re: "
true deep cycle batteries"
Don't get me started on this one ...
If you can come up with an objective and pertinent and useful measure that will discriminate between a "true deep cycle" and a "deep cycle" or even an SLI battery for that matter, please let me know. I've been looking for years and haven't found such measure.
Of course, I am thinking about those batteries commonly used in RV's and readily available at retail. No submarines, specialty purpose thingies, traction batteries, or other such truck, please.
Some things to keep in mind: All lead acid batteries suffer if subject to routine deep (80% DoD) discharging. Cycle life isn't an issue for typical RV use. Energy density by weight for lead acid batteries is within a narrow range.
There is so much money and effort wasted in seeking a magic bullet when it comes to batteries, so many unsupportable myths and assertions, that it is really rather sad. Think and read critically, depend upon good measures that mean something, and watch out for platitudes, sales and marketing labels and hype, and following the herd over the cliff of non-reason.