Converting a 24 V photovoltaic panel output to 12 V

Tom ClearyTom Cleary Solar Expert Posts: 37
Hello:

I am a first time buyer considering buying a 24 V photovoltaic solar charging system. The idea is to eventually power a swampy evaporative cooler along with some other 12 V appliances handy for boondocking in the desert. Is there a way to step down 24 V safely to 12 V in order to power these appliances? I've been browsing for reviews and saw a product that does convert -- it advertises that it will convert 24 Volts DC to an output of 13.8 Volts. Will this work for converting a 24 V photovoltaic panel output to 12 V? Any suggestions would be very welcome.

Thanks.

Tom

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V

    The first thing to explain here is that a "24 Volt" solar panel doesn't put out 24 Volts. It will actually have a Vmp (Voltage at Maximum Power) in excess of (usually) 30 Volts. Furthermore, panels are "current sources" rather than Voltage source: you can't run most things directly from a panel; there needs to be a battery in between.

    However, you can use "24 Volt" panels to charge a "12 Volt" system. This is best done through an MPPT type charge controller. Unfortunately they tend to be expensive (comparing two Morningstar controllers of 45 Amps: $157 for the PWM type, $428 for the MPPT). If you use a standard PWM type controller, you'll lose much of the benefit (power) of the 24 Volt panel.

    You could also set up a 24 Volt system, and use a DC to DC converter to supply 12 Volts for those items that need it, depending on the current requirements.

    And of course there are "12 Volt" panels available, but they tend to be lower Wattage.

    If you could provide some more details about what specifically you want to run, I'm sure we could come up with several viable solutions. There's a lot of folk with a lot of experience here! :D
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V

    A MPPT charge controller will down convert very efficiently.

    Just fyi, I sugggest that you do a bit of reading on PV sizing before you buy any hardware, to avoid the dreaded, "ready, fire, aim".

    Do a real load calculation for things like evap coolers or other 12 vdc appliances. The reality is that PV will yield far less usable power than you might imagine.

    My general rule of thumb is take the name plate rating of the panel, dived by 2 to account for all system loses, then multiply that number by ~4 to account for the average number of hours of good sun you can expect on average.

    So for example, a 100 watt panel (sounds like it should power 100 watts right? Wrong.

    100/2=50*4=200 watt/hours of usable power out of the inverter per day. That would be enough to power a 125 watt TV for under 2 hours.

    So do all your expected load calcs, remembering the second rule of off grid PV, that is most people underestimate their loads, at the same time they are over estimating their power input, leading to a system that performs way less well than they expect.

    One other note, if this is a RV system, panel orientation is very unlikely to be ideal much of the time. It will not quite face the right angle, will be mounted too close to the RV so it will run hot decreasing it's output.

    Good luck and keep in touch,

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 29,887 admin
    Re: Converting a 24 V photovoltaic panel output to 12 V

    Welcome aboard Tom,

    The whole 12/24/48 volt issue -- And you can add 120/240 VAC to that too...

    You are on the right track--Identifying your loads/needs first, before buying anything.

    If you can define your needs (cooling, lights, electronics, etc.) and how much you will use them (watts*hours, amps*hours @ what voltage)--then you can look at the voltage(s) you will want to run for your systems.

    In general, for small off-grid systems, 12 volts is hard to ignore. Lots of inexpensive 12 volt appliances. You can even find some 24 volt appliances too (trucks, boats).

    But, the downside of 12 volts is it is very difficult to send large amounts of power very far. My rule of thumb--if your loads are 1,200 watts or less, 12 volts may be OK. For 2,400 watt or less loads, use 24 volts. For larger loads, look at 48 volt battery systems.

    With 12 volts, you have only ~1.0 volts of wiring/fusing/circuit breaker drop allows (11.5 volts from heavily loaded 1/2 discharged battery--10.5 volts "cutoff" for most 12 volt appliances).

    The other thing to look at is the 12 volt appliances you wish to power. Some may fail when operated on solar off-grid systems because a flooded cell deep cycle storage battery is charged between 14.5 to 15.5 volts. And many appliances don't really work that well near 10.5 volts either. There have been reports of computer 12 volt car adapters failing and such.

