Battery Capacity

The batteries appear to be a GEL type battery designed for long term (10 year life) Float/Backup service. They do not appear to be designed for daily cycling (solar).

https://www.enersys.com/sbsxl/
https://www.enersys.com/WorkArea/DownloadAsset.aspx?id=25769807496
The recommend a 10% rate of charge... And the charging voltage recommended is pretty low--That is common for GEL batteries (to prevent gas bubbles from forming in the GEL/Plate boundaries which can permanently reduces capacity). 

I did not see a voltage set point for cycling applications in the documents I looked at.

You should probably contact Enersys customer support and see what they say.

Most solar charge controllers are programmable for charging different battery types. In this case, the spec. sheet seems to be set really low for charging (2.27 volts per cell @ 25C or 13.62 volts for 12 volt bank) which may be for float charging (again, no cycling voltage charge points are in the spec sheet).

Until you get feedback from Enersys support, probably not a good idea to raise the charging voltage (GELs typically are charged at 14.2 volts for cycling applications--But not recommended for cycling apps).

260 Watt array is pretty small for this bank (if your array is 1x 260 Watt solar panel)... The charging current (best case, solar noon, cool/clear days):

• 260 Watt array * 0.77 panel+controller deratings * 1/14.0 volts charging = 14.3 amps typical max charging current

A 760 AH Lead Acid battery bank would be ~76 Amps charging (at 10% rate of charge per specifications), or at least 38 Amps (5% rate of charge).

And to run just the refrigerator, I would be suggesting a 413 Watt array would barely keep up "in summer" with just the refrigerator. A 1x 260 Watt panel--Your bank would slowly lose average state of charge without a backup gasoline generator (deficit charging).

For the connection of 8 AWG for 5 feet from charge controller to battery bank @ 40 amps (if you get a larger array), the voltage drop would be:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=14&phase=dc&noofconductor=1&distance=5&distanceunit=feet&amperes=40&x=0&y=0

Voltage drop: 0.25
Voltage drop percentage: 1.79%
Voltage at the end: 13.75

That is really too much voltage drop where 0.05 to 0.10 volt drop recommended, not 0.25 volts (if you use 40 amps charging). At 14.3 amps the voltage drop would be:

Voltage drop: 0.090
Voltage drop percentage: 0.64%
Voltage at the end: 13.91

8 AWG cable is just heavy enough (bit less than 0.10 volt drop max suggested).

For a 40 amp rate of charge, the minimum AWG recommended is 4 AWG at 5 feet:

Voltage drop: 0.099
Voltage drop percentage: 0.71%
Voltage at the end: 13.901

4 AWG is pretty heavy cabling--If you can shorten the cable length to 2.5 feet or less (and you can use smaller AWG cable--1/2 the distance then ~ +3 AWG "lighter" wiring).

At this point--Not sure that your present (GEL?) battery is the right pick. The 260 Watt array (?) is too small for your loads, and your wiring from the charge controller to the battery bank is too long/too small for a 40 Amp charging current (too much voltage drop).

-Bill

PS: What is a GEL Lead Acid battery:

https://batteryuniversity.com/learn/article/bu_201b_gel_lead_acid_battery
Again--Contact EnerSys (or your local battery supplier) to figure out the charging voltage needed for your cycling battery bank (I am NOT a battery engineer).

-BB

Generally, we aim for 1% to 3% maximum voltage drop... That is a 1% to 3% (or a bit more) loss of power...

For example, if your fridge draws 4.3 amps @ 14 AWG wire @ 20 Feet, the voltage drop would be:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=12&phase=dc&noofconductor=1&distance=20&distanceunit=feet&amperes=4.3&x=0&y=0
Voltage drop: 0.43
Voltage drop percentage: 3.62%
Voltage at the end: 11.57

It is a bit more drop than I would suggest--But it does work (and wastes somewhere around 3.62% to 7% of your solar energy with the 14 AWG cable... Not the worst thing in the world...

Regarding to switching to 24 volts... The basic power equation is Power=Voltage*Current

So, if you double the voltage, then the current will fall by 1/2... And be the same amount of power (but you need 2-6x heavier cable to carry that same amount of power at the lower voltage).

Sending a lot of power at 12 volts for longer than a few meters--Gets difficult--Expensive copper cable to keep voltage drop down.

Your fridge is not using that much power, but at 20 Feet, it would have been better to use heavier cable. And that is only 42 Watts... When you need to send 420 Watts (like a small well pump), 12 volts becomes pretty difficult to even try and send the power any distance.

You sort of did the cart before the horse here... You installed your load and solar system without really doing much in the way of detailed design (details matter a whole bunch in solar power).

For example, your 12/24 VDC refrigerator is a pretty rare load--Works at both 12 and 24 volts just fine... It will take:

• 42 Watt compressor / 24 volts = 1.75 Amps nominal
• 42 Watt compressor / 12 volts = 4.3 Amps nominal

So in the "solar power" point of view, the power usage is the same. But now a 14 AWG cable @ 20 feet carrying @ 4.3 amps @ 24 volts, the voltage drop would be:
Voltage drop: 0.18
Voltage drop percentage: 0.74%
Voltage at the end: 23.82

And your voltage drop/losses using the same 14 AWG cable is much less at 24 VDC.

Regarding the battery bank... No, the batteries will not charge any faster. However, instead of a 760 AH battery bank with a 76 Amp rate of charge (10%) @ 12 volts, the same energy storage (2s x 2p) is now 380 AH and 38 Amp for 10% rate of charge...

