reasons to use mppt

System · December 2006

i am currently constructing a catamaran on which to extensively cruise the australian coast, as such i wish to be relatively independent of the land and am intending on relying on 4 90w solar panels, i plan on having 400ah of battery and have a 2kw generator to charge batteries and maybe run a small airconditioner?? as i live in the tropics, i have been looking into mppt regulators for a few of the following reasons, i know they cost a bit more however

- if all i gain from the mppt is 5% extra power a day that is 18 w a day, now while this doesnt seem much over a week that is 136w a week which to me might mean the difference between running the generator every week instead of every 3-4 days
-i can run my panels at a higher voltage using the mppt to step it back down to 12v reducing cable size and voltage loss to the controller
-the cost of extra potential energy harvested over a few years would i feel completely offset the extra couple of hundred for an mppt controller
- there is not really a lot more room to put extra panels on the cat so i needo maximize what i have
-i have heard that mppt can really benefit you early in the morning when you the maximum bulk charge to recharge batteries

now firstly is there any problem with my reasoning??, is there a better way?, are there any reasons i should not go with mppt?
is there anything better than mppt out there?
also the 2 mppt regs i like are the solar boost 3024, i like this one as i can use the load diversion function when the batteries are recharged
and the mx60 as i can have all my panels in series and down convert to 12v utilising much smaller cable runs

sean

crewzer · December 2006

Re: reasons to use mppt

Sean,

I’m a big fan of MPPT controllers. While the DC-to-DC conversion feature might be useful in your case, I don’t think the MPPT function will be of significant value to you, if any at all.

The fundamental benefit of MPPT is that is can convert “extra” PV module voltage into additional charging current. If there’s lots of extra voltage, lots of extra current and several hours available, you can indeed harvest some additional energy if the controller is very efficient.

But, when there’s little extra current (i.e., in the morning due to low Sun angle), little extra voltage (i.e., mid-day when the PV modules are hot and their operating voltage drops), little time of optimal conditions, and/or the MPPT controller’s low-power efficiency is but 90% or 95%, then you won’t see much net gain – if any at all.

In other words, MPPT is at its best in large systems in cold weather.

Cost benefit is another factor. For example, compare the cost of a Morningstar Prostar 30M PWM controller (correction from previous 15M suggestion, which is too small for a 360 W 12 V array) against the prices for the controllers you mentioned. You might instead consider using the price difference to purchase a larger PV array.

Also, note that most of the MPPT benefit is realized (pardon the US spelling) when the controller is operating in the Bulk (MPPT) stage. Regardless of environmental conditions, there’s rarely any MPPT benefit while the controller is operating in Absorb- or Float modes, as those two stages both limit charge current.

The DC-to-DC conversion feature is another story. Wiring the modules for “24 V” to charge the “12 V” battery bank can be helpful in hot weather when the PV modules’ voltage is low, and it can simplify wiring between the PV array and the controller.

However, series-wiring of PV modules on a sailboat may be problematic if when shadows from the mast(s) and rigging falls on a module. This will not only reduce the output of the shaded module, but it may also reduce the output of any other modules wired in series.

For an array rated in the 360 W (STC) range, I’d recommend wiring the modules in parallel for 12 V and using a 12 V PWM controller. Look for modules with an STC Vmp rating of at least 17.2 V, and you should be good to go.

HTH,
Jim / crewzer

BB. · December 2006

Re: reasons to use mppt

And if it is windy and/or you can afford the ~50lb drag while sailing, a Wind/Water generator may be very helpful too.

You may be pretty disappointed in Solar PV Panels on a sailboat unless you can get full sun with good angle. pointed north (in your case), and virtually no shading with rigging/sails/railings. Have problems with any of those, and you can probably kiss 50-75% of your expected Solar PV power away. That is why I suggest the wind/water turbine as a more reliable alternative.

Assuming that you will only get 360 WattHours per day (1 solar hour per day instead of 2-5 hours per day) is a good start at realistic expectations--but it could be lower.

