New solar install questions

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svNorthStar
svNorthStar Solar Expert Posts: 47 ✭
in Solar Beginners Corner #1
I am installing 2 Kyocera 245 watt PV panels (KD245-GX), a Morningstar Tristar 60 controller on the hardtop of our boat.

We are a 12 Volt boat. Should I wire the panels in parallel or series? I know the plug for series is being able to use smaller wire size from panels to controller. I also read somewhere that the panels wired in parallel are more resistant to shading. I can't get my head around that concept?? Is there a bigger loss in the controller if the voltage differential is greater? In parallel at max power, per spec, should be about 29.8V at Vmp or 59.6 in series. Well within the specs of the controller either way. I have seen some numbers about the loss in the controllers quoted around, but nothing that says what/if the loss increases/decreases based upon the input voltage. Any help, or direction on this would be appreciated.

Next, can the controller be mounted in our engine room? Ambient temps are usually the same as rest of the boat when sailing or at anchor and up to around 130 max when engine and/or generator is running. I ordered the remote meter for the Morningstar, intending to mount it at the nav station. We'd like to keep the heat out of the living area if possible. As I understand it, the default configuration is to shunt unused power to the heat sink on the controller. We have heard of some boats shunting excess power from their windgens to their water heater element. Any info on that would be appreciated.

Finally, we are located in Colon, Panama, where we are completing an electrical/electronic total refit of our boat due to a lightning strike we took in the San Blas Islands. We don't have access to the normal electrical suppliers, etc. Either order everything and get it in 2-4 weeks or figure something else out. The current dilemma is finding mc4 connectors, or anyone who has heard of them. Can I just cut them off, solder and seal everything back together well? Someone told me that cutting the connector off voids the warranty on the panel. That sounds ridiculous, unless somehow you then hooked them up backwards, or something else related to actually damaging them. I don't have the "tool" people talk about for the connectors and think I'd rather just use standard marine wiring practices for hooking everything up. Ie, marine connectors and crimps and maybe soldering on top of that for the heavy cable from controller to batteries.

Thanks for any and all assistance,
Steve
s/v North Star
http://www.sailblogs.com/member/sv-northstar/
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: New solar install questions

    Welcome to the forum Steve.

    Question: which Morningstar TriStar 60 do you have? If it isn't the MPPT version you need to exchange it. Otherwise the power from higher Voltage panels will be lost.

    After that, parallel wiring of the panels would be fine. The efficiency losses come from having an array Voltage that is much higher than the system Voltage. Vmp 29.8 is above the normal for 12 Volts but not so much so that it would cause any problem (you could have Vmp 35 without trouble). 59.6 Vmp might cost a percent or two. If a shadow fall across one panel it will 'take out' that one panel. If they are wired in series a shadow on one will also limit the output of the other (as it has to send its current through both).

    The ambient temps will be a problem. Nothing likes to work in that heat! Panel Voltage will go down as a result, and the charge controller will be near the limit if it has to operate at 130F! Try and do something to be sure there's plenty of air flow over that controller. Unused panel power isn't shunted at all; it is simply disconnected when not needed. Once the correct battery Voltage is achieved for the particular charging stage the panels are simply switched on/off as needed to maintain that level against varying current demand.

    MC4 connectors are easy to get; just order extension cables and cut them in half. Mail order from our host NAWS: http://www.solar-electric.com/incaforsoelp.html Easy, painless. Do not cut the connectors off the panels as it will void the warranty. Note also the panel multibranch connectors on that page.
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  • svNorthStar
    svNorthStar Solar Expert Posts: 47 ✭
    #3
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    Re: New solar install questions

    Thanks for the welcome and the response. Sorry, should have introduced myself as a noob, I guess. Just sort of wrapped up in this project here....

    Yes, I have the MPPT version, actually didn't know there was one that wasn't by this name.

    As for the wiring, I guess I still don't understand the shadow difference. I thought that parallel would be fine, given these panels are normally operating at a much higher voltage than the batteries. I have one sitting on deck now and the open circuit voltage was 34.5V. I would have thought that since basically each cell is .5V if I'm in series and one is shaded I lose .5/60 instead of .5/30. Your comment about having to send current through other panel is series confuses me. Isn't each panel basically in however many cells in series? It would seem to me that shading on of 60 would produce more total power than one of 30, plus all of the other 30. Although, the more I think about this, the less I think my own sentence makes sense. I understand that if you are working with a panel close to your final voltage, you would want them in series so that it doesn't fall below a shutoff voltage.

