adding a second solar panel

leaf
leaf Solar Expert Posts: 37 ✭✭✭
I have a basic 12V system in my RV. I want to add in an extra portable panel as I don't generate enough power over the winter. Do I need to match anything other than voltage between the 2 panels? I've heard varying opinions on this but can't remember the details.

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: adding a second solar panel

    On the same charge controller?
    In parallel, no; if the Vmp of each panel is within 10% or less they will work together providing the controller can handle the additional current.
    In series, yes; the Imp needs to be near equal to prevent one panel from limiting the current of the other and the charge controller needs to be of the MPPT type to down-convert the higher Voltage of two panels in series.
    Separate controllers is no problem.

    One caution: a detached panel will have more wire to run through to get to the controller. It may be too much V-drop without ridiculously large wires.

    And the usual caveats about detached panels suffering damage and theft.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: adding a second solar panel

    coot, shouldn't that read, "In parallel, yes; if the Vmp.....etc, etc"
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: adding a second solar panel
    niel wrote: »
    coot, shouldn't that read, "In parallel, yes; if the Vmp.....etc, etc"

    That's what I get for trying not to run off at the mouth.
    In parallel, no problem.
    In series, yes could be a problem.

    My mind is getting more Swiss cheesed every day. :p
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: adding a second solar panel

    For our French Speaking Canadian friends: Fromage Suisse

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel

    Um, two 12V panels in a 12V system, so that's in parallel right? I think I followed what you said: if in series, the Vmp of the two panels needs to match, but in parallel they need to be close but not exact. Is that right?

    The Vmp of the existing panel is 17.6V, and one of the new panels I am looking at is also 17.6.

    Two options. One is a panel that has its own regulator and alligator clips straight onto the batteries.

    The other I would need to wire into a regulator, so I can either run that through the existing regulator, or another regulator and then wire that to the batteries? If I use a different regulator (eg if the Vmp is different), is there anything I need to know?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: adding a second solar panel
    leaf wrote: »
    Um, two 12V panels in a 12V system, so that's in parallel right? I think I followed what you said: if in series, the Vmp of the two panels needs to match, but in parallel they need to be close but not exact. Is that right?

    The answer is "It depends".

    If you have a 12 volt battery and a PWM charge controller... The Vmp-optimum is ~17.5-18.6 volts or so.

    But for a PWM controller, as long as both panels are >~17.5 volts, you can put them in parallel. Even if one panel has Vmp~30 volts. The 30 volt panel is not "optimum"--But it will work the best it can with a PWM controller (i.e., the 17.5 volt panel will output almost 100% of its available power, and the 30 volt panel will output a bit more than 50% of its available power).

    If the charge controller is an MPPT type... Placing a 17.5 volt panel in parallel with a 30 volt panel may end up with one panel outputting 100% of its available power and the other 0% of its available power. So, if you have a MPPT controller--Your best bet is to use the highest wattage panel and dumping the low wattage panel (regardless of panel voltage).
    The Vmp of the existing panel is 17.6V, and one of the new panels I am looking at is also 17.6.

    Two options. One is a panel that has its own regulator and alligator clips straight onto the batteries.

    Using battery clips is usually not a good idea for anything other than a low current/temporary connection. If this needs to be reliable, then cut off the clips and use hard bolted connections or a Anderson/similar power plug/connector set.

    If the panel has an "integrated" controller--Your choice is to cut off the regulator and wire the two panels to a new solar charge controller. Or wire up the first panel to the battery bus. And get a second solar charge controller and wire the new panel->controller->battery bus. You can parallel charge controllers (ideally, both controllers connected to the common +/- battery bus).
    The other I would need to wire into a regulator, so I can either run that through the existing regulator, or another regulator and then wire that to the batteries? If I use a different regulator (eg if the Vmp is different), is there anything I need to know?

    You can parallel multiple regulators to a common battery bus (as long as each regulator has a solar panel/array that meets the controller's requirements).

    For example you can mix a PWM+MPPT, two PWM, two MPPT, etc. controllers all together on the same battery bus. The charge controller with the highest set-point will "win" the ultimate battery voltage battle (i.e., high float voltage, or possibly high absorb voltage at the end of the absorb cycle).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: adding a second solar panel
    BB. wrote: »
    For example you can mix a PWM+MPPT, two PWM, two MPPT, etc. controllers all together on the same battery bus. The charge controller with the highest set-point will "win" the ultimate battery voltage battle (i.e., high float voltage, or possibly high absorb voltage at the end of the absorb cycle).