    And with DC appliances--many fans, and others, use 12/24 volt motors with brushes and commutators in them (think electric drill motor). Brushes do not last that long (perhaps a few months before brush replacment is needed--frequently you also need dissasemble and turn the commutator on the armature to make it smooth again).

    There are some DC systems that use electronic commutation (more expensive pumps and motors)--but those are more specialty items (well and irrigation pumps for off-grid systems). And, again, many times 24 or 48 (or even higher) voltages are more useful because of voltage drop / large gauge wiring that would be required for low voltage systems.

    So--At this point, if you are looking at a 24 volt battery bank (with some native 24 DC Loads)--then if you are looking at 12 volt loads with a converter of some sort--You might want to look at changing the 12 volt loads to energy efficient 120 VAC loads and use an inverter instead.

    There are step down converters that can power small DC loads directly, and others that are DC to DC battery chargers (charge a 12 volt battery bank from your 24 volt battery bank). In either case, you are still going to lose 10-30% power losses in the conversion--so losing 10-15% when using an AC Inverter does not look so bad.
    A couple of Inverter FAQs:
    Regarding converting directly from a 24 volt (or other higher voltage) solar panel to a 12 volt device...

    There are several answers to that question. Yes, there are simple voltage regulators that can take higher voltages and regulate them at a lower voltage--but frequently, they are not very efficient (the greater the input:output voltage ratio, the higher the losses).

    Also, solar panels cannot "store energy" (clouds, bird, shadows, night, etc.) all kill solar panel output--and if your device needs consistent energy (TV, Radio, computer, etc.)... Then you really want a battery between the solar panel and the device to store energy for use when the solar panel cannot produce enough power.

    Now, there are solar charge controllers (called MPPT or Maximum Power Point Tracking -- vs PWM pulse width modulation which cannot down convert) that can take 100 Volt solar panels and, very efficiently, down convert the 100 volts (and low current) to 12 volt for battery charging/running your appliances.
    One of the major reasons to run solar panels at "high voltage" is to reduce the current (and amount of copper) needed to move the electricity without excessive voltage drop and power losses from wiring resistance.

    Remember the formula for power is:
    • Power = Voltage * Current
    If you can increase the voltage by 4x (say from 12 volt to 48 volt battery bank), then you can reduce the current by 1/4 -- This allows you to use smaller wire, send more energy a longer distance, etc.

    I will stop here--I am sure you have a lot more questions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tom ClearyTom Cleary Solar Expert Posts: 37
    Re: Converting a 24 V photovoltaic panel output to 12 V

    Thanks, I didn't expect replies for days, and I am slightly overwhelmed by the thorough and detailed nature of the replies. I am grateful for the help as this is my first try at putting a solar RV system together.

    My setup is a Eurovan1993 -- with it I go camping in northern San Diego County without electricity or water hookups. My power budget only needs to cover a few lights, a laptop, a swampy evaporative cooler in the summer -- 200 watts or so ought to be enough I figure to cover that much and have a little extra for backup.

    The main reason that I'm interested in a 24 V system is that I was hoping to buy a single high wattage panel, a 235 W panel, such as a Kyocera or a Sharp. Unfortunately the largest panel in a 12 V system that I could find was 135 W and so if I go with 12 volts that means setting up two panels instead of one. Two panels instead of one might not be so bad, but i want to set up the panels outside the van (about 30 ft away) rather than mount them on my camper van roof. I don't want to mount them on the roof because my van won't fit in the garage if PV panels are installed that way. Also the fiberglass in the pop top shell of the Eurovan camper is a little bit fragile -- and has aged a little bit -- and might not take the weight of the pv panels.

    So my preference would be to go with a single panel employed about 30 feet away from the camper van. So that's the reason that I was interested in a single 24 V panel. it seemed to me it would be easier to keep track of a single panel rather than two of them, but I'm beginning to think it's not worth the extra bother. Finding appliances for example that work with 24 V systems means a bad selection and higher prices.

    The Web reviews I read said that the advantage of 24 volt systems chiefly has to do with reduced wire size requirements, and that over long distances, this can be a significant factor. The distance that I'm considering is minor though, only 30 feet or so. So I am wondering if 24 volt pv panels and batteries would be worth the advantage of only using one panel instead of two 12 volt pv panels.

