Some beginner questions on an RV system, 12v or 24v panels (and all that goes with it)?

TucsonAZ

Wow, the amount of info out there is overwhelming! I have had my head into this for a couple weeks and have changed plans many times so here's what I'm left with rattling around.


1st system:

6 - 100w Renogy 12v mono panels
Trimetric 2025-RV
Morningstar TS-60 charge controller (in case I add a couple panels)

The cost of this system after a shunt and shipping would be $1,150


My goal is a lot of dry camping (limited more by water and black tank capacity than power or propane) so I don't mind spending some additional money provided it will be justifiable. This led me to look for weakest link, where I could get more gains and to see if I was maximizing my money/value which created more questions. Mainly, system two or the pros/cons of it.


2nd system:

3 - 230w Solarworld 24v poly panels (I can also get Canadian solar 250w panels instead for $175 more if that would be worth it)
MidNite Classic 150 charge controller

The cost of this system [provided the MidNite negates my need for the Trimetric] is $1,250


I'm going to be using mostly 12v power and will be running a 625ah (5x125ah Eaton's) 12v AGM battery bank configuration.

The main things I'm struggling to sort out are:

1) Do the mono versus poly panels make much difference?

2) Is there enough added value in running 24v instead of 12v?

3) If yes to #2 would that also apply to running the 6 - 100w 12v panels in a 24v configuration (potentially easier to install on an rv roof)

4) Renogy versus Candian versus Solarworld as I know nothing about panel quality and their ability to stand up to being on a roof while driving 70mph.

Cost per watt is nearly the same so really it's just a question of 24v versus 12v and mono versus poly but I'm really new to solar and while a quick learner there's a lot of conflicting info out there none of which specific enough to my application to be of use.

Thank you for taking the time to help, I respect and value all of your options and hope that you may see something I'm missing or affirm I'm on the right path.

zoneblue

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

TucsonAZ wrote: »

1st system:

6 - 100w Renogy 12v mono panels
Trimetric 2025-RV
Morningstar TS-60 charge controller (in case I add a couple panels)

The cost of this system after a shunt and shipping would be $1,150
...
2nd system:

3 - 230w Solarworld 24v poly panels (I can also get Canadian solar 250w panels instead for $175 more if that would be worth it)
MidNite Classic 150 charge controller

The cost of this system [provided the MidNite negates my need for the Trimetric] is $1,250

For mobile applications the exact panel you choose is often determined by the shape of your roof space. Hence if both fit, then considerations like efficency may come into play (check anyway), for mobile more effciency means smaller panels or more panels per roof space. After that robustness, and brand might give a quality/durability consideration.

I'm going to be using mostly 12v power and will be running a 625ah (5x125ah Eaton's) 12v AGM battery bank configuration.

5 12v in parallel. No dont do that, read up here ad nauseum as to why. Without running your numbers you are probably light on the pv for that bank too.
If its even vaguely possible i'd go up to a 24v bank, and use two strings max. For 600W PV try to drop the AGMs to about 300Ah. OTOH if you have regular access to shore power or willng to use genset (often), then the bigger bank will work ( providing you can get it the 65+ amps it needs.)

The main things I'm struggling to sort out are:
1) Do the mono versus poly panels make much difference?

Not much. Choose on Wp per m2, and Wp /$, which ever is scarcer in your universe.

2) Is there enough added value in running 24v instead of 12v?

For one its two times less battery strings. Then 4 times thinner cabling. Half as much charge controller. Etc etc, you get the idea. 12v loads can be largely avoided with 24v pumps, 24v fridge etc. Any strays use 24-12 converters.

3) If yes to #2 would that also apply to running the 6 - 100w 12v panels in a 24v configuration (potentially easier to install on an rv roof)

We try not to label pv as 12, 24v etc, as it hides important differences in panel parameters. The important one of which Vmp. If you have a particular reason to avoid mppt controllers then you need Vmps in the 18v/36v ballpark. (36/72 cell). With mppt, the vmp is less critical, however you do need to check that a comfortabe mulitple of panels in a string will work for the given controller. As a rule 60/72 cell panels are quite a bit cheaper than 36 cell panels (grid tie market). Its hard to go wrong with 60cell panels and a good controller like say midnite kid, for your application.

