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Old 11-28-2016, 07:20 PM   #1
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12v vs 24v Solar design

This is a bit of advanced solar topic.
My last motor home had solar and was installed by Handy Bob and myself. It worked great. Sold that unit over 3 years ago and never had to worry about going without with that set up. I now have a new coach and Bob no longer does installs so I am doing it myself.
The design is just as important as the install. I have a residential refer this time and it looks like I need 120 Ah just for that. I am unclear as to how I want to go and have some questions. I have a 12v typical RV electrical system. So, going with 12v nominal panels are the easiest way to go. I am trying to eek out as much efficiency as possible though. Currently have the typical 4x6v lead acid batteries with 416 Ah capacity (208 usable). As my username says I am into computers so I am an energy hog for all intents and purposes. Right now my thoughts are 6x160w 12v panels (960w), TS-MPPT-45 or 60. Panels will go along the driver's side edge on the roof.

Here's the questions:
1) 12v (all parallel connections) vs 24v (3 strings of 2 panels each) my understanding is the main difference would be the wire gauge from the combiner box to the solar controller to carry the higher amperage @12v. Looking at <15 ft from combiner to controller. Is that correct?
2) As an alternative could I use 24v grid tie type panels. I believe I will have the room for 255w 24v panels. I would then do 4. Thoughts?
3) Are there any inefficiencies to be had with the MPPT controller and the panels at 24v vs 12v?
4) Maybe a simple answer, maybe a more complex explanation needed. When there is excess capacity provided by the panels over what the batteries can accept can the excess watts be used by the electric consuming devices on during that time? ie the panels producing 45 amps and the batteries only accepting 20 amps to charge. There is "excess" of 25 amps.
5) Will be going to lithium at some point just not sure if I should do it now or wait a few more years. The benefit to doing it now is I can combine the battery cost into the tax credit for the solar and I am still working so am in a much higher tax bracket next year when I retire. Any solid recommendations on lithium? Still concerned about the low temperature charging issues I've read about. Thinking about 600Ah. I will be full time and plan on considerable boondocking.
6) Haven't found any tilt mounts I really like and was considering making my own out of 1" alum angle stock. Anyone find decent tilt mounts that aren't close to the price of the panels? Thoughts I making my own?
Thanks in advance to all.
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Old 11-29-2016, 10:22 AM   #2
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You are fortunate to have Bob helping you in the past. The equipment that Bob had available to him in the past, at reasonable price points, tended to push his work toward 12 volt systems.

The downside of 12 volt systems is wire size. A basic 2 KW inverter needs nearly 200 amps supplied to it in order to operate, feasible, but not simple.

The same inverter in 48 volts needs about 45 amps at full capacity, a dramatically smaller challenge for the battery pack, wiring, and the inverter design.

I am just about finished building up a 48 volt system for my RV / van:
- 8 each, 12 volt, 130 A-Hr LiFe batteries
- Wired into 2 each, 48 volt battery banks
- 4 KW of inverter capacity
- 12 volt - to - 48 volt charger so I can pull from the 12 volt alternator when driving
- Solar panels + charge control

These LiFe packs are roughly equivalent to 8 each, very healthy, deep discharge 8D batteries, but are only size 27, so much smaller and lighter. (400 vs 1500 lbs)

They also make your wallet lighter, but I needed the space and weight savings to make it all work.
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Old 11-29-2016, 10:51 AM   #3
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Every panel has a Vmp and a Voc.

Vmp is the voltage for the maximum power point. In theory the charge controllers try to operate them at this condition. For a panel, example suniva, a 24 volt panel Vmp is around 30 volts.

Voc is the voltage that if there is no connection to the world, they panel voltage will float up to this voltage. For a 24 volt panel, this is around 40 volts.

In order to charge a battery, the voltage going in has to be at least as high as their fully charged state. For a 48 volt pack, this is 56 - 58 volts.

In order to efficiently use a MPPT controller on a pack like this, the series wired panels have to have a Vmp of AT LEAST a 30% higher voltage, so at least 75 volts. If it is less than this, then there is little benefit over a PWM controller.