    Thanks again, in just a few short minutes of reading these responses i learned more than from cruising the internet for hours. Best,

    Tom
  • BB.BB. Super Moderators, Administrators Posts: 29,887 admin
    Re: Converting a 24 V photovoltaic panel output to 12 V

    One thing to think about is the physical size and weight of the solar panels for your application.

    135 watt panels are probably easier to handle/store.

    175 watt panels are probably as large as a single person would want to handle.

    The 225 watt and larger panels might need 2 people to move and setup to limit the chances of damage.

    In any case, make sure the panel(s) are well protected during transport and when setup are well anchored against wind and theft. They are usually constructed from 1/8" (single weight) tempered glass--Tempered glass is pretty tough--but all it takes is one gust of wind to topple the panel and you are left with a shattered panel that is pretty much scrap at that point.

    So, if you can, see if you can look at a couple real panels and see which sizes are appropriate for your needs/abilities.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,506 ✭✭✭✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V

    I would suggest considering 2, 12V - 135W panels, in series, and use a MPPT controller (the Morningstar 15A MPPT comes to mind) to downconvert to your 12V system. It may seem like you are over-powering the the 15a MPPT, but with real life losses, mis-alignment, as long as both PV's are in sun (not shade) you will get close to 200W and any overages, the MPPT throttles itself back to safe limits.
    Or go to the Rogue controller wihich is a little larger in capacity.
    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 ,

  • dak664dak664 Registered Users Posts: 13
    Re: Converting a 24 V photovoltaic panel output to 12 V

    The biggest load would be the DC fan of the swamp cooler? That may not need any regulation, I'd guess a 36 volt motor would draw close to the maximum power from a 24 volt panel across a fairly broad range of illumination and temperature (my shop fan runs this way). The 120-240 volt AC adapters used for laptops and compact fluorescent lights typically rectify the AC into a stepdown DC-DC converter; these will accept lower voltage DC as well, don't know about 24 volts but 70 volts might work. Maybe your best bet would be a 60 volt amorphous panel, connect the laptop charger and swamp cooler and let them sort out their own power requirements.

    I presume you'd want the lights at night though. Tool battery fluorescent lanterns work well. If they don't come with a suitable DC operated stepdown charger you might be able to use the laptop charger for those as well. With conservative voltage limits LiFePO4 is safe and reliable but don't mess around with other lithium chemistries unless you know what your are doing.
  • Tom ClearyTom Cleary Solar Expert Posts: 37
    Re: Converting a 24 V photovoltaic panel output to 12 V

    Hello:

    Thanks for your help so far. I appreciate this discussion forum, it has been very helpful.

    As I mentioned previously I plan to park the RV in the shade and do not want to mount the PV panel on the RV for that reason. To accommodate the longer distance I would like to have 40' foot cables and manually move the panel to point it at the sun.

    Is 200 watts power from a PV panel with a 40 foot voltage drop served best with a 12 volt system or would it be better to go with 24 volt system and an MMPI controller? I asked this question elsewhere in this forum and received some excellent information and reassurance that a 24 volt system is possible to construct without too much trouble.

    I feel like such a well, newbie, asking this but i still don't understand something. Suppose a 24 volt pv panel is plugged into an MMPI controller, modifying the MMPI output to 12 volts and the battery is charged with that. Would there be any difference between the using a 24 volt pv panel and MMPI controller versus using a 12 volt pv panel and a PWM controller to charge a battery? The two set ups seem the same as far as the end result of running 12 volt appliances off a battery.

    Thanks, I am grateful for your time.

    Tom
  • mike95490mike95490 Solar Expert Posts: 8,506 ✭✭✭✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V
    Tom Cleary wrote: »

    I feel like such a well, newbie, asking this but i still don't understand something. Suppose a 24 volt pv panel is plugged into an MMPI controller, modifying the MMPI output to 12 volts and the battery is charged with that. Would there be any difference between the using a 24 volt pv panel and MMPI controller versus using a 12 volt pv panel and a PWM controller to charge a battery? The two set ups seem the same as far as the end result of running 12 volt appliances off a battery.

    Thanks, I am grateful for your time.