Besides saving the cost of copper cabling (80 amp cable around 4 or 2 AWG cable (heavy, expensive, difficult to bend, may not fit terminals easily), whereas 40 amp cable would be around 10 or 8 AWG... Much easier to wire (and cheaper), easier to send same power longer distances, etc.

Also 80 amp solar charge controllers are not cheap (around $600 USD for higher end MPPT controllers). Instead, you can use your existing 40 Amp controller on the 380 AH @ 24 volt battery bank (charge controller are rated for maximum current at 12/24 and perhaps 48 VDC battery bank). So, for the same price charge controller, at 24 volts your controller can support a 2x larger solar array and get your 10% rate of charge (the 40 amp @ 12 volts, only ~5% rate of charge).

But, there are things you have to watch for. Most solar charge controllers are "Buck Type" digital switching power supplies... They can only take higher voltage (lower current) from the solar array and efficiently down converter to the lower battery bank voltage (and higher charging current--Remember Power=Voltage*Current -- Power stays the same, voltage from input to output for MPPT controller--And raises current to battery bank).

This means you have to look at the Vmp-array... For a 24 volt battery bank, that means you need a higher voltage array than a 12 volt batter bank would. For an MPPT type charge controller, the Vmp-array minimum name plate voltage would be:

• 29 volts charging * 1.3 MPPT operating factor = 37.7 volt array Vmp minimum suggested (24 volt bank)
• 14.5 volts charging * 1.3 MPPT operating factor = 18.85 volts Array Vmp minimum suggested (12 volt bank)

Your panels are probably around Vmp~30 volts... So one panel is "enough" for a 12 volt battery bank... For a 24 volt battery bank, you would need 2 panels in series minimum. You have to look at your charge controller Vpanel input specification... Probably 100 VDC (?). So 2 in series is fine, 3 panels in series would "over voltage" the MPPT Controller input.

Note that running your Array at higher voltage Vmp is usually a good thing too... Lets you use a larger array with smaller copper cabling (and you can send more power longer distances with higher voltage array).

There are other details to watch for... Your 260 Watt panel may no longer be available--When you add panels in series, you need to match Imp panel to within ~10% or better.

When you put panels (or strings) in parallel, you need to match Vmp (or Vmp-string) to 10% or better too...

You need to look at the Vmp/Imp specifications for your panel and if you get (for example) 3 more panels (2s x 2p), you need to check the specifications closely (i.e, there may be a 265 or 270 Watt panel with very similar Vmp/Imp specs... And that is fine). However, matching a 260 Watt panel with a 320 Watt panel may not be practical.

Anyway--At this point you need more solar power to run the fridge... Your battery bank is a bit on the large size, but your array is too small to run the fridge through the summer reliably without using a genset during bad weather/occasionaly to bring up the bank charge...

To give you an idea how the battery math works. We normally suggest 2 days storage and 50% maximum discharge for a Lead Acid battery bank. Using 24 volt bank, a suggested "nominal" size battery bank for your fridge would be:

• 1,008 WH per day * 2 days * 1/0.50 max discharge * 1/24 volt battery bank = 168 AH @ 24 volts

So, yes, you could use just 2x 12 volt @ 190 AH batteries for a 24 volt @ 190 AH battery bank... But if you already have the other two batteries, I would keep them (GEL batteries charge slower, so using a larger battery bank is helpful here).

Using a smaller battery bank is not really helpful here--Unless you have need for those two batteries elsewhere. Based on the battery bank size, the "nice" 10% rate of charge array would be:

• 380 AH * 28.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,382 Watt array nominal
• 190 AH * 28.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 691 Watt array nominal

The array size based on your 1,008 WH per day refrigerator load (do you have any other loads such as LED lighting, cell phone charging, laptop, etc.)? Nomrally, suggest that your "base load" be only 50% to 65% of your "predicted harvest" (i.e,. other loads you can turn off during bad weather, run a genset if needed):

• 1,008 WH per day * 1/0.61 DC off grid system eff * 1/4.0 hours of "summer sun" = 413 Watt array "break even"
  
• 413 Watt array * 1/0.65 base load fudge factor = 636 Watt "nominal" array for summer fridge operation

OK, you have some choices here... Strip out 2 of the batteries and install a ~636 Watt to 691 Watt array (very nice match to battery bank and your fridge power)... Or you can keep the 4 batteries (2s x 2p) for the larger battery bank and run a 691 Watt (5% rate of charge) to 1,382 Watt array (10% rate of charge for larger battery bank).

With the larger array and battery bank, you could easily run ~2x or larger loads (AC inverter, laptop computer, water pump, TV, cell charger, etc.)...

The largest (cost effective) array your 40 amp controller can run on a 24 volt battery bank would be:

• 40 amps * 29.0 volts charging * 1/0.77 panel+controller deratings = ~1,506 Watt array "typical" cost effective maximum

A 1,382 Watt array on 4.0 hours of sun per day would generate a minimum average:

• 1,382 Watt array * 0.61 DC off grid system eff * 4.0 hours of sun minimum (summer) = 3,372 WH per day

That is a good amount of power for a cabin/small home... Almost a "near normal" electrical existence.

You have some choices to make... 12/24 volts, 2 or 4 batteries, how large of an array, add AC inverter (slightly less efficient--0.51 AC off grid system eff vs 0.61 for DC off grid system eff).

I am making lots of guesses here---And you need to decide what your loads will be. Having a refrigerator is a pretty good size load for an off grid solar power system. Most people do want more power for LED Lighting, cell/laptop charging, possible Water pump, perhaps even a clothes washer... You are right on the cusp of supporting those loads...

-Bill

You are very welcome foilcap.

And feel free to start your own discussion/thread if you have further questions about your own system.

Sincerely,
-Bill