Designing a space-constrained PV system for a boat. Advice needed.

Boatman · June 2016

I'm new to this forum and recently to the PV world. I'm in the process of assembling a PV system for my cruising catamaran. As you might surmise, I have some tight size and dimensional requirements that are going to cause me to have different panel sizes, and as a result, different panel voltages and wattages. My boat currently has four 31 series 12V AGM batteries that are charged by the two engines while under power or by a three stage Guest marine battery charger while at dock. The batteries can be connected in various combinations through USCG rated blow proof marine switches.

My goal is to become as energy independent as possible and stay away from marinas in our coastal travels. My daily consumption loads are relatively light. The largest 12V draw is refrigeration, followed by navigational electronics, entertainment, lighting and mobile device charging. I'd also like to set up a 12V water heater to take any excess charge I generate and reduce my propane use for water heating. Since I'm planning on using excess charge to heat water, I really can't have too much charge capacity.

Due to the configuration of the boat, the wiring runs are going to be short (none longer than 10 ft.) so I can afford to run large cabling.

I have two large mounting areas, one on the cockpit cover and one on a rear arch. Based on the internet searching I've done on panel ratings and dimensions, to take maximum advantage of the space I have the install looks something like this:

1 @ LG 320 Watt Neon panel

2 @ Sonali 200 Watt panels

...for a total of 720 watts.

To maximize my harvest, I'm going to opt for an MPPT controller(s)

I have a couple of questions...

I understand that either panel voltage or panel wattage needs to be the same if the panels are to be combined (in parallel or series) to input into a single controller. For these panels, the wattages are obviously different, but the voltages are close. For the LG panel, Vmp is 30.7 and Voc is 37.9. For the Sonali panels, Vmp is 30.6 and Voc is 36.6. Are these voltages close enough to combine into one large MPPT controller or am I going to need 2 MPPT controllers?

If I have to run two smaller MPPT controllers, are they going to be able to coexist charging a common battery bank without having to network them together, which my searches indicate entails increased cost and complexity.

Any and all comments, corrections, lessons and advice greatly appreciated. I'm new to this but trying to learn fast and avoid obvious mistakes.

Thanks...Boatman

vtmaps · June 2016

Welcome to the forum,

You can put panels in parallel as long as their voltages are the same. Both of the panels you mentioned are 60 cell panels and will be a good match in parallel.

Boatman said:

I understand that either panel voltage or panel wattage needs to be the same if the panels are to be combined (in parallel or series) to input into a single controller.

Partially correct. When placed in parallel the voltages must match. When placed in series the current (amps) must match. There is no reason for you to have your panels in series.

--vtMaps

Dave Angelini · June 2016

Are you sure you can't fit any more solar on the cat to run the reefer? Double check several times that those engines, the dock, and your solar can't over voltage those AGMs. There is a long list of people who started a cruise and needed AGM batteries very quickly!
Good luck! The cats were just coming out when we finished our cruise. Awesome years!

Boatman · June 2016

VTmaps - Thanks for the info. I was hoping that the panel voltages would be close enough to use a single MPPT controller.

Dave - I know that AGM batteries have more more stringent and narrower charging requirements, but I was under the impression that a good MPPT controller - like a Morningstar or Outback - could meet those requirements without cooking the AGMs. Dealing with perioduic excess capacity on a cruising boat is imperative since the system will be out in the sun and "On" when we're not using the boat. And we're not liveaboards so there may be stretches when the boat sees little use. Are AGMs at risk for the periods when there is no load on the system?

Regarding charging, my cat is a lightweight, fast boat. As such it has twin retractable outboards for power that only generate 10 amps of charge each. No risk of overcharging there. Plus we're trying to make the boat as independent as possible. I figured we could always make use of excess capacity by heating water for showers - a true luxury on a 32 ft. boat.

Thanks for the very helpful information.

Boatman.

Dave Angelini · June 2016

Think of the synergy! All the charging sources must play nice together. Until you test them together you do not know!
Make sure the solar does not over volt the boat plugged into shore and the same with motoring.The over charging is not the same as over volting roger? One is current and the other is the voltage which is what you need to be checking for in the absorption stage.