4) Renogy versus Candian versus Solarworld as I know nothing about panel quality and their ability to stand up to being on a roof while driving 70mph.

Obviously 3.2mm glass minimum, some have a higher rated pressure load specs which you can check. But other than flying stones PVs are suprisingly string. In the flying stone dept copare the hail ratings, usually something like 25mm at 25m/s.

Cost per watt is nearly the same so really it's just a question of 24v versus 12v and mono versus poly but I'm really new to solar and while a quick learner there's a lot of conflicting info out there none of which specific enough to my application to be of use.

Thank you for taking the time to help, I respect and value all of your options and hope that you may see something I'm missing or affirm I'm on the right path.

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

zoneblue wrote: »

For mobile applications the exact panel you choose is often determined by the shape of your roof space. Hence if both fit, then considerations like efficency may come into play (check anyway), for mobile more effciency means smaller panels or more panels per roof space. After that robustness, and brand might give a quality/durability consideration.

This makes sense, I could honestly fit more up there than I can afford and I'm redoing the roof sans AC unit and skylights (leaks, solar gains) so I could fit about 1kw comfortably maybe 1.5kw if I wanted to get creative and make some PV window awnings and patch in 50w panels.

5 12v in parallel. No dont do that, read up here ad nauseum as to why. Without running your numbers you are probably light on the pv for that bank too.
If its even vaguely possible i'd go up to a 24v bank, and use two strings max. For 600W PV try to drop the AGMs to about 300Ah. OTOH if you have regular access to shore power or willng to use genset (often), then the bigger bank will work ( providing you can get it the 65+ amps it needs.)

Well, not what I wanted to hear but exactly why I posted this. Thank you for point out this flaw and after some additional research it seems wise to up the entire coach to a 24v system. This leaves a two part question:

a) Can I use a 24v to 12v DC step down to address my 12v needs (e.g. LED lighting, fans, radio, cell phone chargers, pumps) without any issue or major losses?

b) If I run two batteries in series for 24v and parallel them together for a 4 battery 24v 250ah bank, what am I going to do with the additional 12v battery? I could rotate it into service every 3 months or something or is there another creative idea I can do to make us of it? I may be able to get one more of them too (they were slightly used).

For one its two times less battery strings. Then 4 times thinner cabling. Half as much charge controller. Etc etc, you get the idea. 12v loads can be largely avoided with 24v pumps, 24v fridge etc. Any strays use 24-12 converters.

I can of course get a 24v pump when the one I have goes and the fridge will be a top loading chest freezer/fridge conversion (100w draw after the first second or two for a total of 500w per day) otherwise I would really like to keep the 12v stuff I have until I can replace it with 24v things as I go.

We try not to label pv as 12, 24v etc, as it hides important differences in panel parameters. The important one of which Vmp. If you have a particular reason to avoid mppt controllers then you need Vmps in the 18v/36v ballpark. (36/72 cell). With mppt, the vmp is less critical, however you do need to check that a comfortabe mulitple of panels in a string will work for the given controller. As a rule 60/72 cell panels are quite a bit cheaper than 36 cell panels (grid tie market). Its hard to go wrong with 60cell panels and a good controller like say midnite kid, for your application.

I have my eye on the Midnite Classic 150 MPPT at the moment though I will make certain I'm getting off grid panels and not grid tie panels.

Oh and one other question, if I have an MPPT controller does it matter if I get panels designed for grid-tie or off grid? If so, what specs do I want to pay attention to to be certain I'm getting panels that will work with the Midnite Classic and a 24v batter bank.

Thanks a lot for the response and helping me in the right direction. If anybody else has any thoughts or follow up (to my revised thinking) ideas I would be super thankful for the input.

Cariboocoot

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

For RV use the more expensive mono panels are sometimes worth it for that "how much can you fit" reason. An MPPT controller is a must if you use GT style panels because their Vmp is not correct for battery-based systems. If you buy panels with Vmp 30 (typical of GT style) they will work through an MPPT controller on a 12 Volt system in parallel, but you would need two in series for a 24 Volt system (which should have a Vmp around 35 or better for proper charging).

Which brings up another RV issue; panel shading. You can't always have a big array all pointed in the same direction with nothing blocking any of them. As such parallel panels tends to work better as you don't 'knock out' two (or more) panels with the shading of one.