    -Bill
    Except if one or more of the charge controllers is a shunt type as was/is often used with inexpensive CCs for wind or hydro source systems and even some PV systems. In that case the CC that has the lowest shunt (dump load) voltage setting will win.
    SMA SB 3000, old BP panels.
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel
    BB. wrote: »
    The answer is "It depends".

    If you have a 12 volt battery and a PWM charge controller... The Vmp-optimum is ~17.5-18.6 volts or so.

    But for a PWM controller, as long as both panels are >~17.5 volts, you can put them in parallel. Even if one panel has Vmp~30 volts. The 30 volt panel is not "optimum"--But it will work the best it can with a PWM controller (i.e., the 17.5 volt panel will output almost 100% of its available power, and the 30 volt panel will output a bit more than 50% of its available power).

    The controller is a Sunsaver 10amp (so PWM I think). But at 10amps I won't be able to run a second panel, is that right? I'm thinking something like 175W to go with the 120W existing one, but haven't done the calculations yet (will start another thread on that). I think I have a second sunsaver controller in storage, but they're cheap too so am happy to buy one.

    (I am looking at buying a 175W foldable, but I can borrow a friends portable panel in the meantime, which is why I am looking at two different set ups)

    Using battery clips is usually not a good idea for anything other than a low current/temporary connection. If this needs to be reliable, then cut off the clips and use hard bolted connections or a Anderson/similar power plug/connector set.

    Ah, ok. The panel comes with an Anderson connector.

    btw, it has a PWM 30amp controller, so can I run the original panel through that as well? Or better to just use two controllers?

    To complicate things a bit, the existing controller came with a monitor that I have inside the RV, which is a cheap, no brand Chinese product. If I am running two controllers in parallel, can I use the existing monitor ie will it tell me the total power input?
    If the panel has an "integrated" controller--Your choice is to cut off the regulator and wire the two panels to a new solar charge controller. Or wire up the first panel to the battery bus. And get a second solar charge controller and wire the new panel->controller->battery bus. You can parallel charge controllers (ideally, both controllers connected to the common +/- battery bus).

    What's an integrated controller?

    You can parallel multiple regulators to a common battery bus (as long as each regulator has a solar panel/array that meets the controller's requirements).

    For example you can mix a PWM+MPPT, two PWM, two MPPT, etc. controllers all together on the same battery bus. The charge controller with the highest set-point will "win" the ultimate battery voltage battle (i.e., high float voltage, or possibly high absorb voltage at the end of the absorb cycle).

    -Bill

    Am a bit confused about that - what do you mean by 'win'? I am wanting to increase the total power input into the batteries over the day, so don't I want both panels to be working at the same time?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: adding a second solar panel

    *sigh* This is why my message box keeps filling up.

    Okay, SunSaver 10 is a PWM type charge controller that is capable of handling up to 10 Amps output. Since it is PWM type the input should be no more than the 'standard' Vmp of 17-18 as it can not make use of higher Voltage by converting it to greater current. If you use a higher Vmp panel all that will pass is its current. As such a panel with Vmp 30 and Imp 7 performs about the same as a panel with Vmp 17.5 and Imp 7.

    Panels connected in parallel should have similar Vmp, within 10% at most. Imp ratings can be different; they will add up.

    Panels connected in series should have similar Imp because the current will be limited to that of the lower Imp panel. Vmp ratings can be different; they will add up.

    The SunSaver 10 is capable of handling 175 Watts under typical panel specs (10 Amps @ Vmp 17.5). The main limiting factor is the Imp of all panels connected to it: they should not exceed that Amperage. A 30 Amp PWM controller would by the same reason be capable of handling (30 * 17.5) 525 Watts.

    Whether or not your monitor can be used with more panels depends on the exact nature of the monitor, which we don't know.

    A few panels, particularly of the small portable type, have charge controllers built-in. This is the "integrated controller" Bill speaks of. Morningstar even makes a low Amp (4.5) controller for attaching directly to panels, but that would be for <80 Watts.

    The use of the word 'win' indicates which controller's charging specifications will have precedence over the other. That is if one controller says the battery should Absorb at 14.8 and the other says 14.4 the battery will be brought up to 14.8 (if there is sufficient power to do so). The lower Voltage controller will simply contribute to the Bulk stage until its V set point is exceeded.