The controller is only as good as the person setting it!
Make sure you have temp sensing of the battery from the solar and that when combined with your other charge sources, they do not exceed the battery manufacturer max absorb voltage spec. A 10 amp charger can easily do this to a charged battery BTW! Easily!

It can be a pain because the engines alternator is not adjustable. I have had to add adjustable regulators to generators and alternators to get them to play nice. Sometimes it is easier to just shut off the alternators and be be able to turn them on when you need to.

All of this and the distractions of running a cruising boat are what can ruin the battery.

vtmaps · June 2016

Boatman said:

I was under the impression that a good MPPT controller - like a Morningstar or Outback - could meet those requirements without cooking the AGMs.

On a boat you want a sealed controller. Also take a look at Midnite's kid controller (with the whizbangJr shunt).

Whatever controller you have be certain to disable auto equalization AND set the EQ parameters to be the same as absorb.

Boatman said:

Dealing with perioduic excess capacity on a cruising boat is imperative since the system will be out in the sun and "On" when we're not using the boat. And we're not liveaboards so there may be stretches when the boat sees little use. Are AGMs at risk for the periods when there is no load on the system?

Actually, AGMs are better than flooded at dealing with prolonged floating. It will do them no good to be absorbed daily if they are not being used. I believe Midnite's controller can be programmed to skip multiple days of absorb (just go to float).

AGM batteries also have low self discharge... if you are leaving them unattended for a few weeks the safest thing is to just disconnect them.

--vtMaps

Dave Angelini · June 2016

Add the Schneider Electric MPPT-60-150 to your list. There is no fan like the morningstar and it plays nicely with the freedom series inverter/chargers that are often used in a marine environment. They can be linked by a network to share battery temp. They hold up really well in salt air and are simple to program and will very quickly go to float (5 minutes or so) if not being cycled.

Boatman · June 2016

vtmaps - Actually, the Sonali panels are 36 cell and the LG panel is 60 cell. However, their spec sheets say they put out about the same voltage. Does this make sense? In trying to run different panels in parallel to a single MPPT controller, should I only be concerned about voltage or the number of cells, too?

Advice appreciated...

Boatman

BB. · June 2016

Can you post links to the two panels?

There can be series/parallel connections in a solar panel... For example, a 72 cell panel could have 2x parallel strings of 36 cells each, for Vmp~36 volts output.

A 60 cell panel should have Vmp~30 volts ("standard"), or possibly Vmp~15 volts (rare/no existent?).

In general, solar panel manufacturers seem to be going with higher voltage panels.. GT system typically run at several hundred volts or higher. High voltage strings need much less copper (less costs, easier wiring connections).

Battery based systems are either "fixed" to battery bank voltage (PWM controllers: 18 volts Vmp to 12 volt battery, etc.). We have MPPT controllers, but they are typically limited to Vmp-array maximum voltage of around 100 VDC. There are a few MPPT solar charge controllers that can take higher array voltages (upwards of Vmp-array~400 volts), but they are not cheap.

-Bill

Dave Angelini · June 2016

And 1000 volt controllers battery based and much more are not far away! Not to good on a boat though......

Boatman, are you sure you are not mixing up the different voltage specifications? voc, vmp etc?

Boatman · June 2016

Here are the links and specs for the two panels that fit best fit the available space on my boat to maximize output:

Sonali 200

http://sunelec.com/solar-panels/sonali-200w-solar-panel-12v.html

Watts 200


Panel Voltage	12
Cells	36
Panel Vmp	30.6
Panel Imp	4.9
Panel Voc	36.6
Panel Isc	5.26

LG 315

https://tandem-solar-systems.com/buy-solar-products/lg-neon-315w-modules/?gclid=Cj0KEQjwhZm7BRCUyfS6ho2VjOEBEiQAumpGMvIk4ikXLqm6InnCfUHmql9vm5ci76c_VP_yqjzmaPgaAkdJ8P8HAQ

Cells     60
Vmp   30.4
Lmp   7.58
Voc    37.6
Isc      8.08

Even though the panels have different cell counts, their stated voltages appear to be close, if I'm understanding the spec sheets correctly. Also, It appears that there are different measurement and reporting standards (NOCT vs STC) which is further confusing.