-Bill

System2 · December 2006

Re: reasons to use mppt

i am assuming that i will pick up some extra from the mppt eventhough i live in the tropics as a friend conducted an experiment, he had a a kyocera 85 that had a 17.9v max voltage, he placed it out on the lawn at midday on a 35 c 65% humidity he was still seeing 16.1 volts out of the panel, now this is the hot part of the year. now the panels that i have are sunpower monocrystalline high efficiency panels these are supposed to be slightly better at operating in higher temps than multicrystalline panels, now as i said earlier if all i get is a 5% advantage i would be stoked this would mean that when my panels are running in the hot part of the day i would only need to see about 15.7 volts if i include the effieciency loss from the dc to dc conversion 14.4v *1.09 =15.7v (1.09 = 5%gain plus 1% voltage loss from wiring plus 3% loss from running solarboost 3024 at approx 24a (claimed 97% effieicency at this amperage) )
sean

BB. · December 2006

Re: reasons to use mppt

Sean,

I guess you are also getting some answers over here too:

http://www.cruisersforum.com/forums/showthread.php?t=4760

The Blue Sky controllers seem to be a good controller--good charger and features. On Wind-Sun's web store, they also like Blue Sky too...

http://store.solar-electric.com/chco.html (go to Blue Sky link, and also "power tracking controllers" link)

Normally, I would like to see solar panels in series because with high voltage series strings, the current is kept low--allowing smaller voltage drops and better matching of panel IV curves with battery requirements via a MPPT charge controller...

However--I would think twice about it here... My suggestion is to "home run" each of the solar panels back to an accessible junction box and parallel connect them there. Because:

1. On an open water boat, you want redundancy. If a clip, crank, or something else falls and cracks a panel, you can simply disconnect that one panel from the J Box and still get full power from the other panels.

2. Weight (and to a lesser degree) cost is not an issue here (amount of extra wire for this small system is not going to weigh much or cost that much more)

3. A question that you asked on cruisersforum about how does shading affect solar panels and MPPT controllers... What happens with shading.

Looking at the panels themselves, each silicon wafer is (usually) in series with every other wafer. Shading one wafer will drop will drop the solar panel's output voltage by about 1/2 volt... Shading two wafers, by 1 volt, etc. There are usually about 30-36 series silicon cells in a typical 12 VDC panel (the 30 cell panels are usually intended to connect directly to a 12 vdc lead acid battery, the 36 cell panels are intended to use a charge controller so that batteries are not boiled dry).

And according to this link:

http://www.solarmodules.com.au/solar.php

When a cell is shaded, it behaves as if it is an open circuit (like an open switch or wire)... So the entire current output from that panel is effectively turned off... If you had two panels in series, there are bypass diodes (typically two per panel--per above link)... Which would either allow the shaded cell/panel to pass current (or depending on how the diodes are wired), you might get 1/2 voltage from the blocked panel supplied by the other unshaded cells... So, if you have 2x12v=24 volt series panel system, one shaded cell would reduce the overall panel voltages by 6-12 volts (from 24 volts nominal).

If you have 2x 12 volt panels in parallel, blocking one cell would possibly knock 50% of your solar power off line...

Since you have solar panels available for testing--it would probably be very interesting if you can test the actual results of blocking one or more cells (both lengthwise and widthwise) on a solar panel... Worst case would be that fully blocking one cell (with a piece of foil or wood) would cut the current to zero, or possibly the current would remain the same, but the output voltage would drop by 6 volts.

You could also try some tests with lines (ropes) and wood used for rigging your sail boat (and simulate a mast arm shadow) and see how the the panels would react. My panels are on top of my two story home, so I am not too excited to test that myself here...

From another article:

http://europe.elecdesign.com/Articles/ArticleID/9725/9725.html

The voltage output from a single crystalline silicon solar cell is about 0.5 V. Current is directly proportional to the cell's surface area, and designers count on about 7-A maximum output for a 6-in.2 multicrystalline cell. The typical panel module has 30 to 36 cells that are wired in series to generate a nominal 12-V output, which is actually about 17 V at peak power.

