Updating 12V Offgrid System -- move to 24V or new charge controller?

PorkChopsMmm

I am looking for guidance on how to grow my existing 12V offgrid system. I purchased my inverter and batteries used off of Craigslist and it is time to buy new batteries and at the same time I would like to buy 3 additional 240 watt panels. We are thinking of moving out to the cabin where all of this solar is located and even with extended vacation stays I think I need to buy new batteries, at least. Also, with winter coming, I would like to up my number of solar panels to help charge the batteries in the hard winters in Michigan zone 5 with the panels at roughly 85 degrees tilt.


Below are some specs on my system:
- Xantrex 2.5 Prosine Inverter/Charger w/ battery temp sensor and remote ACS panel
- Outback MX60 MPPT Charge Controller w/ battery temp sensor and MATE
- 3 Chinese 240W solar panels wired in parallel (Vmp) 30.0V, (Imp) 8.0 A, (Voc) 36.0 V, (Isc) 8.96 A
- 8 55amp hour 12V gel batteries (old and tired)


I have identified some Duracell GC2's at Sams Club (Thanks Sam!) that are 6V @225 amp hours for ~$79 a piece + $9 core charge. My plan was to use these batteries regardless of staying with 12V or upgrading to 24V as the price is right and they are easily replaced.

Option 1
Buy 3 additional 240 watt panels, an additional Outback MPPT 60 charge controller , and some number of batteries. This would give me roughly 1440 watts of solar panel -- I was thinking of 12 batteries, configured as 12V, to give me 1350 amp hours. I understand that this configuration would be pushing the boundaries of 12V but I think overall it is the least expensive upgrade, but is it the best long-term solution?

Option 2
This one is more expensive... buy 3 additional 240 watt panels, a new FX2524T 24V Outback inverter/charger and HUB as I already have a MATE, and the same 12 batteries configured as 24V, which would give me 675 amp hours.

Option 3
This one is just like Option 2 but with a FX3048T 48V inverter and buy 16 of the 6v batteries to make a 48V bank that has 450 amp hours.

Which solution would you choose? As a disclaimer, I don't see myself adding more to the system in the near future although it would nice to not be limited by charge controller or inverter if I wanted to add more panels or alter the battery configuration.

And I have a few additional questions...
1) The 3 additional 240 watt panels may have slightly different volts and amps -- could I run 2 series of slightly different panels to the MX60 via a combiner box? I remember reading on this site that if the Imp and Vmp were within 10% of each other than I would still have good efficiency. An aside, at 24 volts I think I would have again reached the limit of the MX60 with the 6 panels.
2) I need to locate the panels roughly 300 feet from the power shed where the charge controller, inverter, and batteries live for better access to unshaded sunlight in the winter months. I have used the voltage drop calculator spreadsheet and if I run 2 strings of 3 panels each in a series it looks like I can get away with using 4 gauge wire to run the 300 feet and still be below 3% loss... actually right around 2%. Any comments?

This site is a great help and I am looking to take my off-grid system up a notch.

Cariboocoot

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

Well I don't like Option #1 because in my opinion 1350 Amp hours of 12 Volt battery is too much current-wise. Coming up with 135 Amps of charge current would require a 2.5 kW array and two charge controllers.

So let's look at Option #4. What do you anticipate for loads? Really, and I know it sounds like a broken record, you have to have a number for daily Watt hours to size a battery bank with.

You've got 720 Watts of panel right now. I use a similar size array to recharge 320 Amp hours of scrap metal ... er, I mean batteries on a 24 Volt system. I would/will go up to 1400 Watts if/when I can (they stopped making the panels so it's "do the math" time).

So if you build your 1440 Watt array what can you expect from it? Check the PV Watts numbers, but under good "typical" conditions it would be about 2.8 kW hours AC per day. Can you live with that?
You could run two parallel strings of four of those batteries for 450 Amp hours @ 24 Volts and only discharge about 25% which would balance fairly well with the panel capacity.