As before, I appreciate all advice and guidance.

Thanks...Boatman

BB. · June 2016

Here is the data sheet:

http://sunelec.com/datasheet-library/download/SS-120-200-Series.pdf

The 200 Watt panel is a 54 Cell panel (Vmp~27 volts, Imp~7.41 amps)... It appears the spec. sheet is correct, and the website is wrong.

The only issue I would bring up is that 54 cell panels are a bit on the non-standard side. If you ever want to add/replace panels, you would probably have to find exactly that brand/model. In general, it appears that solar panels have a short production life (maybe 1.5 to 2 years). If you needed more/replacement panels, you would probably have to get a second charge controller to run in parallel with the first and use different Vmp/Imp rated panels. Nothing wrong to run solar charge controllers in parallel, other than the added expense/space needed for a second controller.

Those panels only have a 10 year warranty--Not long when compared to other "glass based" solar panels (typically 20+ years).

-Bill

Boatman · June 2016

BB: - Thanks for the helpful response. I understand your point and am reluctant to build a system with oddball panels.Should have dug a little deeper for the data sheet and not relied on the distributors information.

I can substitute 4 @ 100 watt panels in the space where the 2 Sonali 200 Watt 54 cell panels were going to go. All of the 100 watt panels I'm finding are 36 cell. Voltages are similar among them. For the Renogy 100 Watt panels, for example, the specs (from Renogy), look like this:

Optimum Operating Voltage (Vmp): 18.9V
Open-Circuit Voltage (Voc): 22.5V

If I had 4 of these panels and ran two panels in series, then both pairs in parallel with the LG 60 cell panel ( Vmp 30.4, Voc 37.6) to the MPPT, would that work well without wasting charge? I tried to find a 72 cell panel that would fit but only 60 cell panels fit in the smaller of my 2 mounting spaces. The LG 60 panel is a newer, high output mono panel. Will it's voltage allow it to coexist with the 4 100 watt 36 cell panels?

Thanks for the help...Boatman

BB. · June 2016

No--Not really,,, The "12v panels" (Vmp~18 volts) would 2x series be Vmp~36 volts. That does not "play well" well with the LG panel(s) of Vmp~30 volts (60 cell panels) driving a single MPPT charge controller (you have 2-3 less than optimum Vmp-array voltages--And no guarantee which voltage will be selected by the MPPT, and none of the voltages selected will maximize the array output). It would take 2x charge controllers (one for the Vmp~36v and a second for the Vmp~30v panels) both charging the same 12 volt battery bank.

Paralleled charge controllers are OK (just be aware that they will not be "in sync" when transitioning from bulk to absorb to float--But that does not really matter. You can also purchase a pair of controllers that can "sync" if that bothers you).

-Bill

Boatman · June 2016

BB - Thanks so much for hanging in there with me on this design. Since I can't fit a 72 cell panel in the smaller area I have on my boat, it looks like I've got to change my design or wind up with two MPPT controllers which will raise cost and complexity and be tough to layout since I'm space constrained inside as well.

Here's the new plan - 4 standard 100 watt mono panels and 3 smaller 100 watt mono panels from AM solar. They are 6 inches shorter (41") than a standard 100 watt panel (47"). The shorter panels are a little pricier, but will fit in my constrained space.

This design will leave me with 7 36 cell, 18 volt panels so all should be able to be wired in parallel to a good MPPT controller.

Any advice on this new design? Any opinions on the quality of the AM Solar panels?

Thanks...Boatman

BB. · June 2016

Yes, there are tradeoffs in any design.

Marine is tough on any metal/electrical stuff.

MorningStar does conformal coat some (most/all?) of their controllers. Can help.

In some ways, buy "cheap/decent" then plan on replacing every 2-5 years vs $$$ and drop a wrench through a 20 year panel in 6 months.

No perfect answers.

-Bill

vtmaps · June 2016

Boatman said:
This design will leave me with 7 36 cell, 18 volt panels so all should be able to be wired in parallel to a good MPPT controller.