Because the cells are connected in series, and cells that are partially or completely shaded by tree branches, chimneys, or even guy wires have a high internal resistance, shading is a problem for photovoltaic modules. If even one full cell is completely shaded, the output voltage of the array will drop to half of its unshaded value to protect itself. If it didn't, the cell would be destroyed by the need to carry the current produced by the rest of the array while in its high-resistance state. If enough cells are shaded by nearby objects that throw hard-edge shadows, the whole module will drop out.

To deal with the shading problem, some modules place bypass diodes across each cell to carry the current when the cells themselves cannot. Otherwise, they would dissipate power from the unshaded cells. A separate blocking diode in each array in a parallel configuration of modules could isolate that panel if it becomes severely shaded. It would prevent other panels' current from flowing through it and being turned into heat.

Well, if there are two bypass diodes in a 2x17vx2.5amp=85watt panels, you are charging a 12 volt battery bank and have two "~17volt" solar panels, and want to generate the most power with sometimes shading of one cell, then two panels in series would generate 17+8.5 volts at rated current of 2.5 amps = 64 watts... Otherwise, with two panels in parallel (standard or MPPT controller), one blocked cell would drop the parallel panel's output to 8.5 volts (useless for charging a 12 volt battery), you would have one panel at 17*2.5=42.5 watts.

With the above paper comparison, a 24 volt string of two panels would seem to be better than a 12 volt parallel string (using a one blocked cell simulation)... But again, I would test it with a real setup of solar panels with real life shadows (in differing orientations) and a controller/battery/load and see what happens in real life for your installation.

In general though, I would not have much hope for good solar PV outputs with hard-edged shadows from lines and masts putting out power at anywhere near rated capacity...

-Bill

Brock · December 2006

Re: reasons to use mppt

I would third the idea of running each panel back to a single junction box and also suggest wiring for 12v instead of 24v for many of the same reasons. Mostly for me is the ability to use three of the panels to get 75% if one was damaged or some how blocked, instead of 50% with only one panel blocked.

Also even if you do wire them all back to a single box you could do a real world test, say run 24v for a week and then try 12v for a week and see if you get more power one way or another, it would be an interesting test.

But I would suggest a MPPT controller. I have a similar situation where I can only get 4 KC 120 panels where I am right now without cutting down my neighbors trees. So the gain I get from the MPPT is worth it. There is also a gadget factor I hadn’t counted on to boot

BB. · December 2006

Re: reasons to use mppt

One other comment I saw from, IIRC, Solar Guppy's forum (or it may have been posted here--but I think Guppy's posts are gone)... And I will probably never find the exact quote to link...

http://www.solar-guppy.com/forum/

But I remember it something along the lines that many MPPT controllers only calculate the MPPT Voltage/current point every 15 minutes (not continuously). And other controllers (it seems) just used a table to set the MPPT voltage. I believe the exact comment involved a very popular brand MPPT controller (don't remember for sure--so I deleted the brand/model number) checking once every 15 minutes and if the check occurred when a cloud passed in front of the solar panel, MPPT measured, then passed, the controller would be operating at a non-optimum MPPT value for several hand fulls of minutes.

So, having a MPPT controller with dynamically changing shadows (like on a sail boat), it may end up not optimally collecting power when compared to a simple 17 volt panel and PWM charge controller.

In any case, testing how the shadows affect the power output over time (and or testing wiring on the boat by switching between series and parallel connections) would be helpful...

You could ask on Solar Guppy's forum about his recommendations for MPPT controllers and wiring with shadows on a sail boat... He is a solar controller design engineer (currently at Xantrex, I believe) and has tested many different models and brands of controllers for performance.

-Bill

mike95490 · December 2006

Re: reasons to use mppt

BB wrote:

You could also try some tests with lines (ropes) and wood used for rigging your sail boat (and simulate a mast arm shadow) and see how the the panels would react. My panels are on top of my two story home, so I am not too excited to test that myself here...