You could do a similar arrangement with a 48 Volt system, keeping all batteries in series.

As for mixing dissimilar panels, the rule-of-thumb is to keep the Vmp's within 10% for parallel connections and the Imp's within 10% for serial connections.

I get the same 4AWG requirement for your 300' wire run with two parallel strings of three panels in series max on the MX60. Obviously a MidNite Classic 200 would help here, but it probably isn't worth $800 to go up a wire size. Any chance you could move your power shed closer to the array and run the 120 VAC for the extra distance?

PorkChopsMmm

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

Cariboocoot wrote: »

So let's look at Option #4. What do you anticipate for loads? Really, and I know it sounds like a broken record, you have to have a number for daily Watt hours to size a battery bank with.

So if you build your 1440 Watt array what can you expect from it? Check the PV Watts numbers, but under good "typical" conditions it would be about 2.8 kW hours AC per day. Can you live with that?

I used the worksheet below and 2.8kW a day should be enough to get by -- I am coming in around 2.4 kW per day. Not much room for expansion of electrical use but my wife is on board with only using appliances during daylight hours. All of my lightbulbs are LED, all fans are energy star, all appliances will be energy star apartment sized or thereabouts as well.
http://www.evsolar.com/worksheet.html

Cariboocoot wrote: »

You could run two parallel strings of four of those batteries for 450 Amp hours @ 24 Volts and only discharge about 25% which would balance fairly well with the panel capacity.

So you think a 24V bank of 450 amp hours, 8 batteries, would be sized appropriately for a 1440 watt array? Is there a specific ratio for this? I have 2 generators up at our location that I can use to charge the batteries. I want to have an appropriately sized bank so I am not reliant on the generator but I don't mind firing it up every now and then either.

Cariboocoot wrote: »

I get the same 4AWG requirement for your 300' wire run with two parallel strings of three panels in series max on the MX60. Obviously a MidNite Classic 200 would help here, but it probably isn't worth $800 to go up a wire size. Any chance you could move your power shed closer to the array and run the 120 VAC for the extra distance?

This isn't too feasible. Our cabin is on the south end of an open field that receives substantial shade from the tree line. I would like to put the panels on the northern edge of the field with a clear shot to the southern sky. Currently they are ~50 feet out in the field and they get more shade in the early morning hours then I would like. I also happen to have a couple hundred feet of 2 gauge wire I got for free. Not sure on the exact length but I may just run the 2 gauge and call it a day.

Cariboocoot

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

The usual "ballpark" calculations I use:

1440 Watts @ 77% efficiency = 1108 Watts "usable" / 28.8 Volts charging = 38.5 Amps.
38.5 Amps * 100 / 450 Amp hour = 8.5% charge rate, which is above the 5% recommended minimum.

The Icarus Formula:
1440 Watts * 4 hours equivalent good sun / 2 = 2880 Watt hours of AC per day.
You will run lower than that on darker days. That's what generators are for. If you look at the PV Watts prediction for your area and decided you'll have too many "generator days" then you need to up the array size or reduce the power usage. It's usually easier to do the latter. Certainly cheaper.

450 Amp hours of battery @ 25% DOD = 112.5 Amp hours * 24 Volt = 2700 Watt hours maximum.
Note that does not include the conversion to AC loss which is largely dependent on your inverter.

The numbers are close. If your panels can carry you through the day while charging the batteries it will work. This is very similar to my planned expansion, only I have 700 Watts right now which perform at 80% to 82% efficiency due to the high elevation.

If you've got enough 2 AWG on hand to make your panel run, go for it. Certainly cheaper than buying a new charge controller.

PorkChopsMmm

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

Cariboocoot wrote: »

The usual "ballpark" calculations I use:

1440 Watts @ 77% efficiency = 1108 Watts "usable" / 28.8 Volts charging = 38.5 Amps.
38.5 Amps * 100 / 450 Amp hour = 8.5% charge rate, which is above the 5% recommended minimum.

