If 690 watts is to be the only charging source, and no more can be fit, I would suggest 4 gc-2 batteries for 440 to 464Ah total capacity.
If one is also going to have the ability to plug in or fire up a generator in the morning and recharge with a 60+ amp charger then I would consider 6.
How long it takes for amps to taper until the level they can be considered fully charged will always be changing.
The variables as to how much it changes, are:
1 depth of discharge
2 how many cycles have accumulated since last true full recharge, and or Equalization charge
3. the amperage rate at which the batteries achieved absorption voltage
4. battery temperature
5. battery overall health
6. initial battery quality
.
How much electricity you use in the morning into early afternoon will play a big part in how soon your batteries get to absorption voltage. I generally try and avoid using power in the morning when I know there might not be enough time in the day to hold ABSV as long as required.
it is best to get to absV as early as possible so the solar can then hold Absv for as long as required. Once ABSV is achieved, then there will be more and more, as time goes on, excess electricity one can utilize to do other tasks. a 600 watt cooktop used in the morning can extend the time it takes to reach absV by hours.
When the batteries are at their lowest first thing in the morning, they can also accept the highest amperage, most efficiently, with little juice lost to battery heating.
The full recharge each and every day is highly desirable, but not always likely to occur. One can learn to tailor their usage by how much sunlight, or other charging sources are expected to be employed the next day. When many days have accumulated only getting to 95% or less, then one should try and be frugal overnight and plan on getting to ABSV asap by whatever means available. many days without a full recharge can significantly extend the time required in ABSv before amps taper to ~2% of battery capacity, or full as determined by a hydrometer, not by the blinking light indicating float voltage on a solar charge controller.
A Vehicle driven early morning with a thick copper path to house batteries can accomplish a lot in the goal of achieving absorption voltage early. Idling unmoving, just to recharge should be limited to 10 to 15 minutes as the alternator can overheat without underhood airflow.
So I say 4 or 6 gc-2 golf cart batteries with that much solar, and you will soon find out if you have enough solar, or need to implement other charging sources, or use less electricity.
The theoretical ideal of AH to solar wattage ratio is different for everybody's system and uses, and changes frequently.
I am of the opinion that many people go for too much battery capacity for the warm and fuzzies/ bragging rights. Lesser depth of discharge does increase battery longevity, but going below 50% here or there is not a death sentence. more important to recharge fully, and promptly, than worrying about occassionally slipping below 50%, especially with golf cart batteries. Now if one were using 12v marine batteries or the less expensive AGM batteries, I say it is more important to not regularly discharge them to the 50% and below region.
The Solar alone approach will not work for everyone. No way could I have 500 deep cycles on my current AGM and have it retain so much capacity at 3.5 years of age, if I could not blast it with high amperage from alternator or plug in charging souorce. Even when my dedicated house battery was a quality group 31 marine battery, it responded favorably to high amperage early in the morning, and much less favorably to a low and slow ramp up to Noon's maximum solar amperage. Even if there was enough daylight to achieve full by afternoon, the days where the battery was blasted with high amperage in the morning, had the battery holding higher voltages during than nights discharge.
I intentionally limited battery capacity in an attempt to charge the deeper dischartged battery at a higher rate, and did achieve a better $/cycle ratio. I've gone from 345AH total capacity 10 years ago, to just 90AH now.
Also with the early morning high amperage blast, the time required to be held in absorption was less , and lesser absorption voltages could be used.
If no higher amp sources are employed, and 50% discharges are regular, then the solar wattage/battery capacity ratio should really be at the 2 to 1 or bigger.
But with a limited size solar array, and that as the only charging source, I would personally not get a huge battery bank just to limit the depth of discharge. There is a balance and the correct balance cannot be determined until it is in actual use. The GC-2 batteries are the best bang for the buck, so 4 or 6 is likely your decision. 4 now and 2 more later is only viable, if later is no more than a month or so, and I would not parallel the other 2 until performing an Equalization on the first 4.
Adding new to old batteries is far from ideal, for the batteries, but it is not an instant death sentence by any means. IF you suspect 6 might be required, and want to start with 4 first, then make sure you have the room and funds to add 2 more within a month/30 cycles or so.
Each pair of GC-2s is 120 Lbs minimum. Depending on the size of the rig and how it is driven, this weight can impact fuel economy, and what might be extended in battery longevity is lost in fuel to move the rig over the life of the batteries, saving no money, possibly.
It is hard to not overthink it, but sometimes one just has to draw that line in the sand, and make it work.
Your questions are the right ones to be asking and show a lot as to your ability, So I would not really freak out or chastise yourself, you are doing well.
If the batteries capacity is proving to not be enough, then try and add more charging sources/use less electricity. When they are too compromised that it impacts one's quality of life, then add more battery capacity when the batteries need replacement.
Usually people are 'just fine' for X amount of time, before they notice the batteries are not just enough. When this is noticed the battery capacity is already compromised, and hail mary recharges in an attempt to regain lost capacity is too little to late, but the knowledge gained during the hail mary attempts add many more cycles to the next set of batteries.
Sometimes you just got to say, Well I'll just have to see how long these batteries will last as I will use them.
No matter what, they will not last forever, even if taken care of perfectly. Often the efforts to maximize longevity costs more $$ and time than simply resigning oneself to replacing them more often.
Basic Summation, achieve absorption voltage as early as possible every day. Do this and the batteries lifespan should be acceptable to good, even if brought to below 50% occassionally. Acheive and hold absorption voltage for long enough 3 days out of 7 will yield good to great cycle life, but those 3 days might require a lot more time than expected.
When the batteries are really down low, and have been low for many days on end, then efforts should be made to ensure they can achieve a true full recharge, and that will require using less electricity overnight, or a longer drive in the morning, or an AC grid powered infusion of higher amps.
Really hard working batteries can often require 12+ hours at ABSV before specific gravity gets back up into the green, so giving them a break by plugging in and letting a grid powered charging source give them a good long massage can usually reset them to an earlier younger age. Hard worked batteries that do not get the 100% recharge are getting ready to jump off of a cliff, but they start running downhill before jumping.
A person who watched a voltmeter, hopefully with an Ammeter, and even better, with an Amp hour counter, will easily notice when the voltage the batteries maintain during discharge is not what it once was. A quick drop off in overnight voltage is a sure sign the time is at hand to ensure a true full recharge, and possible employ an Equalization charge on them.
The EQ charge requires that one bring the batteries to as high as 16.2v, but only after the batteries have spent the required time in the mid to high 14v range. 16.2v is damaging to some electronics and it is best to disconnect loads from the batteries during an EQ charge.
EQ charging is its own subject/thread. EQ charges are hard on a battery. How often they are required is highjly variable and only able to be determined with a hydrometer, but one noticing poor overnight voltages can know the EQ charge requirement is approaching/here. EQ charges are required more often the harder worked the batteries are. They can restore lost capacity, but not always.
Ony EQ flooded batteries. AGMS requiring a capacity restoration requires a different procedure and much more careful observation during the EQ. All but one AGM manufacturer(Lifeline/Concorde) recommend against such procedures.