    I'm not surprised you are confused; people adding in post about equipment not involved with the issue at hand (like shunt controllers on turbines when we are discussing PWM controllers and solar panels) do not help explain things at all. I wish they'd stop it.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: adding a second solar panel

    watch it as 30v is the max voltage for a 12v sunsaver.
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel

    This is getting confusing, I think it might be good to go back to basics so I can understand from the ground up.

    Why would someone connect two panels in series?

    Why would someone connect two panels in parallel?


    I want to add an new panel to an existing panel, to increase the amount of power I am generating each day. Should I be using them in series or in parallel?

    Please bear in mind that I posted in the Beginner's Corner ;-)
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: adding a second solar panel
    leaf wrote: »
    This is getting confusing, I think it might be good to go back to basics so I can understand from the ground up.

    Why would someone connect two panels in series?

    If higher Voltage is needed. For example two "12 Volt" panels in series to charge a 24 Volt system or two "24 Volt" panels in series to charge a 48 Volt system. Also higher Voltage can overcome wiring losses when long distances are involved, but in such an instance would be used in conjunction with an MPPT type controller.
    Why would someone connect two panels in parallel?

    To increase array Watts (and thus current) while maintaining the same Vmp. As in multiple 12 Volt panels on a 12 Volt system using a PWM type charge controller.

    I want to add an new panel to an existing panel, to increase the amount of power I am generating each day. Should I be using them in series or in parallel?

    Please bear in mind that I posted in the Beginner's Corner ;-)

    That depends on the type of controller. With a PWM controller you should use panels with very similar Vmp connected in parallel and not exceeding the controller's Amperage capacity.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: adding a second solar panel

    It is confusing... The basics.

    Obviously, when you have more than one panel, you need to choose series, parallel, or series parallel connections. So, that is the first answer.

    The second is what operational voltage of the array do you need. For a 12 volt battery bank you generally want a minimum voltage Vmp-array of ~17.5 to 18.6 volts or so. As solar arrays heat up, their Vmp voltage falls (by upwards of 20% less than Vmp-rated) on very hot days. For a 24 volt battery bank, you need 2x the Vmp of a 12 volt array (~35 volts minimum). For a 48 volt system you need 4x Vmp-array vs a 12 volt system (~70 volts minimum).

    With PWM type charge controllers (the less expensive type), if the Vmp-array gets too high, you start "giving away" solar panel energy... PWM controllers are "simple" controllers that simply are a computer controlled "on/off" switch. If you put too high of Vmp-array on a 12 volt battery bank, you start wasting panel power. For example, if you put a Vmp-array~35 volts on a 12 volt battery bank+pwm controller, you will lose about ~1/2 the panel wattage.

    So, with PWM controllers, there is a very specific Vmp-array voltage we are looking for that gets us the most energy for the buck. So series/parallel connections are pretty fixed for that type of system.

    For systems with MPPT (maximum power point tracking controllers), the minimum array voltage requirements is pretty much the same as for PWM controllers--So that sets the series/parallel array "floor" requirements.

    With MPPT charge controllers, they are made with a "buck mode digital switching power supply" (in general). This type of "power converter" is sort of like the DC equivalent of an AC variable transformer. You can efficiently take high voltage/low current from a solar array and "down convert" it to low voltage/high current needed to charge a battery bank.

    So, that 35 volt Vmp array connected to a 12 volt battery bank will be about 95% efficient taking the 1,000 watts from the array and sending ~950 watts to the battery bank (where a PWM controller would only be about 50% efficient).

    This ability of the MPPT charge controller makes large arrays (and arrays farther from the battery shed) more efficient to wire. Basically, it is the ability to do "high(er)" voltage transmission. If you have a 1,200 watt array charging a 12 volt battery bank (using even numbers and forgetting losses for the moment)--Look at what needs to happen.

    1,200 watts at 17.5 volts = 68.6 amps, and we want to send that power with a 3% voltage drop (and ~3% losses or ~0.5 volt drop). To send that amount of current 50 feet would take (using a generic voltage drop calculator) would take around 3/O copper wire (almost 1/2 of an inch in diameter of copper). That is expensive amount of cable and difficult to work with.

