reasons to use mppt

System2 · December 2006

Re: reasons to use mppt

i to would be interested in some real world info especially as you have been cruising in the tropics makai
and yes i am the same northerncat i just didnt get as much understanding as i need from my previous questions,
would the 4 90 w panels i have be to close to the limit of the 3024 controller if i run them at twelve volts, can i assume that the max amps i will see will be 90/14.4= 6.25*4 = 25a there fore this is still 5 amps under the load limit of the 3024
sean

crewzer · December 2006

Re: reasons to use mppt

can i assume that the max amps i will see will be 90/14.4= 6.25*4 = 25a

It's actually possible that the PV array could briefly produce higher than rated current. During an "edge of cloud event", the intense sunlight focused by the water in the edge of a cloud can illuminate a PV module that was cooled under the cloud's shadow. This causes the PV modules output power to briefly spike up for a few seconds until the regular insolation is restored, the module heats up, and the power output returns to "normal". The US National Electrical Code (NEC) requires a 25% safety margin to handle this type of event. However, there events are very short lived, and they do not result generation of significant additional energy (power x time).

Using the US NEC requirement as an example, your PV array consisting of four modules with an Isc rating of 5.5 A would require a controller with a continuous current rating of 5.5 A Isc x 4 X 125% = 27.5 A.

HTH,
Jim / crewzer

sv_makai · December 2006

Makai

Actually the output wasn't briefly by sustained for 3-5 hours per day.

The SB50 doesn't require a diversion load as one is built in. One of the next projects is to get a 12 volt water heater element and wire to the charge controller. This can be done according to the manual. We don't expect to heat the water hot but to burn off some of the chill. We found that when we were in areas like Sint Maarten where the average water temp was lower and it was Caribe winter weather in the 70's we would get a chill in the shower. (side project is to build and test a reflective active solar water heater that blends into the boat)

I didn't keep a specific log of charging after the intail setup to determine that we were working and correctly optimized. Once setup it was a matter of the clock and the view of the weather. We are a very electrically intensive boat, watermaker 16 amp 6 gal/hr, semi-automatic washer, Directv with tracker and TV, along with the fridge and freezer with a combined 65 amps per day and SSB/Ham which tough intermidant is 35 amps transmit and 2 amps receive for 3-4 hours everyday. Underway we had radar on full, ssb, autopilot and sometimes the water maker. plus many minor loads such as lights, electric head, etc. This took more attention to battery life as the sail shading made it hard sometimes to get caught up. We added a kiss wingen to address underway.

Our best no engine recharge and living life comfortably and full of fresh water was 6 weeks in the Gulfo de Caricao, Venezuela.

The panels are mounted on the Bimini perpendicular to the for/aft outside of the catwalk. Next set (200 watt 24v grid tie style panels which the controller will step down) will be mounted parallel with the boat. The problem is shading. To reduce that we would travel the boom across the boat in the morning to maximize the recharge. The goal was to bring the batteries up to full charge before starting other heavy loads. This ensure the life of the battery by getting a full recharge and going to bed fully charged. To much deep draws or incomplete recharges reduce the total battery amp output as well as the life of the batteries. And you do not want to buy batteries in the Caribe!

We even though we estimated the size and provided a safety factor we decided to go the next larger one rather than be on the edge of the charger rating. This also gives us an opportunity to add more panels. We can fit another 2 80-100 watt panels on our Bimini without overhands or jungle jim supports. If you are on the edge and you think you might add or upgrade your panels it would be false economy to by a smaller charger as you would just have to replace it if you added or changed anything.

The other factor was the ability to fully equalize. Cruising requires equalization about every 3 weeks. The average time of the day starts to creep towards noon from the norm of 10:30 to 11AM for a floating charge we know it is time. This maximizes the life and provides the greatest amp output. If you don't maintain and equalize (liquid cell) regularly the total amps available falls and you find it takes more power to get it fully charged wasting time and money.

More solar is better. If you can fit larger panels on, put them on. When do the math and then living the life you find more variables to you well thought out power budget that causes you to no have enough.

there is also one non power related benefit of PV. By mounting our panels over the Bimini with a decent air space we reduced the in cockpit temps by 15 degrees, extra shading!

