I think the problem is what LJS alluded to. No battery maintains its capacity forever. If it's 10 hours when new, then it might only be 8 hours in 6 months to a year. 5 hours after 2 years. I think it needs to start with at least double the capacity you want, so that it still has the actual capacity you want after 2 years. That is, of course, just personal preference. But, I want to have enough capacity to do 2 normal (but full) days of diving in a row without having to charge. That allows me adequate margin if I forget to charge it one night. Having 2 days capacity, after 2 years, means it needs to start somewhere around 4 days of capacity when new. At least, that's what I would want. My H3 is rated for 15 hours, I think, and it is fine, but I would not want it to be any less. I normally charge it after every 2 or 3 days in a row of diving. But it's still relatively new.
And obviously, how many hours there are in a "normal full day of diving" TOTALLY depends on the diver and the dives they do. I figure very roughly that 3 hours a day (average) would cover me for multiple days in a row. Often it will be less. But sometimes it could be more.
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