With every new phone, we almost always ask "what will the battery life be" - of course we want something that will go a full day in case we can't charge halfway thru or maybe we are out & about and not near a charger or battery pack. The Galaxy S8 line will not have a removable battery but that should be no surprise, Samsung put that horse to pasture several phone releases ago 
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SamMobile has compared battery life of the S8 & S8 Plus to the S7 and S7 Edge - the results may surprise you as the S7 / S7 Edge topped in a few areas while the S8 / S8 Plus topped others. Of course, everyone's usage varies and these results shouldn't be taken to heart until we see devices out in the publics hands with real customer usage.
Here is the article if you want to see the info...
Here’s how Samsung’s battery stats for the Galaxy S8 and S8+ compare to the S7 and S7 edge
...SamMobile has compared battery life of the S8 & S8 Plus to the S7 and S7 Edge - the results may surprise you as the S7 / S7 Edge topped in a few areas while the S8 / S8 Plus topped others. Of course, everyone's usage varies and these results shouldn't be taken to heart until we see devices out in the publics hands with real customer usage.
Here is the article if you want to see the info...
Here’s how Samsung’s battery stats for the Galaxy S8 and S8+ compare to the S7 and S7 edge