If you want more specifics on the setup of Makai I would be happy to provide additional details in this thread or a new one(don't want to hijack this thread) or via email.

www.sv-makai.com and hit contacts

System2 · December 2006

Re: reasons to use mppt

please yes provide more details in this thread makai, also in another thread on this forum they talk about the power consumption of the unit, do you find that your s uses the 1.8 w they claim or is it higher than this do you think?
sean

sv_makai · December 2006

Re: reasons to use mppt

That or less. We noticed very little overhead when charging. We would see charging start as soon as the sun was on the horizon in the morning. (we have a small tilt on the panels). In the even you could here the positive relay snap as it shuts the unit off. So no noticeable draw at night. If there is one our battery monitor records it lower than the DC muffin fan we run in the freezer.

I will post more shortly

System2 · December 2006

Re: reasons to use mppt

if you are using an mppt regulator

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.

are these expensive or hard to install yourself?
and do the following scenarios work
situation 1
you have 4 panels all in series with a bypass diode between each one the charger which was accepting 48 volts starts to only get 36 volts as the charge bypasses 1 panel that is shaded, will it still continue to provide you charge??

situation 2
4 panels set up in 2 24 v arrays 1 panels charge bypasses a shaded panel we now have one array sending 24v to the reg and 1 sending 12 v will it still use the charge from the 12v
and as side question to this can i wire my 2 arrays in parallel so that i only have 1 pair of wires going to the reg or are there big problems when you get a shaded panel and 12v and 24 suddenly meet in the same wire??

i hope that while these questions display my ignorance they do not make me out to be an idiot as maybe this is really simple stuff!!??
sean
i.e my si

System2 · December 2006

Re: reasons to use mppt

Jim and any others who wish to critique

Appreciate you explanations to this uninitiated one. I am still trying to come to grips on a few issues that have been raised here. The first:

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%

Would I be correct in this calculation given:
> -5mV/0C/cell loss (which appears to be what most manufacturers are quoting in there spec sheets),
> Cell temperature of 30 C above ambient (as you have listed above),
> 40 C area around the charger unit, and
> 4 x 90W panels in a 24V/12V DC system.

-5(mV/C/cell loss) x 32 (cells per panel) x 2 (panels) = -320mV (0.32V)
0.32V x 45 (degrees above spec) = - 14.4V

My next puzzler concerns Power usage of a controller.

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%.

Some quote in Watts and other in Amps (or even mA) and BB has kindly already offered some insight as detailed above. Assuming for this scenario, the unit is charging and using power at its peak, given:
 Same data as in my previous question, and
 20mA unit consumption value.

Would the unit be consuming: 24V x 0.02A = 0.58W
And over a 5 hour day: 0.58W x 5 = 2.88W/hrs per 5hr day

Thank you in advance
James

System2 · December 2006

Re: reasons to use mppt

the interesting thing with temperature and i watched it yesterday was that at 8 it was 22 at 10 it was 28 and at 1 it was 35 and this is in the tropics, now provided you have finished your bulk charge by 10:30 like makai was saying you dont need the efficiency gains after that
sean

BB. · December 2006

Re: reasons to use mppt

James,

I think you are understanding the issues at hand... A couple of minor points. For a MPPT controller (or even a PWM controller), you would probably want to pick a 36 cell solar panel. 14.4 volts is probably not for a MPPT controller to operate (charge a battery at 14.2 volts--check yours, but I would guess that the panel voltages need to be 1-2 volts higher than the battery voltage). And if you need to equalize, 14.4 volts of panels is not going to do it (hot panels and cool batteries)...

Of course, in real life, there are winds on the solar panels (junction temperature are kept lower), batteries get hot as they charge and as outside air temperature rises during day (battery voltage requirements fall). So, to say that a 32 cell panel would never work is probably wrong. But, if you choose a MPPT controller and operate the solar panels at "12 volts" with "12 volt batteries"--it would be nice to use the 36 cell (higher Vmp) panels and make use of the MPPT's step down function (or use several 12 volt panels in series for the same advantage).

Also, my examples have been roughly based on my home system with MPPT Grid Tied inverter (N37 near SF CA)... Operating in the tropics with much nicer sun angles will give you peak power longer because of the better sun angles.

-Bill

System2 · December 2006

Re: reasons to use mppt

Bill

I may given you some incorrect info here somewhere regarding the panels :-(

14.4 volts is probably not for a MPPT controller to operate

The 4 panels are 90W 32 cell monocrystalline - Vmp17.7V (Voc 21.2V) & Imp 5.1A (Isc 5.5A). I’m not sure if this changes the values you have provided above. If so - I’m out of jail :-D, if not- I have a challenge to resolve :?

As you have more than likely gathered, I believe this setup is on that edge (size wise) of going PPM or MPPT and I am trying (with the information everyone has kindly forwarded) to make this calculated decision.

