Because of Excel's 15-digit precision limit, which was implemented to preserve accuracy, long numbers are displayed as scientific notations. Conversion in scientific notation is how Excel deals with that limit.
Again, when opening (importing) your CSV data into Excel, use a special Import Wizard to control how Excel converts your data. For a step-by-step guide, please read this article
So, basically, the only way to keep numbers in their entirety in Excel as they appear in your CSV is to format the column containing this data as text. As a result, you'll be able to view your data in Excel with the full number present, with no weird formatting.