I have a 2 story house, and a high tension line to the east of me. The wires are at least 60 feet above me
(see photo on my web page) and I can plot the drop in solar output, as the 3 shadows traverse my array for about a half hour daily. Creates about a 800W drop in a 3,500W array. Could be fatal on a sailboat with stays and masts.
Hoist a little more sail, and tow a generator pod. (although to be fair, I also heard they don't work too well)
Maybe a wind gennie on the top, or maybe something can be rigged to catch air as it leaves the sail, and spin a generator. Any chance of high rigging a couple of panels on top? (doubtfull)

System2 · December 2006

Re: reasons to use mppt

actually i wasnt getting any real anwers at the other forum but a friend pointed me to this one, what i really want is understanding as to how these things work so i dont make silly mistakes and so i can set it up and repair it myself!!!
but please keep the info coming in it is great!!!
another question is it possible or wise to put all my panels into series but put a blocking diode in between all the panels so that where a panel loses current it shouldnt affect the other panels,
sean

BB. · December 2006

Re: reasons to use mppt

Yes, you can place all of the panels in series, up to the rating of the solar controller... The Outback MX60 controller is rated to 135 volts operational and 150 volts never to exceed.

The standard solar panels already will have bypass diodes installed (you do not need blocking diodes)... Check the specs for the panels you have and/or are planning on using. Also check the Vmp and Voc voltages too and ensure that you don't exceed the specs. of the controller.

Also, check the MX60 efficiencies at the lower power ratings... IIRC, it has about a 10 watt tare (overhead power consumption), so it will never be 97% efficient with smaller panels.

-Bill

System2 · December 2006

Re: reasons to use mppt

sory i meant bypass diodes not blocking, i was thinking these as someone mentioned earlier that if you lost one panels out put you would effectively lose all as they are in series now this would not be a cool thing so i was wondering whether it was possible to put a bypass diode in between each of the panels so that if you do lose production in onme panel you dont lose all
sean

crewzer · December 2006

Re: reasons to use mppt

Kyocera operational values are +10%/-5% of spec. All PV modules degrade with age.

Module voltages can read high when the controller is in the Absorb” or “Float” modes.

You could indeed wire four 90 W “12 V” PV modules in series and connect it to a controller like the MX60. In fact, it’ll easily handle five “12 V” modules wired in series, and perhaps even six in your warm-to-hot environment.

The “suspend operation” spec for the MX60 has been increased from 135 V to 140 V.

I tested my MX60 last year (using separate in- and out shunts) and found it to be 92% efficient when handling ~600 W and converting “36 V” to”12 V”. The efficiency will decrease at lower input power and/or higher input voltage.

Bypass diodes will help with a modules wired in a single series string, and they may already be installed on your modules. For example, my Kyocera 120 W modules were factory equipped with three bypass diodes wired in parallel for each of the modules’ “6 V” sections.

The Sunpower SPR 90 has a rated Vmp of 17.7 V and a power temperature coefficient of -0.38%/C. Allowing for a 35 C operating environment and a cell temp increase of at least 30 C above ambient in full Sun, the specs indicate the module’s power spec (ref 25 C cell temp, not ambient temp) would be derated by ~15% to ~76.5 W, suggesting an operational Vmp of ~15 V. However, the module’s output voltage would increase if cooled by a stiff breeze.

http://www.sunpowercorp.com/pdf/SPR-90.pdf

No matter which controller you buy, make sure it’s circuit boards are “conformal coated” to protect the board from the marine environment.

Good Luck!
Jim / crewzer

System2 · December 2006

Re: reasons to use mppt

is there any way of finding out whether my panels have a few bypass diodes installed ??? and thanks for the confirmation that i could use bypass diodes, would this be a good idea? or just a waste of time? when you talk about conformal coating is this just where they resin coat the circuit boards as i have heard that you can get this done for about 50$
sean

System2 · December 2006

Re: reasons to use mppt

also where it has the temp coeffiecient of -.38%/degrees celsius does this mean that for every 1 degree that the panel is above the 25c temp it loses .38% of its power??
sean

System2 · December 2006

Re: reasons to use mppt

also where it says that an mppt controller uses 8-10w of power for down conversion is this a daily or an hourly figure? also i seem to remember that the solar boost uses 1w of power would this be a daily/hourly figure

if i want to experiment with one of my panels out on the lawn can i just stick itout there with a multimeter connected to each out put lead to see what happens with the voltage at differnt times of the day or is it a bit more complex and i need a battery in the equation?

sean

mike95490 · December 2006

Re: reasons to use mppt

northerncat wrote:

if i want to experiment with one of my panels out on the lawn can i just stick itout there with a multimeter connected to each out put lead to see what happens with the voltage at differnt times of the day or is it a bit more complex and i need a battery in the equation?