You don't necessarily have to avoid a 12 Volt system, just avoid having five batteries in parallel. Look for larger capacity 6 Volt batteries and put two in series instead of two 12 Volts in parallel, so to speak. RV is one of the few applications where staying with 12 Volts is sometimes a good idea.

This is a case where figuring out what you can fit on the roof and how takes precedence over other design factors. And you may have to resign yourself to PV just helping extend battery rather than fully recharging it depending on how much PV you can fit vs. how much battery capacity you need/want.

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

Cariboocoot wrote: »

For RV use the more expensive mono panels are sometimes worth it for that "how much can you fit" reason. An MPPT controller is a must if you use GT style panels because their Vmp is not correct for battery-based systems. If you buy panels with Vmp 30 (typical of GT style) they will work through an MPPT controller on a 12 Volt system in parallel, but you would need two in series for a 24 Volt system (which should have a Vmp around 35 or better for proper charging).

Which brings up another RV issue; panel shading. You can't always have a big array all pointed in the same direction with nothing blocking any of them. As such parallel panels tends to work better as you don't 'knock out' two (or more) panels with the shading of one.

You don't necessarily have to avoid a 12 Volt system, just avoid having five batteries in parallel. Look for larger capacity 6 Volt batteries and put two in series instead of two 12 Volts in parallel, so to speak. RV is one of the few applications where staying with 12 Volts is sometimes a good idea.

This is a case where figuring out what you can fit on the roof and how takes precedence over other design factors. And you may have to resign yourself to PV just helping extend battery rather than fully recharging it depending on how much PV you can fit vs. how much battery capacity you need/want.

Due to my roof (needs some work anyway), some health issues and other factors I have decided I'm going to remove my rooftop AC unit, the skylights, the antennas, and the vent for the propane fridge (I won't be using it). I would like to keep the vents up there to get some fresh air in but I may be able to actually mount them under the panels as I'll never need them fully open.

At that point, affordability is going to be my biggest issue, I can fit 1kw up there no issue at all as the 250w panels I'm seeing are about 70"x40" or roughly 12w per sqft. That would be busting my budget right now but doable with the ability to add another 250w down the road.

I can be into the system for about $1,800 everything included and I already have the 5 AGM 12v batteries so provided I keep those it's money I can spend elsewhere.

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

Okay, I did a ton of research on running the batteries in parallel and from what I can tell found it more dependent on how they're wired than just a simple "bad idea" thing. So, is this guy off his rocker or is it still just a "bad idea"?

http://www.smartgauge.co.uk/batt_con.html

I am open to a 24v system if the gains of running a 1,000 watt 24v inverter will net me more than the losses of running a 24v-12v step down to accommodate my 12v needs. Both are fine with me but having 5 batteries I would like to make use of them all things being equal.

BB.

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

We use the smartgauge link all the time... I personally do not like Method #4--It makes things too complicated and difficult to measure current per battery (or battery string).

Method #3 would be my first choice--Easy to put a fuse per battery string, disconnect a string to work on, easy to put a DC clamp meter on each string to ensure proper current sharing during charging/discharging.

If all you need is a 1,000 watt AC inverter--12 volts is OK (24 volt is fine too--Sometimes it is difficult to find a 1,000 watt 24 volt inverter--they tend to have higher wattage units at the higher battery bus voltages).

-Bill

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

BB. wrote: »

We use the smartgauge link all the time... I personally do not like Method #4--It makes things too complicated and difficult to measure current per battery (or battery string).

Method #3 would be my first choice--Easy to put a fuse per battery string, disconnect a string to work on, easy to put a DC clamp meter on each string to ensure proper current sharing during charging/discharging.

If all you need is a 1,000 watt AC inverter--12 volts is OK (24 volt is fine too--Sometimes it is difficult to find a 1,000 watt 24 volt inverter--they tend to have higher wattage units at the higher battery bus voltages).

-Bill

Okay, awesome! So I can make use of my 5 batteries in parallel using method #3 without an issue? This would give me 625ah @12v instead of 250ah @24v or if I only use 40% of that 3000 versus 2400 watts per day.

When it's time to go with new batteries I'm all for converting to 24v and buying the panels and CC to allow for that but right now would be much less convent of a time for me. Also, configuration #3 would allow me to remove one from service with ease should the need arise.

Thank you!

BB.