I think you might be better off with a PWM controller. MPPT controllers need headroom, i.e. the PV input voltage needs to be higher than the battery charging voltage by several volts. A PWM needs much less headroom. When you panels are hot their voltage may be lower than the voltage you need to charge the battery. If you want an MPPT controller for a 12 volt system, you should have your panels in strings of two... not possible with 7 panels.

--vtMaps

Boatman · June 2016

BB and Vtmaps:

I want to thank both of you for hanging in there with a novice and helping me through my design learning proces. It is much appreciated. Rather than play mix and match and winding up with voltage difference issues, I decided to buy a pair of Panasonic 325 watt 96 cell panels. This configuration (650 watts) gets me close to my original design in output, but lets me run the panels in parallel to a single MPPT controller with no losses from voltage mismatches. As a byproduct, I'll be running at significantly higher voltage so hopefully less transmission loss.

This prompts my next question - how big an MPPT controller do I need? Using the common reduction factor, I'm thinking a 45 amp controller should be sufficient.

Am I correct?

Thanks so much...Boatman.

vtmaps · June 2016

There are two factors to consider. First the wiring and circuit breaker: 650 watts of panels ÷ 14.4 volts (battery charging voltage) = 45 amps. 45 amps X 125% = 56 amps. Your wires (and breaker) from controller to battery must be able to handle that current. The panels should not be able to trip the breaker (that's why the 125%). The breaker serves as a switch. It is important though, because if the controller faults it could be a short circuit across the battery.

EDIT: this is wrong calculation, see correction.

The second factor to consider is your energy budget: 650 watts X 77% = 500.5 watts. The 77% is because when the panels are hot their output declines. The 77% also includes controller losses. Thus you should figure on 500 watts in real world conditions.

A PWM controller will be damaged if you exceed its current (amps) limit. An MPPT controller can be overpaneled a bit....
500 watts (real world output) ÷ 14.4 volts (battery charging voltage) = 34.7 amps. Thus you could get by with a 35 amp MPPT controller and it would clip any power above its 35 amp capacity. Much better to get the 45 amp controller. It will run cooler and harvest any power above 500 watts.

--vtMaps

Dave Angelini · June 2016

I use quite a few of the Panasonic panels lately. Really nice for what I do.
For you, the max power voltage of 58 volts is going to have conversion loss down at a 12V nominal system. Not a terrible amount but it is there. Also, if you are cruising and lose a CC, this panel will be hard to jury rig so you will need a spare CC. I think I would keep looking for panels at a lower MPV and closer to the panels that normally are used with 12V systems.
My boat had a 24V house bank for that reason BTW.

Image: https://us.v-cdn.net/6024911/uploads/editor/1k/bhm1a58ts5r2.png

vtmaps · June 2016

vtmaps said:

There are two factors to consider. First the wiring and circuit breaker: 650 watts of panels ÷ 14.4 volts (battery charging voltage) = 45 amps. 45 amps X 125% = 56 amps. Your wires (and breaker) from controller to battery must be able to handle that current.

Just reread this thread and realize that I made a mistake. You must assume that it's cloudy all morning and that at noon the sky clears and you have 650 watts going into a discharged battery.

Thus:
650 watts of panels ÷ 11.5 volts (discharged battery voltage) = 56.5 amps. 56.5 amps X 125% = 70.6 amps. Your wires (and breaker) from controller to battery must be able to handle that current.

--vtMaps

Boatman · June 2016

vtmaps - Understood. From what I can gather, I'm thinking that an Outback FM60 is probably my best bet. I will wire, switch and breaker accordingly. Should be easy since the controller will be less than 4 feet from the battery bank.

Dave - I'm stuck with an area (the hardtop over my cat's cockpit) that no 72 cell panels will fit. It's either a more standard 60 cell panel or the Panasonic 96 cell panel. To maximize my capacity, the larger mounting area (over my dinghy arch) I can fit either a 72 cell panel or, again, the 96 cell Panasonic panel. So, either I run a 60 cell and a 72 cell panel with two CCs, or I run two Panasonic 96 cell panels. Good call about a spare CC. Being a cautious and prepared coastal cruiser, I was thinking about getting a standard PWM CC for backup and leaving it in a box in a locker for a rainy (or, in this case, sunny) day.