    Tom

    Not sure what your MMPI is, but a MPPT controller is very useful for downconverting panel voltage to battery voltage. http://www.windsun.com/ChargeControls/MPPT.htm is a good page about MPPT controllers
    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 ,

  • BB.BB. Super Moderators, Administrators Posts: 29,887 admin
    Re: Converting a 24 V photovoltaic panel output to 12 V

    An MPPT (Maximum Power Point Tracking) charge controller is typically a "buck mode" down converting digital power supply. It can efficiently (~90-95% efficiently) step down the voltage (and setup up the voltage) from a 24-50-100 volt solar array (depending on specific controller specifications) to a 12 volt battery bank.

    A PWM controller efficiency is basically:
    • Vbatt-charging / Vmp-array = Efficiency
    A "24 volt panel" (Vmp around 35 volts) on a 12 volt (14.4 volt charging) battery bank on a PWM (Pulse Width Modulation type charge controller) will be around:
    • 14.4 volts charging / 35 volt Vmp array = 0.41 = 41% efficient
    It is not that the PWM will not work--it is just "wasting" some of the available energy... Cost/performance/room/etc. all go into making the decision for your particular setup.

    There are "analog" voltage controllers too (typically called 3-T or 3 terminal regulators in electronics too) which have the efficiency issues of PWM and waste lots of heat too... Not typically used in Solar applications.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V

    40 feet is a pretty long run for a "12 Volt" array. I think I'd go with the 24 Volt array (most large panels are 24 Volts these days - with some exceptions of course) and an MPPT controller. But that's just my opinion. :D
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V
    Tom Cleary wrote: »
    I feel like such a well, newbie, asking this but i still don't understand something. Suppose a 24 volt pv panel is plugged into an MMPI controller, modifying the MMPI output to 12 volts and the battery is charged with that. Would there be any difference between the using a 24 volt pv panel and MMPI controller versus using a 12 volt pv panel and a PWM controller to charge a battery? The two set ups seem the same as far as the end result of running 12 volt appliances off a battery.

    Not much difference.

    Either way you need to have voltage higher than the battery voltage in order for current to flow IN to the battery. Since the battery isn't actually 12v, but more like 13.5v (when fully charged), you need to have the voltage up above that to "push" current into the battery. (If the battery is at 13.5v, and the charger is putting out 13.5v, then there is a balance and current doesn't flow.)

    A "12v" PV is usually more like 17v or 18v, so used with a PWM controller the voltage will be far enough above 13.5v that when the PV is connected to the battery power will flow into the battery. The PWM controller will connect/disconnect the PV->battery as needed to get the battery voltage up to full charge and then mostly keep the PV disconnected so as to not overcharge the battery.

    The MPPT will adjust the voltage (PWM won't - it just connects/disconnects the PV to the battery) to get the most watts from the PV into the battery. By lowering the voltage you increase the amperage, so with MPPT you end up with a bit more total watts into the battery.

    BUT,

    For a single panel there isn't very much difference. Also down converting a single 30v panel to 15v doesn't give the same sort of big amperage boost that you would get if you hooked up 3 30v panels in series and then down converted 90v to 15v.
  • mike95490mike95490 Solar Expert Posts: 8,506 ✭✭✭✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V
    dwh wrote: »
    ......
    For a single panel there isn't very much difference. Also down converting a single 30v panel to 15v doesn't give the same sort of big amperage boost that you would get if you hooked up 3 30v panels in series and then down converted 90v to 15v.

    I think I disagree with this. you are comparing the juice from 1 apple, to the juice from 3 apples.

    I like looking at MPPT as Harvest watts - 2% for internal losses.

    A 17V PV and 12V battery would have little mismatch, but a 30V PV would have a lot of mismatch with a 12V battery.

    Example: (102W) 17V panel, 6A

    If you had 5 volts of mismatch @ 6 amps = that's 30w of loss, or only 70W into the battery.

    (102W) 40V panel @ 2.55A
    is 28V of mismatch @ 2.55A = 71.4W of loss, and only 30.6W into the battery.

    The calculation is actually a lot more complicated, but this is the basic idea of how MPPT can preserve your harvest to something the battery can use.
    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 ,

  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V
    mike90045 wrote: »
    I think I disagree with this. you are comparing the juice from 1 apple, to the juice from 3 apples.

    I like looking at MPPT as Harvest watts - 2% for internal losses.