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

Of course, there is the issue that lots of parallel strings of batteries require you to keep an eye on the them to make sure that you don't have problems with current sharing (open/shorted cells, bad electrical connections, battery temperature variations, etc.).

My personal recommendation is ~1-3 strings in parallel.

I would suggest a DC Current Clamp Meter/DMM--It makes it much easier to monitor how your batteries are doing. Just measure the current during heavy charging/discharging once a month to make sure that nothing is going "wrong".

-Bill

Cariboocoot

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

It's still a bad idea even though it can be done because it introduces a lot of extra resistance that would not be present in a system with fewer parallel connections. And every one of those extra wires and connections is a resistance that can increase if it gets loose or corroded, even slightly.

What is more 12 Volt systems will suffer the effect greater than higher Voltage systems because the introduce extra resistance would be the same Ohms for any Voltage but more significant with lower Voltage.

It isn't putting a problem into the system to begin with so much as asking for a problem to occur.

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

BB. wrote: »

I would suggest a DC Current Clamp Meter/DMM--It makes it much easier to monitor how your batteries are doing. Just measure the current during heavy charging/discharging once a month to make sure that nothing is going "wrong".

-Bill

Thank you for the input and I will get one of those meters.

Cariboocoot wrote: »

It's still a bad idea even though it can be done because it introduces a lot of extra resistance that would not be present in a system with fewer parallel connections. And every one of those extra wires and connections is a resistance that can increase if it gets loose or corroded, even slightly.

What is more 12 Volt systems will suffer the effect greater than higher Voltage systems because the introduce extra resistance would be the same Ohms for any Voltage but more significant with lower Voltage.

It isn't putting a problem into the system to begin with so much as asking for a problem to occur.

I see the problems which is why I'm seeking a solution. The 5 batteries would each have a 1 foot lead and another foot to get me to the charge controller for a total of 11' of wire total. Are you saying the resistance added will negate the additional 125ah of the 5th battery?

I'm just trying to understand the best way to attack the problem, I can do a 24v system but the only known in this is the batteries and the 5th wheel as I already have those. I can do four batteries but removing the things that would be equal no matter what like loose connections and corrosion all the other batter adds is a 20% higher chance of those issues and 2 feet of wire.

I'll do more research but I've yet to find a viable solution which is of course what I'm after. I'm also trying to add key words in describing this so others can find it (and hopefully a solution) in Google searches.

BB.

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

In general, you want the wires from the charge controller to the battery bank to to short and heavy (roughly 0.05 to 0.10 volt drop at maximum charging current for 12 volt battery bus).

You want the "longer cables" from the charge controller to the solar array (hopefully, that is your ~11' of cables).

-Bill

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

BB. wrote: »

In general, you want the wires from the charge controller to the battery bank to to short and heavy (roughly 0.05 to 0.10 volt drop at maximum charging current for 12 volt battery bus).

You want the "longer cables" from the charge controller to the solar array (hopefully, that is your ~11' of cables).

-Bill

I worded that poorly, I meant based on the #3 diagram in the above link I would be looking at 5 batteries x 2 terminals each at 1 foot long leads on each terminal for 10 feet total for the interconnecting wires and another foot to get those leads to the charge controller.

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

I have been doing a ton of reading on resistance and please correct me if I'm wrong but it seems like the gains I would get with a 24v system would be in the ease of converting 24v to 120ac for the inverter (which costs more) and less loss to the things being run off of the battery bank as they use slightly thinner wiring to the lights and so on.

The cons would be 24v items are 2x to 4x the cost of 12v items and there would be a loss with the DC-DC step-down converter as well.

In terms of the battery jumpers, they are needed in any configuration so that's a wash but from what I can see, with say 12 combined feet of #2 wire (I already have 100 feet of it from other applications) for all interconnects and wiring between the CC and batteries I'm looking at 0.0012 ohms total. That isn't a difference between batteries however but total for the wiring in the battery bank and that number would be 0.0002 between the CC and any given battery terminal which if not mistaken would result in a 0.16% voltage drop between the CC and batteries?

So from what I'm seeing, the biggest gains in the 24v system is in the ability to run the higher voltage from the panels to the charge controller where with #8 wire you're reducing your loss on a 20 foot run from 4.7% to about 2.35% or the ability to stay under the acceptable 3% loss with #9 wire instead of #6 wire.