    If you put 6 panels (Vmp~17.5 volt panels) in series for 105 volts and ~11.4 amps (higher voltage, lower current, same power) and now 3 volt drop, you would need 12 AWG cable--Much cheaper and easier to wire (basically normal house branch circuit wiring). But the MPPT charge controller may cost 3-5x more money--So there is a trade off.

    Of course, there are different controllers that have different maximum input voltage... A high end controller may have ~140-150 maximum input voltage (Voc-cold) and some controllers have even higher voltage inputs (upwards of 600 Volt Voc-cold, or Vmp~400 volts). These are good for large systems that have to put the panels a fair distance away from the battery shed.

    One of the other "advantages" of a MPPT charge controller is you can get "Grid Tied" panels that have "non-standard" (for a battery bank) Vmp voltages. One typical voltage is Vmp~30 volts... Too high for a 12 volt battery bank (PWM controlelr) and too low for a 24 volt battery bank... An MPPT charge controller can match these panels Vmp-array to your battery bank very efficiently (at a cost). And the GT panels are (roughly) about 1/2 the price ($$$/Watt) of the Vmp~17.5 volt panels. So, pay less for the panels, and more for the charge controller.

    For the typical MPPT controller, roughly the most efficient Vmp voltage is about 2x the battery bank working voltage... I.e., a 12 volt battery bank around 35 Vmp array. As the Vmp-array goes higher, the MPPT charge controller may lose a couple percent of efficiency and run a bit hotter... It is all trade offs.

    And you need to check the price delivered of your solar panels. Around 140 Watt panels are the maximum size you can send via UPS/FedEx/Bus, etc... Larger panels need to ship by truck. Also panels >~175 Watts (typical GT panels) need two people to move them around safely.

    Many vendors have created a string sizing tool for the MPPT charge controllers to help you with the math. In cold climats, the Voc-array-cold must not exceed the charge controller's maximum input voltage or it will be damaged (~140-150 volts for a "typical" high end MPPT controller). Here is an example for Midnite Solar:

    http://www.solar-electric.com/midnite-solar-classic-sizing-tool.html

    In general, for arrays smaller than ~400 watts, a "simple" and cheap PWM controller is "good enogh". For 800+ watt arrays (and arrays with long wire runs), an MPPT controller is usually worth the expense.

    Do a couple "paper designs" with different panels and charge controllers... See what works out best for you. In general, the new features (such as integrated battery monitor, programmable charging voltages, aux control outputs), Internet/Web communications, etc. are going to be found on the MPPT type charge controllers. That is where the money and new development is.

    Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel
    If higher Voltage is needed. For example two "12 Volt" panels in series to charge a 24 Volt system or two "24 Volt" panels in series to charge a 48 Volt system. Also higher Voltage can overcome wiring losses when long distances are involved, but in such an instance would be used in conjunction with an MPPT type controller.



    To increase array Watts (and thus current) while maintaining the same Vmp. As in multiple 12 Volt panels on a 12 Volt system using a PWM type charge controller.




    That depends on the type of controller. With a PWM controller you should use panels with very similar Vmp connected in parallel and not exceeding the controller's Amperage capacity.


    Thanks, that's pretty much what I thought. Since I already have a 12V system, and there are no voltage drop issues, I think it would help to focus on what works with my system i.e putting the new panel in parallel.


    BB, thanks for the detailed reply but I think that is way beyond the basics. I'd like to come back to your post when I have a few more things sorted out in my head.


    It turns out I don't have the sunsaver regulator in the existing sysytem (I have that one as a spare). I will have access to a panel to borrow tomorrow, so will post the specs of what I have in the system and with the new gear and then see if I can figure out here what will work.

    thanks.
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel

    Just doing the research on the panel I can borrow. The only information on it is on the unit that the wires come out of on the back of the panel. That says:

    ZJRH 4mm2 1000V IP67

    PV-RH06-70

    i.e. http://www.renhesolar.com/products1-pid-2.html

    Is there any way to tell the panel specs from that, or could that unit be used on any panel?

    Or from the panel itself:

    The panel itself is 1250mmW x 808mmH including frame. There is a row of 9 hexagonal shapes along the top, and 6 going down ie total of 54 hexagons. Each hexagon is 125mm x 125mm at its widest point. The hexagon is in two parts (a split down the middle).

    Which sounds like a larger version of this http://www.sopraysolar.com/english/products.asp?type=17 I'm guessing it's a 120W panel, does that sound right?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: adding a second solar panel

    Well the PV-RH06-60 seems to refer to the junction box, so that's no help.

