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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.
I no longer have a old phone but I found that disabling bluetooth and mobile data when your not using it can help with battery drain.
You can also go into developer settings and set the maximum backround process limit to 3 and that does a good bit on its own.