# Balanced MPPT system?

Solar Expert Posts: 180 ✭✭✭
Hey All,
First off, I have learned an enormous amount reading through all the information contained within these forums. Thanks!

My first system project/proof of concept will be on my RV. It is a compact 23' 8" class A (Aero Cruiser) with very limited roof real estate. I've ordered 3 panels from china to fit the dimensions I can use. 2 140w 36v and 1 170w 36v.

Cable runs are minimal. At the moment I have selected the Morningstar TS-45 MPPT as the charge controller.

The basic calculations (without losses, etc) shows ~37.5a at 12v.

Given the panels spec at:
Peak Power(W) 170
Open Circuit Voltage (V) 44.00±1
Short Circuit Current (A) 5.30±0.2
Max. Power Voltage (V) 36.00±1
Max. Power Current (A) 4.72±0.25
Max. Rated System Voltage (V) 1000
Working Temperature -40℃——90℃
Net Wight (kg) 8.5
===================================
*2 of these*
Peak Power(W) 140
Open Circuit Voltage (V) 42.12±1
Short Circuit Current (A) 4.16±0.2
Max. Power Voltage (V) 36±1
Max. Power Current (A) 3.89±0.2
Max. Rated System Voltage (V) 600
Working Temperature -40℃——90℃
Net Weight (kg) 7

*Will I ever encounter a situation where I would exceed the amp rating of the TS-45 MPPT and waste precious power??

Due to space constraints, I will only have 3 batteries (Trojan SC225 130a/hr @20) for a total of 390amp hours. Divide by 2 gives me 195 amp/hrs per night of use.

I will divert power to non charge loads during the day once charging is complete. Ultimately I will be adding either 1 (for a total of 4) or an additional 3 to get to 6 (though I do not believe that will be needed.)

Of interesting note, these panels are "semi-flexible" and will conform to my roof. They are light weight and VERY thin. I will be using a 3M silicon based adhesive to attach them to the fiberglass roof (after proper prep of course!)

My A/C loads ( very few) are currently handled by a 1800w Xantrex inverter with a transfer switch build in. It is wired directly into the AC breaker system and powers a set of outlets along one side of the RV. This includes the Microwave, all important Coffee maker, TV/video/satellite (LCD small flatscreen system <1/2 amp/hr) I am also equipped to use propane for cooking and heating as is usual in an RV.

Diverted loads will likely include: Hot water, appliance use during the day, etc.

I believe I have everything figured out, but I always remember:

Murphy was an Optimist!

Any comments or suggestions would be greatly appreciated.

Thanks,

Rick "Wanderman"

• Banned Posts: 17,615 ✭✭
Re: Balanced MPPT system?

Let's see ... 450 Watts total panel, right? At around 77% efficiency on a "12 Volt" system should produce a peak charge current of about 24 Amps, so there's no problem with the Morningstar 45 Amp handling that.

And you've got 390 Amp hours of battery, which is more than I run the whole cabin on daily. So that should also be good. 25 Amps is about 6% of the battery bank; maybe a bit low but certainly within range and "doable".

Microwaves are murder on power, but only for a few minutes and if you use them when the batteries are full and the panels still producing it will work. Coffee maker ... not so good. Bound to be used early AM when batteries are at their lowest. Got propane? Yes, you do. Always a better way to heat things than electric. Make your coffee on the stove.

It all looks to me like you thought it out well and it all should work.
• Solar Expert Posts: 8,196 ✭✭✭✭✭
Re: Balanced MPPT system?

panels flat on roof, you can expect less than 70% nameplate, because you will seldom have proper sun alignment.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

I have 3 x 36v panels going into the TS-45 MPPT. My wire runs are quite short ~12ft or less. How should I wire them 36v parallel or 108v series to maximize efficiency?

Thanks,

Rick
• Solar Expert Posts: 1,959 ✭✭✭
Re: Balanced MPPT system?

Parallel for sure, your panels are different amperage, also 3 in series in cold weather cold exceed the maximum input voltage on the charge controllers.

Your still going a 3 to 1 step down 36v to 12v, so the mppt has plenty of room to work with
Re: Balanced MPPT system?

Vmp=108 VDC @ STC is pretty high... The controller's Maximum Voc is 150 VDC...

We need to know exactly which solar panels (name/model) and your expected minimum temperature for your area--Then calculate the Voc-cold and see that it is Voc-cold<150 VDC.

It is close with Vmp-stc conditions = 108 VDC.

