Good points Almot. I am not aware of the lighting characteristics of cheap controllers. Mine (BS 2512i)is solid for bulk, slow blink absorb, fast blink float.
My wording should have been... just because the charge controller indicates a full charge, does not mean the batteries are 100%.
If the goal is to get the solar controler to indicate a full charge, I could get there a half hour after sunrise every day, and replace batteries several times each year. I could brag about the 1/2 hour and not mention the necessary replacement, and do a serious disservice to anybody less than informed and reading about such results. An Extreme example for illustrative purposes, but one must be wary of all internet claims.
Cheap controllers might not allow absorption voltage adjustments. If said controller only allows 14.4v on a battery that needs 14.8v, the full charge indicator means very little, and the observer should be made aware of this, rather than place blind trust in any given and perhaps very well marketed product, and claim excellent performance, when all they can realistically claim is they experienced no failures, yet, and the system Seems, to be perfect.
Trojan's PDF lists a 10 to 13% bulk rate, but does not say this is a recommended minimum. Besides, Solar ramps up slowly and does not conform to any of the usual charging profiles which apparently assume a flick of the switch charging source delivers instant bulk amps to a battery resting somewhere below 80% SOC.
Regarding my own apparently Acid starved Screwy 31, well I wont be replacing it, when necessary, with another 31. However my previous house bank, 2 27's which were cycled shallower hardly gave me an excellent lifespan either. Their location made hydrometer dipping a serious inconvenience, and I remained ignorant of how my Solar was actually doing replenishing them daily. I trusted my soothing blinking green light on my charge controller way more than I should have, and want to make others aware that they can be making this same mistake.
If I get a year of daily cycles from this 31, I'll have a better return on investment than I did with 2 different sets of 2-27's.
I believe, in my situation/usage that higher wattage to capacity ratios, and having figured out the voltage setpoints and durations on this screwy 31, will yield better return on investment.
But I can't say for sure until December. Lately I've been taking the poor thing to 40% or below nightly, making it work for me, and I have not been having enough Solar wattage or time to see much of my blinking green light, not that I believe it anyway.
I do believe that higher solar wattage to capacity ratios will always make the batteries happier for longer, but the person who sizes a system to replace only the capacity removed, +10%, and only cycles them for a week or 2, and then goes home and plugs in, will likely never notice the difference another hundred + watts will make.
Perhaps the timespan of the outing requiring daily replacement of overnight usage should be factored into recommendations. Each battery user is going to use/cycle their batteries differently and this difference requires different recommendations. There are few absolutes in life, yet many make well worded statements in absolute terms which can easily do a disservice to the Novice who takes them as gospel, and perhaps worse, repeats them as gospel, convincing others to follow the same path.
To the OP, in your case I'd go with GC batteries over the 31, and if 100 watts of solar is all you have to recharge during your outing, then look it is as negating some of your usage, but that when you get back home, you should have a good method of insuring you can return those batteries back to maximum Specific gravity before storing them.
