OGM,
Trojan’s new warranty for solar energy applications is seven years. Accordingly, five years out of your Trojan T-605’s (“small†6 V deep-cycle batteries made for floor scrubbers) may be a bit on the short side. As Bill indicated, depth of discharge can affect battery life. Other factors that can affect battery life include charge voltage settings, maintenance, and ambient temperature.
Trojan recommends fairly high voltage settings for their batteries. The best standard TriStar 12 V battery charging setting for the “short†Trojan batteries is probably setting #6 (14.8 V Absorb, 13.4 V float, and 15.3 V EQ; manual page 27). A PC is required to use Trojan’s exact settings via the TriStar’s “custom†setting (#8 ). A remote battery temperature sensor is also a good idea.
Periodic equalization is also an important maintenance procedure for flooded-cell batteries. I didn’t see any discussion of this essential maintenance step in your posts. Here’s a link to Trojan’s on-line maintenance manual:
http://www.trojanbattery.com/Tech-Suppo ... nance.aspx
A battery bank with a large number of parallel strings can also be problematic. I suspect that five parallel strings is a large number, no matter who you ask. However, good, heavy interconnecting cables, and "diagonally opposite" connections of the main cables, can alleviate many performance problems associated with many parrallel strings.
When looking for new batteries, a smaller number of larger batteries (i.e., L-16's or 8D's) would allow for fewer parallel strings and battery-to-battery interconnects. Another advantage to larger batteries is that there would be fewer cells to maintain.
Finally, it can get a tad warm in Alabama. Optimal battery operating temperature is 77 F, and higher temperatures reduce battery life, so do what you can to keep the batteries cool and especially out of direct sunlight.
niel brought up a good question: How big is your PV array (Watts STC)? Similarly, how big (V x Ah) is your new battery bank? The issue is that a small PV array and/or large loads can prevent proper battery recharging. Such "deficit recharging" can contribute to reduced battery performance and longevity.
In summary, I’d recommend you program the TriStar to Crown’s recommended voltage settings, that you follow Crown’s maintenance recommendations (including periodic equaliization), that you consider larger (and therefore fewer) batteries, that you keep the batteries cool (you may need an insulated battery box), that you use an RTS, and that you try to limit average discharges to ~20% of bank capacity.
HTH, and good luck with your new batteries!
Jim / crewzer