24vdc Solar to 12vdc Battery Bank

[Pelagic]

Originally Posted by Windsun

Not all 24 volt panels are really 24 volt panels

Look at the spec sheet, a true 24 volt panel will have a Vmp of around 34 volts. A lot of the "24 volt" panels are made for grid tie systems, and are actually more like a 23 volt panel.

And a lot of the higher voltage panels are much larger physically, which can be an issue on a boat.

Ok, this is where I start to get confused, since I have never installed a solar system, so have no first hand experience.

On Star Gazer, my onboard DC system is 24v with 1040AH of AGM Fullriver Batteries (8 x 260ah) for my 24V House Bank.

What size Voltage solar panels would be best suited to charge those batteries?

[bill good]

I run 80volts @ 5Amps into a MX60. 2x 200w panels as used on house installations. works well (when there is sun!!). I expect about 50% with panels mounted flat.

[Seahunter]

Originally Posted by Pelagic

Ok, this is where I start to get confused, since I have never installed a solar system, so have no first hand experience.

On Star Gazer, my onboard DC system is 24v with 1040AH of AGM Fullriver Batteries (8 x 260ah) for my 24V House Bank.

What size Voltage solar panels would be best suited to charge those batteries?

It doesn't matter as long as it adds up to 24V in the end. Example: Using 2- 12V panels in series gives you 24V or 2 - 12V in parallel gives you 12V; WHAT changes? Answer: WATTS output. My choice use 2- 24V panels in series giving you 48V. Why doesn't it matter what voltage; because an MPPT charge controller does all the work. Remember that your AGM's can quick charge at up to 29V. If your house battery system was 48V your charging voltage could be as high as 62V. This voltage is easier for the inverter to convert ergo less heat/energy loss. Again, this is why it's important to have a good charge controller, because it can divert incoming solar voltage directly to the inverter for immediate AC loads after the batteries have been charged with little loss in the system.
For your system; I would leave the batteries alone and install 4 Sharp ND-224U1F panels in series with a half decent MPPT charge controller EG Outback Power Flex Max 60 or Xantrex XW SCC. By the way these links are for example only; I'm not affiliated in any way with this company.
Solar systems have become very sophisticated in just a few years. As far as I can see and experienced these "on dash 12V" type of systems with cracker-jack charge controllers are weak and prone to failure and in the end can compromise the whole electrical system.

[senormechanico]

Originally Posted by senormechanico

Even with so called 12 volt panels, a MPPT controller will help.

I'm in the middle of adding an MPPT controller and a 120 watt panel to my 2 existing 32 watt flexible panels.

So far, I have added the MPPT controller (I don't live aboard) and even though it's January, the MPPT Bluesky controller goes into the MPPT phase during the day.

I just got the 12 volt 120 watt panel mounted this afternoon, and will get it hooked up in a day or so. I'll post the results.

t

With batteries mostly charged, but down a little I got almost 13 amps from 184 watt rated solar array.

Without the MPPT, I got about 9 1/2 amps under the same conditions.
Definitely worth adding that MPPT !

[btrayfors]

The typical "12V solar panel" actually puts out more voltage when not under load. Usually, this runs around 17 volts. A 24V panel will put out 34 volts or so.

In many/most installations, it makes good sense to arrange solar panels in series rather than in parallel, in order to increase the voltage output from the panels and to reduce the size of needed wiring.

Let's say you have two 12V panels which actually put out 17 volts each, open circuit (i.e., no load). Hook these up in series and you have 34 volts open circuit.

If you take these 34 volts and feed them to a competent MPPT controller, it will make very good use of all the volts/amps coming it's way on the input circuit, and will output the needed 13.2 - 14.4 volts needed for charging your 12V battery system.

MPPT controllers help make the best use of all available energy, rather than casting off some as "excessive".

The "best practice", then, is to:

1. hook up your panels in series, not parallel; and
2. use a suitable MPPT controller which can convert the incoming volts/amps to the needed level for charging your house batteries.

Bill

[hellosailor]

"What size Voltage solar panels would be best suited to charge those batteries?"
Not "size voltage", just "voltage". If you want to charge a nominal 24V battery, you need about 29 volts. A "nominal 24v" solar panel would be rated to put out 24V when under load, and would probably put out ~35 under no load. Since there are apparently "24v" panels meant to really just put out 24V into grid arrays, you really need to check the output voltage rating of your panels, to make sure they will put out MORE than 24V so you can charge a 24V battery.
Every panel has a "nominal" output voltage rating, and a maximum output rating. Any good vendor should be supplying you with all that information. If they don't ot can't...walk away.