I have installed cabling that will allow PV arrays of 12V, 24V or 48V. My first choice is 24V input to 12V output, second 48V input to 12V output and last 12V input to 12V output.

Would appreciate your/anyones thoughts here too. Thank you

James

crewzer · December 2006

Re: reasons to use mppt

James,

I’m not sure of where you’re trying to go with your -5 mV/ C / cell discussion. I’ve not seen that spec used before, although it could certainly be derived from modules specs. Typical temperature coefficients of power loss are ~0.5% / C ref a PV cell temp of 25 C for polycrystalline modules, and less (i.e., -0.35% / C) for monocrystalline modules. Virtually all of the power reduction is manifested as lower voltage. The detailed specs for any particular module are usuall included in the spec sheet.

Borrowing on Bill’s example above, I believe part of the MPPT mystique is that it can somehow recover all lost power. For example, if a PV module rated at 17 V x 5 A (85 W STC) was connected to a PWM controller and a 12 V battery, you might see 14.4 V x 5 A at the battery, or 72 W. And, manufacturers like to claim “up to” 30% current boost.

Would the MPPT controller recover and restore all of the lost 13 W (15.3%) of power and deliver 14.4 V x 5.9 A? The answer is not very often, and, if yes, not for very long.

Early in the morning, a 85 W STC PV module at 25 C cell temp might deliver 17 V x 0.5 A due to the large azimuth angle (i.e., the Sun rises in the East but the PV module is likely facing South, so the PV module temperature remains nears ambient and its output current is low). Assuming perfect wiring and a perfect MPPT controller and a partially discharged battery, that could be 13 V x 0.65 A at the battery. That would indeed be a 30% improvement in power over the PWM controller’s delivery of 13 V x 0.5 A. Over an hour, the benefit would be an additional 0.15 Ah of “energy”.

See: The manufacturers are telling the truth!

However, at mid-day, the power from the hot but well illuminated PV module may be 15 V x 5 A. Assuming perfect wiring and a perfect MPPT controller and a fairly full battery, that could be 14.4 V x 5.2 A at the battery. That would be a 4% improvement in power over the PWM controller’s 14.4 V x 5 A. Over an hour, the benefit would be an additional 0.2 Ah of “energy”.

However, on a very cold day, the mid-day power from the cool but well illuminated PV module may be the full 17 V x 5 A. Assuming perfect wiring and a perfect MPPT controller and a fairly full battery, that could be 14.4 V x 5.9 A at the battery. That would be an 18% improvement in power over the PWM controller’s 14.4 V x 5 A. Over an hour, the benefit would be an additional 0.9 Ah of “energy”.

So, if there’s a useful voltage differential between the PV array and the battery, MPPT will convert the “extra” voltage into additional charging current. But, if the voltage difference is not there, usually due to hot modules, wiring losses, internal controller losses and/or high battery voltage, then there’ll be no MPPT benefit.

Make no mistake: I like MPPT. But, while it can “transform” available “extra” voltage into additional charging current, the extra voltage actually has be available in the system and not just a manufacturer’s nameplate specification.

HTH,
Jim / crewzer

System2 · December 2006

Re: reasons to use mppt

i am going to reask this question on the proviso that if it is just moronic you will tell me
situation 1
you have 4 panels all in series with a bypass diode between each one the charger which was accepting 48 volts starts to only get 36 volts as the charge bypasses 1 panel that is shaded, will it still continue to provide you charge??

situation 2
4 panels set up in 2 24 v arrays 1 panels charge bypasses a shaded panel we now have one array sending 24v to the reg and 1 sending 12 v will it still use the charge from the 12v
and as side question to this can i wire my 2 arrays in parallel so that i only have 1 pair of wires going to the reg or are there big problems when you get a shaded panel and 12v and 24 suddenly meet in the same wire??

sean

crewzer · December 2006

Re: reasons to use mppt

Situation 1: Yes.

Situation 2: If one module of a "24 V string" is shaded, the string's voltage could drop to "12 V". This would cause the other string's voltage to drop to ~12 V. Not all shading will cause this much of a drop.

There may be a work around for this issue. If your module's are actually two "6 V" sections sharing a frame, you can install a single bypass diode around each 6 V section (two diodes per module). If but one section was to be shaded, the drop would be just 6 V.