Measure away, one panel at a time. One panel is safe, not too much current, most meters can take 10A in the current mode. HArd to shock ones self with 12-24-36V After 36V it starts to become more of a hazzard, it's enough to get your skin to begin to conduct.
Open circuit voltage should be very close to what the panel nameplate is, and the same when you place the meter into current (Amps) mode, about 70-80% of the max nameplate amps, when well lit.

crewzer · December 2006

Re: reasons to use mppt

also where it has the temp coeffiecient of -.38%/degrees celsius does this mean that for every 1 degree that the panel is above the 25c temp it loses .38% of its power??

Exactly. Just to be clear, the STC power spec is based on a PV cell temperature of 25 C. However, cell temperatures typically run anywhere from 30 C to 35 C above ambient when the module is exposed to “full” sunlight. So, if ambient is 35 C, and cell temp is 30 C above that, cell temp will be 65 C. That’s 40 C above the 25 C spec, and the effect could be 40 C x -0.38% / C = -15.2%

HTH,
Jim / crewzer

crewzer · December 2006

Re: reasons to use mppt

is there any way of finding out whether my panels have a few bypass diodes installed and thanks for the confirmation that i could use bypass diodes, would this be a good idea? or just a waste of time? when you talk about conformal coating is this just where they resin coat the circuit boards as i have heard that you can get this done for about 50$

You'll have to check the manufacturer's specs, other manufacturer documentation, or the modules themselves for info on the bypass diodes. For example, Kyocera's installation manual says that "All KC series modules are supplied with factory installed (non user serviceable) bypass diodes", and they're clearly visible inside the junction box on the back of each of my KC-120's.

I think bypass diodes are a good idea, especially on single-string arrays. My array is two strings, and I installed bypass diodes on the non-Kyocera modules.

Check controller manufacturers' literature re conformal coating. The coating's main purpose is to protect circuit board traces (and the gaps between them) from accumulation of conductive material.

HTH,
Jim / crewzer

BB. · December 2006

Re: reasons to use mppt

Remember when measuring solar panel output that the voltage is, more or less constant and the output current is proportional to the amount of light hitting the panel (unless you are shading the panel, then the voltage will change dramatically too).

Connecting the panel through a controller and to a partially discharged battery and monitoring the current and voltage over time is the best way to understand the issues of sun, shading, and temperature.

Also, for example an MX-60 will consume ~10 watts of the panel power before the first watt of power will be sent to the battery... If the solar panels are generating 200 watts, the MX-60 will output 190 watts, maximum, to the battery. If the panels are generating 11 watts, 1 watt will be going to the battery.

This is one reason that MPPT controllers may not recover as much power as everyone expects... For example, your 360 watts of panel do not generate 360 watts for 5 hours = 1,800 watthours of sun (and the controller consuming 10 watts for 5 hours=50 watthours), the panels will generate closer to 800-1.800 watthours of sun over 10 hours (and the controller consuming 10 watts & 10 hours = 100 watt hours) for an efficiency of 88%-94%.

The 5kWhrs would be cool days, full sun, panels tilted towards sun, no shading (like on a "north" facing roof of a home)... The 0.8 kWhrs is just a guess at where you might be with shading, non-optimal pointing at sun on a boat. Your could be better or worst (and will probably be all over the map because of weather, shading, rocking, boat pivoting on anchor, etc.).