The Icarus Formula:
1440 Watts * 4 hours equivalent good sun / 2 = 2880 Watt hours of AC per day.
You will run lower than that on darker days. That's what generators are for. If you look at the PV Watts prediction for your area and decided you'll have too many "generator days" then you need to up the array size or reduce the power usage. It's usually easier to do the latter. Certainly cheaper.

450 Amp hours of battery @ 25% DOD = 112.5 Amp hours * 24 Volt = 2700 Watt hours maximum.
Note that does not include the conversion to AC loss which is largely dependent on your inverter.

The numbers are close. If your panels can carry you through the day while charging the batteries it will work. This is very similar to my planned expansion, only I have 700 Watts right now which perform at 80% to 82% efficiency due to the high elevation.

If you've got enough 2 AWG on hand to make your panel run, go for it. Certainly cheaper than buying a new charge controller.

Thank you for the replies. This is extremely helpful. Is going to a 48V inverter out of the question? Any reason not to?

Cariboocoot

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

PorkChopsMmm wrote: »

Thank you for the replies. This is extremely helpful. Is going to a 48V inverter out of the question? Any reason not to?

You can go 48 Volt if you want to. In this instance there's no reason not to, as the OB 3648 is the same price as the 3524. Basically you'd just be swapping current for Voltage on the battery bank. If you want to be picky about it there is a tiny efficiency improvement that comes with that (from reduced DC current). Probably 0.5%. :roll: Gives you lots of expansion capacity too (maximum battery bank size on the MX60 @ 48 Volts works out to about a 14kW hour capacity).

PorkChopsMmm

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

Cariboocoot wrote: »

You can go 48 Volt if you want to. In this instance there's no reason not to, as the OB 3648 is the same price as the 3524. Basically you'd just be swapping current for Voltage on the battery bank. If you want to be picky about it there is a tiny efficiency improvement that comes with that (from reduced DC current). Probably 0.5%. :roll: Gives you lots of expansion capacity too (maximum battery bank size on the MX60 @ 48 Volts works out to about a 14kW hour capacity).

Great, thank you. If I go 48V and stick with the same number of batteries mentioned earlier, 8, that would give me a 48V 225 amp hour bank. If that number of batteries was fine at 24V would you see any reason for me to add more batteries in a 48V set up?

niel

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

putting my 2 cents in here i'll say minimally that a 24v battery bank would be recommended. you could go 48v with no problem, but this is a minimum of 2 pvs in series to charge the battery bank with and i suspect the need for 3 in series due to your long wire run to reduce the v drop. i don't recommend you go to 4 pvs in series for each string as you reach the critical area of the controller.

the best efficiencies will be had with a 48v battery bank and will lessen wire losses some too. the pv vmp to battery voltage ratio is lower so that improves the cc efficiency too not to mention that the cc will operate with higher watts at 48v. 48v x 48a = 2304w in pv by nec standards for 1 controller so this lowers your cost by negating the need for a 2nd controller as 24v x 48a = 1152w in pv by nec standards. the controller cutoff is 24v x 60a = 1440w which would be borderline for your proposed 6 240w pvs at a 1440w with no room for more later. losses may swallow that borderline margin some so this isn't out of the question if one goes against the nec's wishes. as to the inverter wattage, the added watts of the 3048 gives a tad more room to expand your load capacity maximum, but smaller wattage ratings are workable and teach how to manage one's power better. even the best managers of power tend to increase their loads so this buffer is good in going with the 3048.

as to your wire, with 72v nominal @ 8a per string with 2 strings feeding the main wiring run i get 4.23% using #4 copper wire. using #2 copper wire i get 2.66%. now i don't know if you have 300ft of that #2, but using 1 wire of #4 and 1 wire of #2 both at 300ft of length gives a 3.44% v drop. none of these are great, but the all #2 is the only one less than 3% and some recommend under 2%. if you went with #0 so as to use 3 strings of 3 pvs later you'll get 1.67% v drop with the 2 strings in the meantime.

