5% minimum charge to the batteries and preferably around 10% or a bit more.

    Does that refer to the charge rate or the actual charge of the battery?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 24V Panels for 12V System and Battery Bank

    that would be a charge rate.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 24V Panels for 12V System and Battery Bank

    Battery capacity in Amp*Hours at 20 hour rate... (i.e., 140 AH battery discharged at 20 hour rate = 7 amps for 20 hours).

    5% of battery 20 hour capacity:
    • 0.05 rate of charge * 140 AH (20 hour capacity) = 7 amp rate of charge minimum

    Numbers happen to work out that C/20 is 1/20 = 0.05 = 5%

    Just picking rules of thumb that happen to match other rules of thumb/measurement conditions. If we picked C/10 as our "battery capacity standard", then we may have chosen 4% / 0.04 as the minimum rate of charge.

    I think Surrette/Rolls uses C/10 capacity numbers for their rules of thumbs (as I recall).

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