Just going to chime in here to reinforce what ghurd said about the charging voltages being the biggest factor in the lifespan of an SLA.
I used to let them 'cook' at a higher voltage too until I found out what the skimpy (if even present) labels on a battery are really trying to tell us.
Most SLAs indicate something like 15V for cyclic use, and 13.5-13.8 for standby use.
That's great and all, but ONLY if you know how to properly interpret the info.
The problem is, any significant gassing that occurs is NOT a transient event like it is in a flooded battery. In FLA, bubbles form, and are displaced by the electrolyte when their buoyancy is sufficient to take them to the top of the cell to be vented. The electrolyte returns to the surface of the plates, and everybody is happy.
This does not happen in an SLA, because of the absorbent separators (or gel, depending). The gas gets trapped, combines with other bubbles, creates a large pocket, and nothing short of a miracle brings the electrolyte back in contact with the plates (read as "ain't gonna happen"). The result is a permanent loss of capacity.
The "cyclic use" they refer to means that avoiding the gas voids involves shutting the charger down completely when this voltage is hit. Most SLA chargers don't have a provision for this, and the result is the battery gasses itself to death. And you don't want to lollygag your way TO that 15V either. Use current limiting, and "ride it hard and put it away wet". This is the key concept. Wish someone would have explained it to me years ago...
The same concepts apply for standby, but again, the "how long" part is not made clear by the label. The range is given to account for this somewhat, but its too vague for most people to avoid capacity loss from improper charging methods. The rule is, the longer you expect it to sit on float, the lower the voltage needs to be. A few hours? 13.8 wont hurt anything. A few days? 13.8 will dry it up by a few percent (as pointed out with the UPS issues, typically 13.75). 13.5V is a much better holding voltage where a week or more is expected. Prorate for in between. If more than a couple of weeks is expected, take it down to 13.2 or so (after its full, ie <C/500 current at 13.8V). So for a 7AH battery, this would be 14 mA.
Don't go straight from a "cyclic" charge to a float either, the gassing will continue to the point damage will still be done.
Even for cyclic use, I personally don't take it above 13.7V. Its better to not quite hit completely full than to over charge and gas it to death. I fill my SLAs by going to 14.1V and holding it for an hour about once every 10 cycles, or after a deep (<50% remaining capacity) discharge. This helps counteract the tendency toward sulfation, but doesn't hold it for so long at an elevated voltage for the gassing to kick in significantly.
Another important factor in SLA lifespan is charge rate. I've learned the hard way to keep it at C/20 and lower; higher than that and you're still gassing, before the battery even hits full!
The trick then becomes recovering your losses on a daily basis. Obviously, a C/20 rate means you have more battery than "sun" (since to slow the charge rate, you need longer days). The workaround is to size your bank (or panels) so that your battery is 20x your panel output in full sunlight, and only use the top 30% or so of the battery. With charging losses, this means about 8 hours of sun per day. This would be adjusted according to your weather, use, and location, but only to a degree - don't go too much higher than C/20 for your rate. To account for non-ideal circumstances, have more input available, but use current limiting. The above example also doesn't take into account powering any loads during the day - too many variables to consider here, but for a continuous load, only 10x the battery is a closer match, but then current limiting is a must if you want to have the batteries hang around for a while.
Something else that I have learned, but is difficult to keep track of, is that capacity loss is a self perpetuating problem, since what is important is not what the label indicates the capacity is, but what the EFFECTIVE capacity is. The label is only of use when the battery is new. Once it begins to lose capacity, the charge rate needs to be lowered to account for the reduced capacity, or the effective charge rate will increase, further perpetuating the failure.
Moral of the story is, as someone else here put it (can't remember now who) - SLA needs to be babied and RE is not naturally suited for the job. Despite this, many (myself included) have a successful SLA system up and running.
All of the above considerations are of an "ideal" nature. Many systems have some compromise in the design to balance complexity, cost, and requirements. In my case, my system is based on a single large flooded battery (8D, 220ish AH on 60W of panel), which then in turn is used to charge the SLA assortment. Its not the most efficient way, but I don't have problems with SLA damage anymore either. There are multiple ways to approach this, many probably better than my methods, but eh, whatever.
FWIW
Oh yeah - almost forgot - ghurd's connection method is the best, second only to the "multi equal length common tie point" thing. Common point is superior, by far, but when you have that many batteries tied together, the wiring can become a total clusterfsck and lends itself readily to a migraine with an itch for something to go wrong (or worse!) if detailed wire management is not employed...
Steve