[JayH]

Originally Posted by Pelagic

On Star Gazer, my onboard DC system is 24v with 1040AH of AGM Fullriver Batteries (8 x 260ah) for my 24V House Bank.

What size Voltage solar panels would be best suited to charge those batteries?

I used 24V nominal panels to charge my 24V house bank. My goal was to avoid having panels in series since this makes their output more susceptible to shading or failure of any single panel. I didn't want to go higher than 24V nominal since the panels would then be useless for charging if the MPPT died. You can, of course, make it work any number of ways.

[Pelagic]

Thanks guys for your input, without first hand user experience or a local supplier it is hard for me to prioritize the solar issues for a Star Gazer installation.

Here is my situation:

• Being schooner rigged my aft boom extends right to the tender davits
• To minimize shading I plan to build a solar frame above the strong davits as I show in yellow
• I will have to import everything, so totally open to equipment suggestions
• The usable area above the davits is approx: 2.3m (w) x 1.8m (l)
• I would like to keep the SSB whip antenna where it is because it works so well.
• Based in the Philippines where temperatures are high

I know everything is a compromise, but would appreciate some kind of consensus from users on….What should be my Priorities for an SG installation?

1/….Probable Shading: Have read pros and cons over parallel or series hook up re shading or maximizing Voltage for series.

In my situation which way would you wire?

Would that also influence your Panel selection and MPPT?

2/….Temperature: Some panels have a better Temperature co-efficient, should that be a priority for the Philippines?

3/….Output Voltage: (Thanks Hellosailor and Seahunter) My AGM’s float at 28.9 volts so having that minimum voltage while under load condition is preferred…Right???

Just to clarify using Seahunter’s Sharp 24v panel example… when it states 224 Watts producing 8.33A that is at 36.6 volts?? or 24volts??

Is JayH’s point about choosing 24v “nominal” output so as to have redundancy in case of MPPT failure a practical priority in the big picture of maximizing reward?

4/…Equipment suggestions: I have no problem receiving buying advice from others as I will not get a chance to touch or see them till they arrive.

Big investment so Salesmen are also welcome to pipe in…. so

If you were doing Stargazer what specific equipment would you recommend for panels and controllers etc…

Thanks again for any advice

[JayH]

Originally Posted by Pelagic

Is JayH’s point about choosing 24v “nominal” output so as to have redundancy in case of MPPT failure a practical priority in the big picture of maximizing reward?

Well, if you're concerned about redundancy, it is probably most practical to just carry a spare MPPT.

However, I had never previously heard that higher voltages maximized reward. For the MPPT to work you need the panel's temperature compensated Vmp to be higher than your battery's charge voltage. This should be the case for most panels, which is why the voltages are referred to as "nominal". Other than that, I don't think that an MPPT controller can do more with 500W @ 48V than it can 500W @ 24V.

[Seahunter]

Originally Posted by Pelagic

• Being schooner rigged my aft boom extends right to the tender davits
• To minimize shading I plan to build a solar frame above the strong davits as I show in yellow
• I will have to import everything, so totally open to equipment suggestions
• The usable area above the davits is approx: 2.3m (w) x 1.8m (l)
• I would like to keep the SSB whip antenna where it is because it works so well.
• Based in the Philippines where temperatures are high

Based on your size requirement you could install 2 Sharps there.

Originally Posted by Pelagic

1/….Probable Shading: Have read pros and cons over parallel or series hook up re shading or maximizing Voltage for series.

In my situation which way would you wire?

Would that also influence your Panel selection and MPPT?

Shading isn't an issue; I would still wire in series. Panels actually work better with a little shade. A decent MPPT will adjust for shade and temp as well as voltage.

Originally Posted by Pelagic

3/….Output Voltage: (Thanks Hellosailor and Seahunter) My AGM’s float at 28.9 volts so having that minimum voltage while under load condition is preferred…Right???

Floating is just one stage of the process and means exactly what it says. Charging volts can be much higher on AGM's. Depending on the manufacturer (check your battery manual) http://www.fullriver.com/products/ad...ile/charge.pdf based on this chart it's actually 29.4V in absorb.

Originally Posted by Pelagic

Just to clarify using Seahunter’s Sharp 24v panel example… when it states 224 Watts producing 8.33A that is at 36.6 volts?? or 24volts??

