# newbie mppt controller sizing question

snokid
Posts:

**6**Registered Users ✭✭
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

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

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## Comments

1,903Solar Expert ✭✭✭✭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

963Solar Expert ✭✭✭✭2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

6Registered Users ✭✭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

1,903Solar Expert ✭✭✭✭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