    On the temps, wasn't sure and still haven't found anything that says what temp the controller shuts off or dials down. I think maybe what I'll do is wire everything up temporarily and see how much heat is actually produced. I understand now about the controller shutting the panels off instead of shunting it away. That is good to hear and maybe I'm worried about nothing here. We just don't want to add heat to the interior of the vessel. We are presently at 9 degrees north and headed to the equator. Heat inside the living area is bad already. The engine room vents to outside when engines/generators are running and when nothing is running it is the same temp as the rest of the boat but again vented to the outside.

    I know the connectors are probably easy to get in the states. It is a full day trip from where we are to Panama city where I'd have to go look for them. What is the rationale behind voiding warranty on this basis. That is crazy. Not that the connectors are bad, but you obviously are going to be doing your own wiring at the next connection anyway. Many cruisers would just not want to add another type of connector and requisite tools/spares to the inventory we already have to keep on board. A package shipped here is $75 minimum for a letter by fedex. Is this warranty position a Kyocera thing, or all the manufacturers that way?

    Anyway, thanks for all the good info and the link and I look forward to participating in the forum here,

    Steve
    s/v North Star
    http://www.sailblogs.com/member/sv-northstar/
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: New solar install questions

    Don't think in cells, think in panels. Each panel is its own entity, and that entity is a current source. Shad one and instead of it contributing it becomes an obstruction; the output current for two (or more) in series is limited to the lowest amount any one will pass. In parallel if one is shaded the other(s) will continue to supply all the current they can.

    It isn't that the controller will shut down due to high temps, it's that higher temps are hard on components. So I guess it would shut down 'prematurely' at that. As in drop dead before its time. Electronics do not like to be cooked. As a rule 40C (104F) is the upper limit although MS actually lists 45C (113F) for this unit.

    The thing about cutting the connectors off is a legal issue; it constitutes a modification to the panel. At that point no manufacturer is required to honour the warranty because the unit has been altered. The same goes for drilling holes in the frame. Technically it won't affect operation at all, but warranty is more about rules than reality. If this isn't a problem for you, go ahead. In general panels are amazingly dependable anyway.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
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    Re: New solar install questions
    svNorthStar wrote: »
    I would have thought that since basically each cell is .5V if I'm in series and one is shaded I lose .5/60 instead of .5/30. Your comment about having to send current through other panel is series confuses me. Isn't each panel basically in however many cells in series?

    What you are not considering is that when a cell gets no light, it does not become a 0 volt battery which will let the current flow through unobstructed. Instead it becomes a resistor combined with a diode which will break down or be damaged at high voltage/current.
    So to let the other cells send their current through the shaded cell, you have to put an external diode, called a bypass diode, around that cell. In practice a single bypass diode may be used for every group or 12 or more cells ( more than 6 volts). And when the current is flowing through the bypass diode, none of the limited current that the shaded cell would produce on its own can be used.
    So, one shaded cell in a 12 volt panel will cost you at least half of the output power of that panel. Two shaded cells in different bypass diode groups will cost you the full power of that panel, as well as adding in a voltage drop of 1 volt or more through the two bypass diodes.
    If you then try to connect a string of 4 panels with one shaded in parallel with another string of 4 panels, all in full sun, the MPPT CC may well end up not drawing any current at all from the shaded string.
    SMA SB 3000, old BP panels.
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  • svNorthStar
    svNorthStar Solar Expert Posts: 47 ✭
    #6
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    Re: New solar install questions

    Ok, thanks. This and the comment above make it clearer for me. I had forgotten about the internal diodes and dead cell issue. I'll do this in parallel. I can always change it around later if desired. Ok on the temps for the MS. I hadn't seen that anywhere, just several comments about making sure there was airflow, etc.

    On the connectors, I'll have to check on availability down here. I sent an email to the link you provided earlier, but haven't heard anything back yet. I'll see what I can find in Panama city on Monday.