This could totally be some GW Bush math so feel free to correct me on this but that's what I'm picking up on from this thread.

BB.

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

Pretty much it... More or less, American Wire Gauge changes cross sectional area by a factor of 2x for every 3 gauge steps.

So, doubling the voltage (12-24 volts) cuts the current by 1/2. And you have ~2x more voltage drop at 24 volts vs 12 volts (~2 volt wring drop vs ~1 volt wiring drop).

1/4 the cross section of wire (roughly) or ~6 AWG gauge drop (use a 12 AWG wire at 24 volts vs 6 AWG at 12 volts).

Also, I like to keep the nominal current at 100 amps maximum (load/charging current)--To keep the copper wire diameter to a "manageable" size... 100 amps * 12 volts = ~1,200 watts.... 24 volts * 100 amps ~ 2,400 Watts.

There are other issues too--But, in general it is voltage drop and copper wire sizing. 24 volt wiring/components should not cost much more than 12 volt (other than 12 volt stuff for cars/trucks/boats is more common and can be cheaper).

Because a true deep cycle battery system runs at a pretty wide voltage range (from ~10.5 volts to ~15.5 volts or more for a 12 volt battery bank)--It turns out that much of the 12 volt DC appliances really do not work well on that wide of range (they are typically designed to run on a car electrical system from 12.0 to 14.2 volts or so)--I usually like to suggest using an AC inverter to run all of your loads (120 VAC, 10x the voltage, 1/10th the current, ~15 AWG smaller wire gauge for same wattage loads) and let the AC inverter give you "stable voltage" for your appliances (many 120 VAC appliances have similar power consumption to 12 VDC versions--Due to the Energy Star/conservation programs out there these days).

-Bill

unyalli

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

TucsonAZ wrote: »

a) Can I use a 24v to 12v DC step down to address my 12v needs (e.g. LED lighting, fans, radio, cell phone chargers, pumps) without any issue or major losses?

b) If I run two batteries in series for 24v and parallel them together for a 4 battery 24v 250ah bank, what am I going to do with the additional 12v battery?

Get a victron energy orion 24 - 12 volt converter and connect it from your 24 volt bank to the remaining 12 volt battery and connect your rv 12 volt to this battery. Set the output voltage to around 13.4 and the single 12 volt batt will always be topped off and provide any surge requirements like a pump starting.

-Jeff

TucsonAZ

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

unyalli wrote: »

Get a victron energy orion 24 - 12 volt converter and connect it from your 24 volt bank to the remaining 12 volt battery and connect your rv 12 volt to this battery. Set the output voltage to around 13.4 and the single 12 volt batt will always be topped off and provide any surge requirements like a pump starting.

-Jeff

Now this is a damn good idea, I can run the inverter at 24v to get the up side to that and I still get all of the advantages of having the extra amp hours on the 12v side. Is there any down side to this other than I see it's only 96% efficient. The converter would be acting as a charger and step-down pretty much correct? Would I have to worry about the 24v bank always wanting to equalize with the 12v battery? This seems like too easy of a fix to be viable.

-Brew Town native (as a side note)

Cariboocoot

Re: Some beginner questions on an RV system, 12v or 24v panels (and all that goes with i

TucsonAZ wrote: »

I see the problems which is why I'm seeking a solution. The 5 batteries would each have a 1 foot lead and another foot to get me to the charge controller for a total of 11' of wire total. Are you saying the resistance added will negate the additional 125ah of the 5th battery?

No. I'm saying that lots of wiring in the battery strings means lots of resistance which may not be equal between all strings. As such one ends up doing most of the work while others contribute varying amounts and all of them die prematurely; some due to handling most of the load and others due to sitting around relatively inactive.

I'm just trying to understand the best way to attack the problem, I can do a 24v system but the only known in this is the batteries and the 5th wheel as I already have those. I can do four batteries but removing the things that would be equal no matter what like loose connections and corrosion all the other batter adds is a 20% higher chance of those issues and 2 feet of wire.

I'll do more research but I've yet to find a viable solution which is of course what I'm after. I'm also trying to add key words in describing this so others can find it (and hopefully a solution) in Google searches.

If staying with 12 Volts is required you can use larger capacity batteries rather than multiple parallel strings.

Otherwise there's this explanation for different system Voltages: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power