For example (watch units--C vs F, Tcoeff in volts, millivolts, or even % change):

STC temperature is 25C, so all values have to be adjusted by -25C. Using -0.16 V/°C (-160mV/°C because I do not know yours:
• Voc (cold) = Tcold * Tcoeff V/°C + Voc @ 25C
• Vmp (hot) = Thot * Tcoeff V/°C + Vmp @ 25C
Use -10C, Vmp=33.6 volts and Voc=44 volts per panel, assume hot panels maximum of 35C over ambient:
• Voc (cold) = (-10C-25C) * -0.16 V/°C + 44 Volts = 49.6 Volts (@-10C)
• Vmp (hot) = (45C-25C+35C) * -0.16 V/°C + 33.6 Volts = 24.9 volts (at 45C ambient + 35C rise)
Three panels in series:
• Voc-cold * 3 = 49.6 volts * 3 = 148.8 volts (@-10C--pretty close)
• Vmp-hot * 3 = 24.8 volts * 3 = 74.4 volts (@+45 ambient)
• -10C = 14F
• 45C = 113F
Does that make sense to you (and meet your temperature requirements)?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

The temps are basically what I will see in the Northeast...and while traveling. Deep winter where I store the RV is in Upstate NY (12428)

The panels are custom made in china and I should be receiving them in a few weeks. The specs that are listed are all that I have at the moment. Will be testing extensively when they arrive.

Rick
• Solar Expert Posts: 452 ✭✭
Re: Balanced MPPT system?

Go with the Guppy! But do look at the gauge of your 12' (24' round trip) cables.
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

OK assuming I need 24' of cable, at the ~36V what would be a good gauge to go with. Ideally I'd like 2 conductor shielded UV outdoor....flat would be a bonus.

I've been looking online and haven't come across a good source....any ideas?

Rick
Re: Balanced MPPT system?

Assuming 3x panels in parallel with Imp-array~15 amps @ 36 volts and using a generic voltage drop calculator, with ~1-3% voltage drop on a 24' one way cable run:
• 36 volts * 3% = 1.08 volts max recommended drop
• 36 volts * 1% = 0.36 volts min cost effective drop
And using the calculator (trial and error) with 24' one way run:
• 24', 10 awg, 15 amp=> 0.9 volt drop
• 24', 8 awg, 15 amp=> 0.55 volt drop
• 24', 6 awg, 15 amp=> 0.34 volt drop
-Billl
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

The panels aren't in series they are natively 36volt nominal. Each panel wired into the controller independently.

Better?

Rick
Re: Balanced MPPT system?

Sorry, that was a typo... Should have read "parallel" (fixed in post). Everything else is correct.

You have the choice of one pair of heavy wires from the array combiner (with fuses/breakers) or running three parallel sets of smaller gauge wires to the battery/controller shed with the combiner mounted there--Your choice.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 9
Re: Balanced MPPT system?

Off the subject, but I have a question.

You divert electric load to your RV water heater....so your water heater isn't propane? Did you replace the original, and is your electric one standard (and how many gallons), heat pump, or on-demand?

Thanks, just curious.
Re: Balanced MPPT system?

By the way, I did 24' one way run---I bet I should have done 12' one way run... The wire gauge would be 1/2 the diameter (basically ~3 gauges smaller).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

OK...no problem...I'd prefer to go heavier gauge anyway. I am likely going to forgo the combiner and run 6 wires from the panels into the controller. Originally I was going to use the AMSolar.com refrigerator vent combiner, but the more that I think of it the easier it will be to have the connections out of the sun and other elements. That's why I'd like 2 conductor wire, then it's 3 runs, not 6. Still can't find a heavy gauge source of 2 (or 3) conductor flat-ish uv outdoor rated cable...help!

As for the water heater, it is a typical 6 gallon Atwood DSI (Direct ignition) propane unit. I have fitted a HOTT ROD 120v heating element so I can use either or both. I have a 12V version as well, but that is for experimenting with down the road.

On another wild scheme, I 'll wager it wouldn't be too difficult to do a passive solar water heater "booster" simply pump the water to the roof "collector" and let gravity fill the main tank (or a header tank). Very little weight while travelling and more efficient hot water heating while parked in the sun.

The overall goal for this RV is to be as off grid as possible for the longest period. This would include water, sewage and food. If I can get 10 days I would be very happy. 5 would be good.

Rick
Re: Balanced MPPT system?

10 AWG 3 conductor + ground direct burial (which is also UV rated if I recall correctly) was available at my local Home Depot a year or so ago... 12' or so should not be that expensive.

For fusing/breakers--Normally for 3 or more parallel panel strings, you need a series protection fuse in each panel string (typically ~2xIsc -- short circuit current). This prevents two panels from back feeding a shorted third panel and causing a fire.

It is a good idea? Yes. Is the fuse police going to arrest you if you don't? No.

Could an insurance inspector (heaven forbid) deny a claim if there was ever a fire--possibly. Would it a fire ever occur for lack of series fuses--unlikely.

But some wise person on the board--to Quote:
Murphy was an Optimist!

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

That 3 conductor is it solid core??? Stranded? Copper?

Thanks!

Rick
• Solar Expert Posts: 334 ✭✭
Re: Balanced MPPT system?

I just bought a bunch of 10/2UF at my local home depot last month, as I had to run a line to power my toolshed. Solid Copper, and somewhat flat (but rather thick too with the added insulation). They also had plenty of 10/3UF as well. Sorry I can't remember the price off the top of my head, but for 12' it's going to be pretty cheap in the overall installation cost.
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

Just got confirmation that my custom semi-flexible panels are on the way from China via DHL!

Watch, I'll get them in a few days instead of weeks and not be able to do anything until spring.

Murphy WAS an optimist!

