The only batteries I've found with more usable capacity that might fit in my existing battery compartment are of the Lithium chemistries. Here's a site that peeks my curious side:
12V RV Batteries | Deep Cycle Lithium Ion Batteries | Smart Battery
I've been messing with various Lithium battery types for many years and really like the usable capacities but I've had mixed results for longevity. I've actually had the best results with the more volatile chemistries.
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