also note that 2 strings won't need a fuse or circuit breaker, but any expansion will need them. going to a 24v or 48v pv strings will have more than 2 inputs to the controller so there must be fuses/breakers and these will be combined and then go to the cc.

just to show what it would be if you added another future 3 pv string with 72v nominal v at a 24a total imp and this represents 9 of the 240w pvs your site and the following are the v drop percentages,
all #4 = 6.34%
all #2 = 3.99%
all #0 = 2.51%
1/2 #4 and 1/2 #2 = 5.16%
1/2 #2 and 1/2 #0 = 3.25% (just in case you have enough #2 and buy a 300ft length of #0)
1/2 #2 and 1/2 #00= 2.99% (as above, but you buy a 300ft length of #00 to improve the v drop even more)

unfortunately, there is no getting around the resistive losses of a long wire run and i don't recommend skimping on the wire if you can help it. at least you wouldn't need another controller with a 48v battery bank so maybe some of the saved $ could go to some heavy wire.

Cariboocoot

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

PorkChopsMmm wrote: »

Great, thank you. If I go 48V and stick with the same number of batteries mentioned earlier, 8, that would give me a 48V 225 amp hour bank. If that number of batteries was fine at 24V would you see any reason for me to add more batteries in a 48V set up?

No; the potential Watt hours is the same between 450 Amp hours @ 24 Volts and 225 Amp hours at 48 Volts. As Niel & I both said, there is a small improvement in efficiency with the 48 Volt system over the 24. The big concern is still the 300' run from array to charge controller. That is nasty.

PorkChopsMmm

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

Thank you all for the guidance. So here is what I think I need, most of which I will be buying from NAWS. I didn't have the funds to buy this a year ago when I started but I wish I could get the all in one of the Flexpower preassembled kits.

1) Outback FX 3048T (this will be in a shed so I figured I would go with vented -- no reason for sealed) http://www.solar-electric.com/fx3048t.html



2) Outback 4 port HUB to connect the inverter and my existing MX60 http://www.solar-electric.com/hub4.html



3) Not from NAWS, but I picked up 3 240 watt panels last night from a guy who deals in new uninstalled one off panels. The three panels and the necessary cabling was $900 cash. I think I did alright.

4) 8 Sam's Club GC2 6V 225 amp hour batteries. I already have my current battery bank set up for 8 batteries so I shouldn't need any additional cabling. From the Smart Gauge website I will use Method 3 -- which I believe is normally recommended here. I am assuming for a 48V bank I would have alternating positives and negatives from the batteries going to the positive and negative copper buss bars?
http://smartgauge.co.uk/batt_con.html

5) Pole mount -- I am stuck on this one. I was hoping to just buy the pole mount and enough aluminum struts to build the frame that holds the solar panels myself. So far I have only seen aluminum struts that are custom built for the panels you have e.g. Sharp, etc. This may not work for me as I have 6 somewhat similar panels (in amperage and voltage), I basically have 3 pairs of 2 like panels. Has anyone done a pole mount where they have built the aluminum "rack" to suit their needs?

Thanks again, I am at peace with the 48V set up that will allow me to grow in the future.

BB.

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

If you have "bugs" in the building, you may wish to double check the screening.

We have lost a few inverters here due to warm environments drawing in little nesting creatures.

-Bill

Cariboocoot

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

An Outback FX3048T is sealed. The VFX 3648 is vented.

You got a good price on those panels, but be aware that "one-off" panels may be particularly difficult to find matches to if you want to add more later.