    Sounds like fifty-four cells @ 0.5 Volts each is a 27 Volts if they are all in series, so that would be "GT style". Probably a 26-ish Vmp rating.

    I think you'll have to measure it. You can check the Voc easily with a multimeter. The Isc would be helpful, and that can usually be checked with a multimeter that can do 10 Amps DC. Very few panels are higher than that.

    It doesn't sound hopeful for working with your system.
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel

    "It doesn't sound hopeful for working with your system."

    How so?


    How do I measure the Isc? Is that just the amps output?

    I've taken a voltage reading and an amps reading just before noon. Voltage was between 20V and 21V. Amps were 6.8. Is that voltage equivalent to the Vmp?
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel

    Ok, here is what I have:

    Existing system:

    120W panel, mounted at a low angle.
    Voc 12.8V
    Isc 7.2A
    Vmp 17.6V
    Imp 6.82A

    2 x 6V (= 12V) batteries, 225 amp/hrs (@20hr)

    A no name controller, 10amp, PWM

    (specs here, PDF, http://www.google.co.nz/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCYQFjAA&url=http%3A%2F%2Feccsun.com%2Fuploads%2FEPIP20-D_user_manual.pdf&ei=bztkU7DXDYSolAXgvIGgBw&usg=AFQjCNHja5_HRVl8rJUQ-Z7RJLs4mmAtZg&sig2=_-Zx302ktA-TBrVp9agXDg )


    I also have a spare Sunsaver 10amp PWM controller.


    In the short term I'd like to add in a borrowed panel, specs unknown, but measurements just before noon:

    - Voltage was between 20V and 21V.
    - Amps were 6.8.

    Can I add this to the battery bank with its own controller?


    I should add, I am in the southern hemisphere heading into winter. 7 - 8 months of the year my current system is adequate, but stops being adequate by the end of April.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: adding a second solar panel
    leaf wrote: »
    "It doesn't sound hopeful for working with your system."

    How so?

    It appears the Vmp may be in the mid 20's which means if used in conjunction with a PWM type controller you will not get full power from this panel. It is basically incompatible with a 12 Volt system & PWM controller or even a 12 Volt panel & MPPT - the Vmp's will be too far apart.

    How do I measure the Isc? Is that just the amps output?

    Ammeter between the the two panel leads, panel in full sun. It should read close to Isc.
    I've taken a voltage reading and an amps reading just before noon. Voltage was between 20V and 21V. Amps were 6.8. Is that voltage equivalent to the Vmp?

    Unless there was a load on it the Voltage would be Voc. 20-21 Voc is normal for a 12 Volt panel. If we extrapolate a Vmp from that and multiply it by the 6.8 Amps current (Isc?) then you have close to 120 Watts.
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel

    Ok, thanks. Does that mean I could use it with my existing system, but it wouldn't be as efficient as if properly matched? How much less efficient? I am thinking at this point that any increase on power is good. My friends use the panel in the winter as a back up to their 12V RV system, and they do get extra power from it. Not sure if they are using it inefficiently or if I have measured something wrong...
  • leaf
    leaf Solar Expert Posts: 37 ✭✭✭
    Re: adding a second solar panel

    btw, 21 x 6.8 = 142.8, closer to 140W. I'm still not totally clear on the Voc vs Vmp thing. Can I assume that if take a voltage reading (with nothing connected) that the Vmp can't be less than that reading? (but could be more if I had the panel angle wrong or it wasn't the best sun etc).
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: adding a second solar panel
    leaf wrote: »
    Ok, thanks. Does that mean I could use it with my existing system, but it wouldn't be as efficient as if properly matched? How much less efficient? I am thinking at this point that any increase on power is good. My friends use the panel in the winter as a back up to their 12V RV system, and they do get extra power from it. Not sure if they are using it inefficiently or if I have measured something wrong...

    With a PWM controller all you get is the current. So if you use a panel with a Vmp higher than typical for a 12 Volt system (17-18 ) you get the power equivalent of its current at the typical Vmp. For example a 240 Watt panel with a Vmp of 30 and Imp of 8 would produce power equivalent to 17.5 Vmp 8 Imp or 140 Watts: a 100 Watt loss!

    But we have yet to establish the exact Voltage and current of this panel.