    A 17V PV and 12V battery would have little mismatch, but a 30V PV would have a lot of mismatch with a 12V battery.

    Example: (102W) 17V panel, 6A

    If you had 5 volts of mismatch @ 6 amps = that's 30w of loss, or only 70W into the battery.

    (102W) 40V panel @ 2.55A
    is 28V of mismatch @ 2.55A = 71.4W of loss, and only 30.6W into the battery.

    The calculation is actually a lot more complicated, but this is the basic idea of how MPPT can preserve your harvest to something the battery can use.


    The OP's question was how much difference between single 12v panel w/PWM, and single 24v panel w/MPPT.

    A. Not much.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V
    dwh wrote: »
    The OP's question was how much difference between single 12v panel w/PWM, and single 24v panel w/MPPT.

    A. Not much.

    Yup.
    'Til you send the power down 40 feet of wire. That could make difference.:p
    But gee MPPT is expensive for just one panel.
  • Tom ClearyTom Cleary Solar Expert Posts: 37
    Re: Converting a 24 V photovoltaic panel output to 12 V

    I have read through a lot of posts and especially the responses to my question and just want to say thanks .. hope i am not being repetitive saying that, but the comments have been a huge help and spurred me to do a lot of reading in this forum.

    Regarding your reply Cariboocoot, you suggested that the voltage drop over 40 feet could well turn out to be significant and that a 24 volt panel probably would be better for that reason, but also suggested also that a drawback would be the cost of an mppt controller, which for a single panel would be prohibitive.

    the NAWS website features an MPPT product for about half the usual cost called the Morningstar SunSaver 15 amp MPPT solar charge controller. It has less capacity but i wonder if this might be okay for a single panel.

    This proposed use of keeping the panels 40 feet away from the controller and batteries also raised two other questions for me:

    First, the panels will be moved around a lot, and I have heard that this can stress wiring. I am wondering if I would be better off with a particular type of wire that will be able to handle the strain of being handled.

    Second, say 40ft, but do I need a few more feet going into the rv? How much extra should i figure?

    Am trying to learn as much as I can to do this right. Thanks for your help.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V

    Yes the Morningstar http://store.solar-electric.com/mosumpsochco.html is a good solution at about $250. That's still more than a PWM-type would cost, but it will allow you to run a "24 Volt" panel and reduce line loss.

    How much wire do you need? That's a problem. "Enough to go from the panel to the controller" is the answer. How far that will be only you can tell. Obviously you're going to want stranded wire, which is more flexible than solid, and it's still going to have to be fairly heavy gauge (depending on the length - see the Voltage drop calculator http://forum.solar-electric.com/showthread.php?t=29 ) and you'll want it to be fairly resistant to weather/UV. That's not going to be cheap either.

    It will also be important to build a stand for your panel that is sturdy and can be made safe against wind, accidents, and vandalism. Seems to me this has been discussed around here before, but I can't think how to search for it. Guys? :confused:
  • BB.BB. Super Moderators, Administrators Posts: 29,887 admin
    Re: Converting a 24 V photovoltaic panel output to 12 V

    The Morningstar 15 amps MPPT charge controller is probably the best out there at that current level...

    The most solar panels I would suggest for that controller (from a price/performance point of view) a rough maximum of around:
    • 15 amps * 14.5 volts charging * 1/0.77 system derating = 282 watts of solar panels
    You can put more than 300 watts of panels on the controller--but it will not be able to convert all of the power available during the middle of a sunny day.

    Say you want to put the panels 100 feet away and use and extension cord to carry the power. There are 14 gauge cords and even 10 awg extension cords available. Lets say you want a pair of 135 watt Kyocera solar panels. And you want to look at parallel and series connections (parallel for PWM or MPPT type controller and series for MPPT). Use a voltage drop calculator to find the drop vs current and wire size.
    Specifications for Kyocera KD135GX-LPU Solar Panel Max Rated Power (Pmax) 135 Watts
    • Voltage at Max Power (Vmpp) 17.7
    • Current at Max Power (Impp) 7.63 Amps
    • Open Circuit Voltage (Voc) 22.1
    • Short Circuit Current (Isc) 8.37 Amps
    Using the voltage drop calculator for a few different setups for parallel panels:
    • 2*7.63a=15.26 amps => 9.3 volt drop for 100' of 14 awg cord
    • 2*7.63a=15.26 amps => 3.7 volt drop for 100' of 10 awg cord
    Same thing for series connected panels:
    • 7.63 amps => 4.6 volt drop for 100' of 14 awg cord
    • 7.63 amps => 1.8 volt drop for 100' of 10 awg cord
    Now, the panels themselves--assuming 95F summer day, under full sun with a 63F rise from hot sun and no wind, your Vmp will drop down to around 17.7 volts * 0.80 = 14.6 volts ... Note this is a worst case situation as most of the time your panels will have higher than that output voltage...