HTH,
Jim / crewzer

sv_makai · December 2006

Re: reasons to use mppt

the SB50L uses both MPPT and PWN so as noted earlier about the varing effecinceis dependent upon the sunlight and charge state. You get the best optimization based on charge and sun condition. If you you use either system independently, ie a strict MPPT or PWN, you will be trowing some power away.

The biggest advantage of MPPT is seen when the battries are heavily discharged. This is where you will see the 30% gains as the battery moves from bulk charging to Float. As the battery charges the regulator will taper the charge off which is just throwing away power anyway this is the point the SB50L will charge to PWM to finish charging and keep the float. . Generally a large bank (properly sized) will accept almost everything a boat sized solar array can put into it. So the advantage is faster morning recharges and improved recharge under lower light, clouds, haze, etc. In our view once the battery is floating then everything else is excess and can be used without drawing down the bank. Ie water maker. We use a lot of water, regular showers even underway, liberal use of the SSB, and even a semiautomatic washing machine. We dislike going to bed if the batteries ar less than 90% capacity.

To answer any earlier query by northerncat

I designed and installed Makai's solar array. There is a lot of good material on optimizing the setup and boards like this to help resolve some questions. Built the mounting hardware from aluminium and ran the over sized 1/0 cabling, hiddden in the arch, (tinned) purchased at a wire ware distributor rather than the Marine store (differnce of $5 a foot, on a cat the is lot 25 feet feed lines). As always more is better. I spent much of my time rseraching equipment in the solar homes and off grid sites. The Marine sites are old thinking and most direct you to over priced underperforming systems. You will learn more here which is why I am lurcking these sites. I want to increase and upgrade to ~1KW and there is much to learn.

As far as going the mainre route. Stay away from West marine or boat stores to get you equipment and hardware. When we bought our Kyrocea 120's we paid just under 500 a piece, WM wanted 1,100 for the exact same panel. The panels were not seconds, but it was just before the shortage, so prices we very good compared to today. RV Power products, now BlueSky Energy was great in answering my questions and helping me configure and optimize my setup.

The panels are hard mounted a a slight angle on the bimini, 2 per side and no blocking diode is installed. They panels are configured as 12 volt rather than 24 and stepped down. this was done as I was the first person I knew who had installed one on a boat. I was not sure how long the contoller would last in the sea conditions, salt air is hard and pounding waves are hard on equipment. This allowed me to manually control my charging should the regulator fail by direct wiring, or install some other regulator in a pinch.

niel · December 2006

Re: reasons to use mppt

for the record i do not know of any mppt controller that isn't based in being a pwm controller. if somebody knows of one please let me know.
as to the sb50 i do have one as well and i am curious if bluesky gave you any instructions for weather sealing the unit. it is designed to be in a standard indoor electrical box with knockouts for various wire sizes and numbers of wires so it would give some degree of a headache and limited success to weatherproof the controller.

sv_makai · December 2006

Re: reasons to use mppt

Makai's controller is not weather proofed. Our Cat was designed with an electrical room and an interior battery room if we choose to install the controller there. It is very dry and has nor exterior exposures. This same room houses all of our electronics as well, radios, circuit control board, etc Even the wiring runs out of the room down and out in to another compartment before splitting off. So in our case the standard installation worked without the need for weatherproofing.

System2 · December 2006

Re: reasons to use mppt

in looking at one of the brochures for the sb50 they said that it came conformal coated which i believe is what they term as marinised
sean

System2 · December 2006

Re: reasons to use mppt

out of curiousity what happens when 2 differetn voltages travel down the same wire, if my panels were as cruzer said and actually 2 6v panels joined together then if they are all set up in parrellel and one panels cops a bit of shade then there will be three panels sending 12v down the wire and one only sending 6 v but all at the same amperage,
does anything happen? or is it just a case of the regulator not being able to use that 6v for charging as it is below the 12 needed also will an mppt regulator be able to use the 6v if a standard pwm cant
sean

System2 · December 2006

Re: reasons to use mppt

ok i assume the above question was silly
sean

crewzer · December 2006

Re: reasons to use mppt

if they are all set up in parrellel and one panels cops a bit of shade then there will be three panels sending 12v down the wire and one only sending 6 v but all at the same amperage...does anything happen?

Actually, it's not a silly question. It just took me a while to get past the part about "what happens when 2 differetn voltages travel down the same wire". :? Unfortunately, the combination of a "6 V" module and several "12 V" modules wired in parallel will result in an array with an output voltage of a just a bit over "6 V".

HTH, and I hope you're enjoying your Summer down under! 8-)
Jim / crewzer

reasons to use mppt

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