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Why Charging Your Gadgets Over 80% Is Such a Bad Idea | iFixit News
(www.ifixit.com)
This is a most excellent place for technology news and articles.
Waterproofing is a lame excuse that I won't accept from these manufacturers. It may be not as easy as just permanently gluing the thing together, but it's definitely possible to have a sealed battery compartment.
For example cameras have been weatherproof for decades now. And you can both change the batteries and plug a bunch of stuff in them no problem.