    But, even if you assume the panels have 15.5 volts, and your charge controller will have about a 1-2 volt drop, and you want to charge your battery bank to 14.5 volts or so--that leaves very little room for voltage drop from your wiring.

    For a 100' 14 awg wire, we get a 9.3 volt drop @ 15.26 amps -- Obviously that will not work at all (14.5v batt charge + 9.3v cable drop = Vmp of 23.8 volts minimum). Plus all of that voltage drop goes into heating your cord instead of charging your batteries.

    If, instead you use a 10 awg 100' cord, with the two panels in series, and an MPPT charge controller, your voltage drop is only 1.8 volts... Vmp is around 2*17.7v*0.80=28.3v on a hot day---plenty of voltage to charge your 12 volt battery bank...

    You asked for 40' cable range--and the voltage drop will be 40% (about 1/2) that of what I calculated worst case--but you can see that a PWM controller (or MPPT) running the two panels in parallel:
    • 2*7.63a=15.26 amps => 3.7 volt drop for 40' of 14 awg cord
    • 2*7.63a=15.26 amps => 1.5 volt drop for 40' of 10 awg cord
    For 14 awg cord, that is way too much voltage drop.

    For 10 awg cord--it will work on cool days--but would be less than ideal if your idea of fun in Death Valley in the Summer (don't get me wrong--that is one of my ideas for fun ;)).

    So, long way around... MorningStar 15 amp MPPT charge controller (or you can look the Rogue 30amp MPPT controller--it has an LCD display included), and as heavy extension cord as you can justify (and keep length short as practical if you use smaller AWG wire to reduced price/bulk of cordage). And wire your pair of panels in series.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Converting a 24 V photovoltaic panel output to 12 V

    Bill has reminded me what the other issue was with this sort of set-up; using outdoor extension cords for wiring. The wire and insulation is entirely suited for the application (you can get up to 12 Gauge).

    But ... there is the safety matter of not using the standard "120 VAC" connectors lest someone plug the wrong end into the wrong thing. You don't want to accidentally tie the back-up generator in to the charge controller or the panels! FWOOM! :cry:
  • BB.BB. Super Moderators, Administrators Posts: 29,887 admin
    Re: Converting a 24 V photovoltaic panel output to 12 V

    I am of mixed thoughts about using a standard extension cord for remote placement of solar panels...

    Yes, you don't really want the idea of using 120 VAC plug on your 12/24 volt battery charger system. plugging in 120 VAC to it will ruin your day (at least).

    On the other hand, carrying around two extension cords--one for solar and another for AC power when available (like a trailer park, visiting friends) is also a waste of space and money.

    Also, I like non-locking plugs if there is the chance that somebody forgets and drives away with the cord still connected (it happens to us all)... Non-locking plugs stop you from dragging $1,000 worth of solar panels down the road. :cry:

    So, if you place a 120 VAC receptacle on the RV (behind a locked cover if you wish)--you cannot connect a "live" 120 VAC cord to the RV (wrong sex). And if somebody plugs a 120 VAC appliance into the 24 volt outlet, probably not the end of the world.

    The bigger issue is if you use a 120 VAC plug on the solar array--that could be plugged into 120 VAC 60 Hz (that is the correct sex)... Nothing good would come from that...

    You could do things with a twist lock adapter, Anderson Power Pole connectors, etc. to adapt the panels to the extension cord--but you will still end up with the same issue with an unmodified cord set.

    If you are the only person setting up / breaking down--you will probably be careful enough. If you have "help" -- Then you may wish to have a dedicated "DC extension cord" for your solar panel array...

    -Bill "mixed messages" B. :confused:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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