    Thanks again
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  • svNorthStar
    svNorthStar Solar Expert Posts: 47 ✭
    #7
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    Re: New solar install questions
    What you are not considering is that when a cell gets no light, it does not become a 0 volt battery which will let the current flow through unobstructed. Instead it becomes a resistor combined with a diode which will break down or be damaged at high voltage/current.
    So to let the other cells send their current through the shaded cell, you have to put an external diode, called a bypass diode, around that cell. In practice a single bypass diode may be used for every group or 12 or more cells ( more than 6 volts). And when the current is flowing through the bypass diode, none of the limited current that the shaded cell would produce on its own can be used.

    I remember reading something about that now, but had forgotten. This makes more sense to me now. Thanks for the explanation.
    So, one shaded cell in a 12 volt panel will cost you at least half of the output power of that panel. Two shaded cells in different bypass diode groups will cost you the full power of that panel, as well as adding in a voltage drop of 1 volt or more through the two bypass diodes.
    If you then try to connect a string of 4 panels with one shaded in parallel with another string of 4 panels, all in full sun, the MPPT CC may well end up not drawing any current at all from the shaded string.

    Obviously, that would seem to make this decision a no brainer. Not sure why there is always so much discussion about it. Several other people advised that series was the way to go, saying that I'd get more usable power out of the system that way. Obviously, that sounds great, but I had forgotten about the internal diodes and the voltage drop across them bringing down the entire panel.

    Thanks again.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: New solar install questions

    Shading issues aside, series/parallel makes no difference in power except for:
    http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    People often think that running panels in series will increase output on cloudy days. This is not so because panels are a current source, and it is the current that drops off with lowering of insolation. You can get Voc from a panel under almost any amount of illumination, but without that square-on intense sun you don't get the full current and Voltage (Imp * Vmp) that means full power.
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  • svNorthStar
    svNorthStar Solar Expert Posts: 47 ✭
    #9
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    Re: New solar install questions

    Ok, the link to the other thread was helpful. I will be doing the parallel install.

    Now, if you could just verify that my thinking/calcs are correct on sizing the wire I think that should just about wrap it up. I'm not sure which spec to use, but I used Imp and Vmp for the voltage drop calc. With my two panels in parallel I should be getting 16.4A and again using Vmp at 29.8V I get the following:

    Assumptions: 20 foot/one way run panels to controller and 10 foot one way run controller to batteries. Assuming 13.5 V float charge rate.

    Panels to controller: AWG 10 Vdrop=.678V / 29.8V = 2.3%

    Controller to batteries AWG 6 Vdrop=.295V / 13.5V = 2.2%

    Calced using http://www.powerstream.com/Wire_Size.htm

    These get me under 3% loss. May go larger one size on each to get under 2%??? The Voc is 36.9V and Isc is 17.82A but were not used in the calcs.

    Finally, is there any reason not to run the output of the charger to the battery bus on the boat? I've heard people say you should only hook directly to the battery, but never heard why. We have 4 separate battery banks onboard. All which can be selected by large Perko switchs to drop off certain banks. We have a total of 1200Ah Lifeline AGMS. The only thing that comes to mind is that during the switching there could be some issue that the controller wouldn't like? Transients? Any thoughts on this?

    Thanks again for all the help!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
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    Re: New solar install questions

    Panels to controller: 20 feet, 24 Volt nominal, 17 Amps maximum; 10 AWG gives 2.9% V-drop. Perfectly acceptable.

    Controller to batteries: 10 feet, 12 Volt nominal, 34 Amps maximum; 6 AWG gives 2.3% V-drop. Perfectly acceptable.

    Notice that this Voltage drop is at its maximum under full power conditions; as the current drops off and the Voltage rises the efficiency improves. If you can be under 3% loss at full power it should be good enough. If you are planning on adding more panel later on the same controller, wire for that now; it's easier than trying to retrofit the wire later.

    It is all right to connect the output to a battery bus. But, you have 1200 Amp hours all together? Don't expect 32 Amps to do anything for that massive battery bank. Full current would only be trickle charge for it. Separated as four individual banks of 300 Amp hours each it would work fine on any one.

    The only issue with switching is to be sure the controller doesn't get disconnected from battery Voltage entirely between changing banks; if you remove the battery power without disconnecting the PV first it may confuse the controller. Otherwise, no problem.
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