PorkChopsMmm wrote: »

4) 8 Sam's Club GC2 6V 225 amp hour batteries. I already have my current battery bank set up for 8 batteries so I shouldn't need any additional cabling. From the Smart Gauge website I will use Method 3 -- which I believe is normally recommended here. I am assuming for a 48V bank I would have alternating positives and negatives from the batteries going to the positive and negative copper buss bars?
http://smartgauge.co.uk/batt_con.html

Um, if you have eight 6 Volt batteries there's only one way to get 48 Volts from them: all in series. There will be no "alternating" posts or buss bars.

Custom-built panel rack is possible. You just have to buy the aluminium stock that suits your panels and carefully drill the holes. Okay, it's somewhat more complicated than that. But it's also hard to advise on by "remote control".

PorkChopsMmm

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

Cariboocoot wrote: »

An Outback FX3048T is sealed. The VFX 3648 is vented.

Thank you, I went with the vented model.

Cariboocoot wrote: »

Um, if you have eight 6 Volt batteries there's only one way to get 48 Volts from them: all in series. There will be no "alternating" posts or buss bars.

Thank you too for this clarification. Brain lapse.


Here is a screen shot of my predicted line loss. I would just like to verify I am doing it correctly. 300 foot run, 48V battery bank, 2 strings of 3 panels. Going off of this info, if I did it accurately, I will change my plans and use 4 gauge wire instead of 2 gauge wire to save costs.

Cariboocoot

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

Niel can best tell you if you're using that right. I think the array Voltage should be the Vmp (90) or even the nominal (72), not the Voc as Voc isn't the Voltage present when the circuit is complete.

I still get 4AWG for the 300' run.

Not sure the post controller wires need to be that large, though.

niel

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

first thing wrong is that should be calculated with the temp at 90 degrees c. we are talking temperatures the wires can reach and not what your typical ambient temps are. nec usually has the wires rated to 90 degrees c so that is a good point to place it at for high temps increase the resistance of the wires some.

2nd thing wrong is the 300ft length for the wire total. a 300ft run length means 600ft of wire as you have 2 wires going that 300ft length, one positive and one negative. you must add the lengths of the + and - wires together to get the total length in feet. you have to watch how you handle this stuff as for example when doing the pv interconnections as you can forget to add the wire lengths between the seriesed pvs as they do add to the v drop too.

although not necessarily wrong, the 3rd thing was i used an approximate nominal voltage of 72v for those are 24v pvs with 3 in series. you could use the vmp of the pvs and it will show a lower v drop % than the nominal voltage. never use a voltage higher than the vmp of the pvs though.

as i roughly calculated all of this for you in a previous post, a red flag should've arose when your figures were nowhere near mine.

another note is that the wire from the controller to the batteries is critical as a voltage drop here could fool your controller into thinking the voltage is actually much higher than it is due to the voltage drop. you list a wire total length as 5ft, but if that is 5ft for both the + and - wire that the total is actually 10ft and the voltage drop is double what you are calculating it too be with just listing 5ft. add to the fact you used a different temp and array voltage and it is artificially swinging the v drop % much lower than it really is. i can say it to be around about .2v rather than your rounded .08v if you made the error on the total wire length for this section. fortunately .2v isn't terrible for that wire section.

PorkChopsMmm

Re: Updating 12V Offgrid System -- move to 24V or new charge controller?

niel wrote: »

as i roughly calculated all of this for you in a previous post, a red flag should've arose when your figures were nowhere near mine.

Neil, thanks for the help! The temp and distance info was good. My apologies for not referring to your earlier post. After seeing how I will need a long run of cable and costs associated I am going to keep my panels closer to my power shed until Spring/Summer and come up with an alternate plan. I may build a small shed out on the northern end of our field which will give me great southern exposure and allow me to just run AC to my cabin.

I was able to get the VFX3648 in the mail, the HUB, the Flexnet DC battery monitor, 2 500 amp shunts, and other needed supplies like gloves, face mask, hydrometer, and battery ON-OFF switch in the mail. I should be all good to go for wiring everything up this weekend. I will take pictures and post them next week.