House size panels are a lot lower cost than the same capacity in 12 volt panels, you just need to watch for flex. They are so much cheaper that in many cases, you can throw them away every 5 years and still be ahead.
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Old 11-29-2016, 10:55 AM   #4
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Quote:
Originally Posted by computerguy View Post
1) 12v (all parallel connections) vs 24v (3 strings of 2 panels each) my understanding is the main difference would be the wire gauge from the combiner box to the solar controller to carry the higher amperage @12v. Looking at <15 ft from combiner to controller. Is that correct?
Upside less losses and smaller cable, downside more susceptible to shading issues and requires an MPPT controller.

Quote:
Originally Posted by computerguy View Post
2) As an alternative could I use 24v grid tie type panels. I believe I will have the room for 255w 24v panels. I would then do 4. Thoughts?
If you have the room it can be a good option, grid tie panels can be even higher voltage than 24v nominal as well. Downside, find mounting space and getting a good mounting system as they have more surface area per panel. Also may not be warrantied for mobile use vs the 12v nominal smaller panels.

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3) Are there any inefficiencies to be had with the MPPT controller and the panels at 24v vs 12v?
MPPT is generally more efficient the higher the voltage from the panels, PWM works best when panels and batteries are closer in voltage. At this point you might as well just go MPPT, a good one like the TS-MPPT-60 should work better with either 12v or 24v panels, just costs more.

Quote:
Originally Posted by computerguy View Post
4) Maybe a simple answer, maybe a more complex explanation needed. When there is excess capacity provided by the panels over what the batteries can accept can the excess watts be used by the electric consuming devices on during that time? ie the panels producing 45 amps and the batteries only accepting 20 amps to charge. There is "excess" of 25 amps.
Yes pretty much, the solar controller just see's everything as the batteries to charge, including live loads on the 12v system. For instance I am a computer guy too and have some core electronics (Router, NAS, Camera) running right off 12v and the total draw in the MH from 12v is usually around 8 amps. When the solar is going and batteries charged I will see 0 coming out of the batteries through my BMK, when the sun goes down I will see 8 amps coming out. Same when charging batteries about 12 amps going into batteries even though 20 coming from solar due to house loads.

Also I have a Magnum 3000 Hybrid inverter running the whole coach and when the sun is out and batteries charged and on shore power or gen the Magnum will go Load Support VDC and invert excess solar to help run the A/C or other loads. I only have 320W of solar now so it ends up just being about 1 amp of 120v extra, but with say 960W it could be significant.

When off shore or gen the inverter is just another 12v load and the solar will drive it like the others.

Quote:
Originally Posted by computerguy View Post
5) Will be going to lithium at some point just not sure if I should do it now or wait a few more years. The benefit to doing it now is I can combine the battery cost into the tax credit for the solar and I am still working so am in a much higher tax bracket next year when I retire. Any solid recommendations on lithium? Still concerned about the low temperature charging issues I've read about. Thinking about 600Ah. I will be full time and plan on considerable boondocking.
For me too much extra cost / complexity for Lithiums, need to keep them in a protected environment, things like engine alternator charging need to be changed etc. Sure would be nice for weight and capacity and charge/discharge charactaristics. Instead I just went with really good AGM's (Lifelines) that have no maintenance and can accept higher bulk charge but also tough and still much cheaper than lithium.

Quote:
Originally Posted by computerguy View Post
6) Haven't found any tilt mounts I really like and was considering making my own out of 1" alum angle stock. Anyone find decent tilt mounts that aren't close to the price of the panels? Thoughts I making my own?
Thanks in advance to all.
I went ahead and spent the money on the AMSolar kits with panels and mounting hardware, not cheap but top notch including cabling too. They have nice tilt bar option too. Sounds like you didn't want to go that route though. I didn't bother tilting yet, just don't see myself getting up on the roof to do so, would love to do something remote controlled with actuators, but then things get really expensive.
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Old 11-29-2016, 10:59 AM   #5
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960 watts is a bit over the max input of a TS-MPPT-60 controller running with a 12 volt system.

It will probably be fine unless you park on the equator at noon, but with the 45 amp controller, you may be loosing some output.

I believe MPPT controllers work better in low light conditions, with higher panel voltage inputs. I wouldn't spend the money on an MPPT controller with parallel 12 volt panels.