Rick
• Registered Users Posts: 9
Re: Balanced MPPT system?

My husband says you can use SO stranded cable that's UV resistant, comes 1, 2, or 3 conductor, available at an electrical supply, but it's round, not flat. It's also hard to find.

How did you install the 120v heating element on your 6 gallon propane water heater, and is it just plugged into an outlet like an appliance? It would be so nice to have an on-demand on our RV, but hard to justify the cost.

I thought a solar water heating system which directly heats water to be used by circulating it through roof panels can't be used in locations with freezing weather?
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

I will see if I can source the SO style cables...I really believed that 2 conductor side by side semi-flat cables would be easier to find!

The Atwood 6 gallon water heater has a drain plug. It can be replaced with a heating element (see the HOTT ROD) A thermostat is installed below the cardboard and fiberglass insulation container. I've wired it into a handy dual outlet box that was close by. I've also wired in an on/off switch to control usage. The kit comes with a standard 3 prong plug (and no switch) so you can simply plug it in to a handy outlet when you'd like to use it.

The passive Solar (roof) water system i was thinking of designing will only contain water when in use. There would be no static water to freeze. Also, it could have a small circulation pump that would prevent standing water. I'll likely use gravity to keep the system drained and lightweight for travel.

Rick
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

OK, so the panels have arrived from China.

As you can see by the last photo they are THIN, about 3mm. 2 140Watt Panels and one longer rectangular one at 170watt.

There is a plastic protective coating on the face of the panels similar to the one you get over chrome or watch faces. I will not remove this until I am ready to install.

They are an aluminum and plastic silicon "sandwich" designed for outdoor/vehicle/marine use. I will be using adhesive (likely 3M 760 Hybrid Series) as long as it meets the temp requirements.

Any idea how hot these will likely get flat mounted to the fiberglass roof??

Just for fun, I connect a DVM to the outputs of each panel, one at a time. At night, with only room lighting, these 36v panels were producing 17 volts. Held up to an overhead recessed 100w bulb they went up to 22.5v. Not telling us anything really, but at least they work! No amp info yet. Too much snow outside!

I did have some edge damage in shipping from China via DHL, no damage to the cells, but one of the panels had 2 corners bent and the edge a bit turned down. Should be able to bend it flat again. I MAY want to trim these a bit smaller as there is a lot of waste edge space on one panels long edges and one panels short edges. If anyone knows RVs every inch on the roof helps!

Any idea how to cut them smooth and what tool to use???

I am going to mock up the install inside before the spring and work out the wiring and details.

Stay tuned!

Rick "Wanderman"
Re: Balanced MPPT system?

I cannot tell--but I would be leery of trimming the edges unless it is aluminum only--If there is any bonded plastic--I would not cut through that (may weaken hermetic seal).

Panels are usually rated to around 85C/185F maximum--I would plan on reaching nearly that temperature if you are in any reasonably hot weather (over 100F).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

OK... The 3m 760 is good up to 212 deg F so I should be OK bonding-wise.

I emailed the manufacturer and asked about trimming. We shall see. I MAY not have to, but if I do I would like to make sure it is done properly.

Rick
Re: Balanced MPPT system?

Also, be very careful in what you bond the panel too... there can be significant differential expansion if there are temperature differences between the panel and the substrate--or if there are different materials (example: aluminum panel bonded to plywood).

Using a flexible adhesive (like silicon based products) can frequently be a much better solution vs a "hard/thin" adhesive like epoxy.

I would be very tempted to build a "frame" to capture the edges of the panels with a couple globs of high temp silicon (typical red in color) in the middle of the panel vs gluing the entire panel to the roof of the RV with epoxy/thin/low flex adhesive.

What does the MFG recommend?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

Bill,
These panels are designed to be adhered directly to the mounting material. The manufacturer said to use a silicone based adhesive. I have found the 3M 760 Hybrid to be the best bet so far.

Your concept of "capturing" the panel is a sound one, but defeats the purpose of having a 3mm thick flush mountable unit.

The 760 will have a certain amount of flex and give like most silicone based products.

I cannot do anything until we get some warmer weather, so I may have found a better solution by then.

Thanks,

Rick
• Solar Expert Posts: 1,959 ✭✭✭
Re: Balanced MPPT system?

The Jbox is going to cause allot of shading issues ...that why they are always mounted on the back, not the front of a solar panel
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

THE J-Boxes can be easily moved. They are simply adhered to the top of the panel near the leads. I'll see what my overall output is and move them if need be.

I will be painting them white...don't know why you'd want black boxes on the top side!

Rick
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

OK, just about to order the Charge controller and cables.

Everyone still like the Morningstar TS-45 MPPT for my setup?? Anything newer better, lower standby consumption in the wings???

Rick
• Solar Expert Posts: 180 ✭✭✭
Re: Balanced MPPT system?

OK...so I guess everyone likes the Morningstar TS-45-MPPT controller.

Now, who has the best price, delivered? Ideally, I'd like to purchase from our forum hosts, but their online pricing seemed a bit high since it does not appear to include the remote temerpature sensor.

Thanks,

Rick