-Bill

mike95490 · December 2006

Re: reasons to use mppt

graph of an average day of output, fixed panels on a roof. Peak power when the sun is perpendicular to the panels, as angle changes, the power drops off
Screen shot attached. red:power blue:heatsink temp
Xantrex GT inverter (MPPT) I don't know how a "plain" system would work under the same conditions, I was given to understand this would add about 10% to overall output.

System2 · December 2006

Re: reasons to use mppt

is it possible to buy a solar regulator that can run your boat after the bulk charge stage has finished or is the closest they have come to this load diversion
sean

System2 · December 2006

Re: reasons to use mppt

sorry but i am full of questions, the solarboost 3024 has a claimed efficiency of 97%@24A would this efficiency figure include the amount of power the unit uses or do i have to factor this in as well?
sean

BB. · December 2006

Re: reasons to use mppt

I don't have the numbers for the SB--But I would sincerely doubt 97% efficiency... And to be honest, don't even get too lost in a 97% vs 94% (or whatever) efficiency... The variability in solar is huge and would swamp a 3 point discussion.

Just for discussion, I ran a standard deviation (P) and Average on 1+ years of daily data (numbers are approximate--occasional holes in data)... 3.5 kWatt on a 3kW Grid Tied Inverter (near SF CA), Ave:14.8 kWhrs/day +/- SD:5.1 ... That is almost a +/-50% deviation in energy (fixed home, good and bad weather, etc.)... You would not even be able to see a 3% change in the data without a lab...

If you are really concerned about energy efficiency... Look into AGM type batteries instead of standard Lead Acid type... AGMs are about 98% efficient, whereas lead acid are around 80-85% efficient (amount of energy released to load vs energy required to charge).

-Bill

BB. · December 2006

Re: reasons to use mppt

Sean,

And just to complete the discussion, efficiency also applies to your loads too; lighting, computer, navigation, communications, autopilot, fridge, etc...

Lastly, remember too that if you have enough solar power to fully charge a battery, that the last bit of 80-100% (final) charging and float charging is energy tossed overboard (battery is not accepting the full ability of the panels/controller ability to supply energy). I.e., Panels can output 300 watts, but batteries are only accepting a 100 watts during final charge...

I am not sure what you are asking about diversion... Yes, there are controllers out there that would take the excess power available (typically used with wind turbines to prevent overspeed in high winds) into other loads (typically electric heating elements for water or air heating).

You could use the diversion energy for other uses on your boat--But at this point, I am not convinced that your 350 watt boat system will generate very much (if at all) useful power for diversion use.

-Bill

crewzer · December 2006

Re: reasons to use mppt

Xantrex GT inverter (MPPT)... I was given to understand this would add about 10% to overall output.

Allowing for higher PV module voltage due to lower ambient temperatures in the late Fall, Winter and early Spring, the 10% gain may be true when considered from the perspective of total annual energy production.

is it possible to buy a solar regulator that can run your boat after the bulk charge stage has finished or is the closest they have come to this load diversion

Once a controller has completed the bulk stage, it'll switch to absorb and then to float, both of which are current-limiting functions. So, as Bill indicated, the battery charging will use less energy than what's usually available. There are several ways to harvest this otherwise lost energy:

1) Manually switch on an opportunity load such as a water pump.
2) Purchase/build a voltage controlled switch that activates a relay when the voltage reaches a preset level.
3) The MX60 offers a VENT FAN mode for it's AUX output... just set the ON voltage
4) The MX60 offers a FLOAT mode for its AUX output... the AUX is activated when the controller is in the float mode
5) The Morningstar TriStar can be configured as a diversion controller... check the user manual for details

AGMs are about 98% efficient, whereas lead acid are around 80-85% efficient

In general, I agree with Bill on this issue, as AGM batteries offer many advantages. They can usually be stored in almost any position, and that may be beneficial in tight sailing quarters. They're sealed, so they need less maintenance, and there's little risk of salt water mixing with the battery acid. They typically don't require any special ventilation. Finally, they are indeed more efficient, but the energy recharge efficiency (Wh out / Wh in) is ~90% vs. ~80% for flooded cell lead acid batteries. The coulombic efficiency (Ah out / Ah in) of AGM batteries is indeed ~98%.