Thanks...Boatman

vtmaps · June 2016

Boatman said:

I'm thinking that an Outback FM60 is probably my best bet.

I like my FM60, but I'm not on a boat. The FM60 has a fan which pulls corrosive marine air into the unit... Have you considered any of the sealed controllers?

How about putting one of these on each of your panels? It costs more than a single FM60, but you did say you wanted some redundancy.

--vtMaps

Dave Angelini · June 2016

If you get hit with lightning there will be no way to use those panels and if you know this then at least you can plan.
The Schneider XW CC is conformal coated, has no fan, and is available world wide!

Vic · June 2016

Dave Angelini said:

If you get hit with lightning there will be no way to use those panels and if you know this then at least you can plan.
The Schneider XW CC is conformal coated, has no fan, and is available world wide!

Did look at the spec sheet and the Manual for the XW SCC, and saw nothing about conformal coating ...

Am with vtmaps, the MidNite KID, is Certified to UL 1741 & UL 458, comes in a Marine version, has no fan, but is a much more modern CC than the dated XW SCC (IMO).
http://www.midnitesolar.com/pages/kid/index.php

FWIW, Vic

Dave Angelini · June 2016

Always been conformal coated. I really have not had to take many apart because they just keep on working, well one that water leaked on from a solar hot water system. They have a very nice scroll of all the main things a non technical crew member would look for on a boat. No buttons to push as it just scrolls the functions. and the only charger that I know of that is more efficient is the Morningstar, also a good choice on a boat.

Boatman · June 2016

Vtmaps - Excellent suggestion. I checked into the specs and having 2 small units like that instead of one larger one mkes sense on a lot of levels:

1. Redundancy. If one goes down, the second unit will still give me half capacity.
2. Midnite Kids are much smaller and easier for me to install with my limited available surface mount area.
3. Seem to have a terrific reputation and have coated boards - good for a marine environment.
4. Not much more money than one larger one.
5. Allows me flexibility to upgrade to a different panel if more efficient ones come along that would only fit one of my mounting areas.

Going to order two Midnite Kids today.

Thanks for all the help and assistance.

Boatman.

littleharbor2 · June 2016

Boatman said:

BB - Thanks so much for hanging in there with me on this design. Since I can't fit a 72 cell panel in the smaller area I have on my boat, it looks like I've got to change my design or wind up with two MPPT controllers which will raise cost and complexity and be tough to layout since I'm space constrained inside as well.

Thanks...Boatman

A little late chiming in here but wanted to mention, The older gen 72 cell modules use smaller cells than todays 300+ mods do. The typical 165/190 watt Sharp, Schott, Solar Tech, etc. modules are in the 32" x 65" size range. Might work for your fitment situation.

z0xal · July 2016

Boatman have you considered bendable thin 100w or 50w panels for any areas where you can't fix on a flat one. They can be walked on and bent upto 30deg if you could get two 50s on the cat and you're using multiple controllers anyway this would bring you up to where you initially intended to be watts wise

z0xal · July 2016

Also if you leave the boat unattended and are worried about overcharging you can use a 12v timer to run your fridge during the early afternoon keep it fresh even if it's just got some bottles in it saves you cleaning and propping open

SOMF · September 2016

Wow, quite a bit to consider. I have a similar situation, my boat currently has 3 135w Kyocera panels with the following specs Vmp17.7 Imp7.63 Voc22.1 Isc8.37 I want to add 2 smaller panels in to the mix Everbright 80w specs Vmp17.2 Imp4.65 Voc21.6 Isc5.17. The plan is to run all panels parallel into an Outback FM60 feeding 4 Trojan L16h batteries totaling 870ah. Will the FM60 handle the variances in the panels? I already have all of the above and will be heading South in a few weeks to start putting it together so if this won't work the alternative is to make some other cruiser happy with a couple of 80w panels for sale. If it will work what would be the recommended circuit breaker, I believe it's 25 amp currently.

Designing a space-constrained PV system for a boat. Advice needed.

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