newbie mppt controller sizing question

snokid

here's the hardware, later I will ask my upgrade path...
Morningstar mppt sunsaver 15
2x 180 watt 36 volt panels
6x 100 ah agm batteries 12volt system
This is mounted on a sailboat.

So right now I have the 2 panels wired in parallel and the batteries have held a charge no problems.

Ok so here's the question.....

The sunsaver mppt 15 says it will charge up to 15 amps.  

According to my understanding my calculations say 1 panel will output 5 amps @ 36v but when it goes thru the charge controller and drops the voltage for my 12v system I will get 15amps @ 12v.  Right now it's getting 2 panels so the 12v charging amps should be 30amps.  Well that's more than the controller is rated for.

So my question is first am I figuring this right? If so am I only get half of what the system should put out?  Or is the system putting out 30 amps (in a perfect world) 


OK second question.

I'm going to add 1 more panel very soon (more later but don't know the time frame).  Remember this is going on a sailboat and I'm leaving land for an extended period of time.  I'm thinking I'm going with another 180watt 36 volt panel (that's what I have room for)  and another sunsaver 15  The reason I'm doing it this way is spare parts, all I need to carry as a spare is 1 more sunsaver for a total of 2 in use and 1 just incase.

Should I just have 1 charge controller per panel?  So that would be 3 controllers in use and 1 more for a spare.
I know I can get bigger controllers does that make more sense?  say a 60 amp controller then I more for a spare?

thanks in advance 
Bob

mcgivor

The controller is designed to charge at 15A maximum  per nominal voltage, in your case 12V, using a charge voltage of 14.35V multiplied by 15A the maximum PV input would be ~215W, with 2 × 180W panels derated 25% the wattage would be ~270 W, so you are over paneled, the controller will only process 15A so whatever extra is available would be wasted. The best way to determine the optimum configuration would to use Morningstars string calculator http://string-calculator.morningstarcorp.com/ 

Note only one MPPT charge controller can be used on a single array, or single panel, so using 3 panels is not an option with 2 controllers unless you have 1 panel on one and 2 on the other, if Morningstar allows the latter. With controllers it's best not to run at maximum capacity so using the 3 controllers opinion would probably be the best.

The 6 batteries in parallel, an assumption, would be something I'd be more concerned with, not an ideal arrangement even when wired correctly. Just an observation 

littleharbor2

If you were planning on having an extra (3rd) controller anyway just run one per panel. Morningstar products are very robust and the odds of one breaking down are slim. Having the redundancy of 3 separate controllers, and not having any series wired strings are a good thing. Shadows on a sailboat are probably a fact of life with the constant motion, cables, masts, etc. Series wired strings will suffer from any, even minimal, shading.

snokid

thank you guys!!!
I kind of figured it was the charging amps not the input amps.

So is it better to parallel 3 panels to 1 controller like a 60 amp or have 3 smaller units like the 15 amp fed individually by each panel?

mcgivor---
you say 6 batteries in parallel is bad, but I'm not sure I could do anything else, my boat runs off a 12 system, I have an inverter but it's only used to run things I can't get on 12v.  ie. charge laptop.  everything else is run off the batteries straight.  fridge, gps, radar, lights, pumps, etc....

So is that something I could change and still have a 12v system as far as charging the batteries?

littleharbor2---

I'm going to have a spare no matter which setup I'm running...  Crossing the Atlantic ocean takes about a month on a sailboat, then visiting small islands there's no way to just order a new one if it breaks!!!!

thanks Bob

mcgivor

mcgivor---

you say 6 batteries in parallel is bad, but I'm not sure I could do anything else, my boat runs off a 12 system, I have an inverter but it's only used to run things I can't get on 12v.  ie. charge laptop.  everything else is run off the batteries straight.  fridge, gps, radar, lights, pumps, etc....

Having  batteries in parallel can be problematic due to uneven charging and dischargeing, especially if connected incorrectly. Just attempting to prevent possible disappointment, let us know how the batteries are wired with diagrams if possible , perhaps in a new thread to avoid confusion with this thread, it's really important you understand why and how, if you are crossing the Atlantic,  again this is VERY IMPORTANT