Finally, I also tend to agree with Bill over not getting hung up on the controller efficiency. The total rated current output of the four 90 W modules is 20.4 A, and you might see that ~mid-day if the system is in bulk mode. The system will otherwise operate at lower charge current levels, so the efficiency will be less.

HTH,
Jim / crewzer

BB. · December 2006

Re: reasons to use mppt

Thank you Jim, :-)

I tend not to remember that AHout/AHin is a better way to measure "battery capacity" in terms of current... Not in terms of power.

If it is a constant current load (tends to be lights, perhaps some motors--as voltage falls, current use remains the same or drops), AH efficiency is fine. If it is a constant power device (modern electronics, devices operated off of inverters) then constant power is more appropriate for planning capacity (constant power p=i*v, as voltage goes down, current goes up to keep power used constant).

-Bill

System2 · December 2006

Re: reasons to use mppt

when i say load diversion i understand that as a battery enters the acceptance float mode it requires less current, what i would like to know is can you then buy or set a regulator so that instead of wasting the potential current that the panels use can you get it so that it runs things like your fridge etc?
oh and what does vent fan mode do?

3) The MX60 offers a VENT FAN mode for it's AUX output... just set the ON voltage
sean

BB. · December 2006

Re: reasons to use mppt

I would not use load diversion for running a refrigerator. Especially for such small panels.

Diversion is for opportunistic use where a few minutes to a few hours per day (and sometimes days without diversion power such as bad weather or heavy use of loads). Refrigeration would not not meet that as a load definition for me (generally, you need to run a fridge every hour +/-. Running once a day is probably not good enough). Pumping well water into a tank would be a good use.

Also, your panels are pretty small (just not much excess room on a boat) and running a heavy load like a fridge might draw so much power that it would just knock the controller out of diversion as the batteries may start suppling power to the fridge through the diversion output.

Running bilge pump, trim/fuel transfer pump, charging the starting battery, charging a laptop battery may work as diversion loads. You may be able to think of others.

-Bill

System2 · December 2006

Re: reasons to use mppt

aha i see, so i would be better then to program my fridge and dc watermaker to run in the morning when the solar panels are supplying the first stage of charge(my fridge is a home made eutectic 12 volt where you can program when you want the motor to run) so as to maximise the amount of charge my batteries can take
sean

sv_makai · December 2006

Re: reasons to use mppt

I am a newly registered user but just returned from 3 years in the Caribe on our cat. Makai carries 4 Kyrocea 120's and the SB50 mppt regulator. The link to the SB50 is http://www.blueskyenergyinc.com/. It works superbly.

1st question. Is this the northerncat who post on the cruising board? If it is I have outlined the setup on Makai there, if not I can reproduce it here.

Quickly; the panels are configured as 4 separates 12 volts connecting to a junction box in the arch. The junction box then has 1/0 (major overkill-bought the cabling direct) to run to the MPPT controller and then 18 inches to the battery master bar.

The peak output we saw with the combonation was 33 amps vs a projected 28.4 from the panel spec pages) starting about 10AM until about 2:30 PM. We run a 12 volt fridge and separate 12 volt freezer both 24/7 and a 12 volt watermaker. We found with the 680 amp bank our battieres would be fully charged and floating by 11AM most days. At this point we would run the watermaker to make water and burn the now wasted output.

BB. · December 2006

Re: reasons to use mppt

Sean,

No, you would probably be better to do as SV_Makai suggests and turn on your "optional" loads as the battery charging starts tapering off... My concern was with using the diversion control possibly turning on and off as the boat rocked, shadows crossed the array, and changed headings (when the loads would exceed the panel outputs)... Just monitoring the average time when the batteries were getting near fully charged and turning on the optional loads might be better than trying the diversion control method.

Makai, were did you mount the panels and how much power (AH or WattHours per day/week/etc. per season) do you get from them and how did shadowing affect their outputs?

Real life is always a useful data point. :-)

-Bill

reasons to use mppt

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