36.6V is the max output for this 24V panel. Output/input voltage is managed by the MPPT. I prefer the Outback one because there is good control over the settings.

Originally Posted by Pelagic

Is JayH’s point about choosing 24v “nominal” output so as to have redundancy in case of MPPT failure a practical priority in the big picture of maximizing reward?

The MPPT doesn't care what voltage (that's why I recommended the previously noted controllers) your input voltage will be selectable. However, your inverter will work a lot better with less line loss if closer to the inverted voltage.

Most people think you can install solar on a whim and a shoestring budget. In order to install a low maintenance system, without screwing up your batteries and receiving a long term cost benefit addition to almost any boat, is going to cost about US$2000.00 plus labor, but realistically, probably closer to $4000.00.

[MoonlightSailor]

Originally Posted by Windsun

The Blue Sky controllers are analog MPPT, nearly all others such as Outback Power, Xantrex, and Morningstar are digital. In general, the digital controllers track and convert the power coming from the solar panels better and more efficiently under less than perfect conditions. On the other hand, analog is cheaper and easier to manufacture.

I knew there had to be a reason for the difference in price! Seems the Blue Sky units have external panels too (optional). Personall, I like eveything in one unit.

[hellosailor]

"However, I had never previously heard that higher voltages maximized reward."

Because MPPT controllers use the magic of pulsed DC, which is essentially AC, through a transformer or charge capacitor, apparently some or all of the designs DO get better efficiency as you raise the input voltage. IIRC Blue Sky mentions that going from 12VDC input to 24VDC can give you a 4% gain in efficiency. That may not sound like much--but 4% here, 4% there...pretty soon you can find 20-25% gains if you tweak a system, and that counts! (Gain 10% just by angling the panels back toward the sun every hour, they lose that much for every 15 degrees they are off, and that's the shift of the sun over one hour.)

"when it states 224 Watts producing 8.33A that is at "
The numbers are based on Ohm's Law. Volts x Amps = Watts. So, 224 Watts divided by 8.33 Amps, is based on 26.89 volts coming out of the panels, under load.

[cal40john]

Originally Posted by Seahunter

Based on your size requirement you could install 2 Sharps there.

Shading isn't an issue; I would still wire in series. Panels actually work better with a little shade. A decent MPPT will adjust for shade and temp as well as voltage.

Can you explain this better or source it? This is opposite to every thing that I've read. A cooler panel puts out more power, but even a partially shaded panel becomes a high resistance. If the panels are in series the high resistance is in the entire path reducing all the panels output. An MPPT controller cannot fix that.

I would think that if you're going to have shading problems it would be better to at least have one panel putting out full power by having them in parallel rather than having the entire output reduced by having them in series.

John

[hellosailor]

An MPPT controller won't "fix" anything, it will just optimize whatever power is able to get through a system.

[cal40john]

Originally Posted by hellosailor

An MPPT controller won't "fix" anything, it will just optimize whatever power is able to get through a system.

The statement made was "Shading isn't an issue; I would still wire in series. Panels actually work better with a little shade. A decent MPPT will adjust for shade and temp as well as voltage." This says to me that an MPPT controller will "fix" or put as much energy into the batteries whether the panels are wired in series or parallel.

The point I was trying to make was that a shaded panel puts out very little power, an MPPT optimizing, gaining 3-5% on panels that are putting out 50% or less of their power because they're in series is a poor return when you could get 100% out of one panel and the 50% or less out of the other panel, so maybe 75% of the total power if they were in parallel.


Wiring two 36 cell modules in series is equivalent to a single 72 cell module. Shading one cell reduces the power output of the entire 72 cell module.

From Kyocera:

Kyocera | Products | Kyocera Solar Energy | Understanding Solar | Modules

Partial-shading even one cell of a 36-cell module, such as the KC120, will reduce its power output. Because all cells are connected in a series string, the weakest cell will bring the others down to its reduced power level. Therefore, whether ½ of one cell is shaded, or ½ a row of cells is shaded as shown above, the power decrease will be the same and proportional to the percentage of area shaded, in this case 50%.

When a full cell is shaded, it can act as a consumer of energy produced by the remainder of the cells, and trigger the module to protect itself .The module will route the power around that series string. If even one full cell in a series string is shaded, as seen on the right, it will likely cause the module to reduce its power level to ½ of its full available value. If a row of cells at the bottom of a module is fully shaded the power output may drop to zero. The best way to avoid a drop in output power is to avoid shading whenever possible.