As far as excess power, if the batteries are accepting only 20 amps, the rest of the output is avalable to draw off of.
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Old 11-29-2016, 01:36 PM   #6
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The choice of panels with today's MPPT controller are primarily dependant on the physical non-shadowed space available. Higher voltage panels/strings enable the efficiencies in power production and cabling. In any case multiple strings with panels disbursed about the roof can mitigate shading losses. As for the selection of controller capacity if flat mounting the panels a 15-50% loss from rated can be used when determining oversubscription margin.
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Old 11-29-2016, 01:53 PM   #7
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As for the selection of controller capacity if flat mounting the panels a 15- 50% loss from rated can be used when determining oversubscription margin.
15 to 50% ? Quite the range.
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Old 11-29-2016, 04:25 PM   #8
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Just for fun I ran the design through the string calculators at morningstar and midnight solar.

Where you can get into unexpected limitations are at the high and low temperature extremes. You likely will not go where it is super cold, but I ran it from (-40 C) to (+60C). You might be surprised at just how hot a black panel on a roof in the middle of AZ can get, so I use this number.

The Vmp drops pretty far at these high temperatures, so I had to do a minimum series voltage of 70 volts to make it work at the high end.
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Old 11-29-2016, 04:35 PM   #9
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The charging of LiFe batteries when at lower temperatures needs to be thought about, but it isn't any more complex than anything else.

I just mount the batteries in a box, insulate, and add a heater to keep them above the minimum temperature charge spec. It isn't that hard to keep them "warm enough".

Good quality LiFe batteries produce nearly no heat when sized correctly, but I do have some cooling for very hot days if needed.
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Old 11-29-2016, 05:29 PM   #10
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15 to 50% ? Quite the range.
I should have added the range encompasses the difference for time of year and latitude.
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Old 11-29-2016, 05:51 PM   #11
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The charging of LiFe batteries when at lower temperatures needs to be thought about, but it isn't any more complex than anything else.
Certainly more complex than my Lifeline AGM's that are rated -40F to 160F. I may go Lithium next time but I wouldn't fool myself into thinking they are just as simple as a mil-spec AGM.
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Old 11-29-2016, 06:56 PM   #12
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Our son mounted a blower in our Roadtrek 190 that transfers air from the living area to the LiFePo battery comparment (under the bed) It turns on at around +90F and at a few degrees above freezing. We keep the living area above 50 when sleeping in cold weather and run the A/C when it gets above 90 ambient. We were sleeping in it the other night with the temperature down to 26 F and the blower did keep turning on.

We just put a couple of 30 W bulbs in the battery compartment of our 34' 5th wheel and it keeps the battery compartment well above freezing at 20 F. We have about 9 kW-hr of LFP so the watt-hrs to run these bulbs is not a concern. We left this at son's place at 7600' in Sangre de Cristos as we are on way down to Yucatan. He will turn off the battery this week.

The 5th wheel has the power go from panels to controller at 90 V and then to battery suite at 48 V. This greatly reduces power loss/and/or use of thinner cabling. Another advantage to 48 V (nominal) battery suite is that mini-split a/c usually runs at 24 or 48 V.

50% in power in winter is reasonable. Tilting is a good idea then - better idea is to head south as in:

Do I want to build a snow man. No I do not I prefer to build sand castles on a warm beach with no snow.

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Old 12-01-2016, 10:03 AM   #13
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Certainly more complex than my Lifeline AGM's that are rated -40F to 160F. I may go Lithium next time but I wouldn't fool myself into thinking they are just as simple as a mil-spec AGM.
You are completely correct, doing a good, safe LiFe system is significantly more complex than an good quality AGM. I should have said that the complexities can be overcome with some thought and engineering. (and money)

Which Lifeline AGM are you using? Concords or the GPL?

I found capacity vs temperature information on a few of them, but not as much as I hoped to.

Harry
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Old 12-01-2016, 10:21 AM   #14
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Which Lifeline AGM are you using? Concords or the GPL?

I found capacity vs temperature information on a few of them, but not as much as I hoped to.
Currently 2 Concorde Lifeline GPL-4CT's. I can recharge them from 50% to full in about 2 hours using my Magnum 3000's 125 amp charger. It does 125 amps up to about 67-70% SOC then hits absorb V, continues to accept more than 50 amps through 85% SOC. They have no issue running big loads including A/C although will drain quick doing so being only 220AH total.

Nearly all of the info for all models is contained in the tech manual: http://lifelinebatteries.com/wp-cont.../12/manual.pdf

Capacity vs temp is on page 36 for all models.
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