The following applies to SharePoint 2013:

1.   Restarting ECS will force a wipe and reload of PowerPivot databases.

2.   Any service account needing to interact with the PowerPivot data model needs to be an Administrator on the PowerPivot AS instance.  This is less obvious when you are in a client application but consider these scenarios:

              a.   PowerView client (in Excel) > SQL Reporting Services service application needs to be an admin

              b.   Performance Point > Performance Point Service application needs to an admin

3.   This table shows backward compatibility for workbooks in SharePoint 2013 and what service application performs a scheduled data refresh.  Interactivity refers to the user-level interactivity allowed with the PowerPivot data model.  Only Excel 2013 workbooks allow for full permissions, which equates to full back-end connection-based data refresh in browser (which means writing/updating the data model in PowerPivot-AS):

 

* Note:  Data refresh schedules for PowerPivot 2012-Excel 2010 workbooks, the PowerPivot System Service account requires modify rights to the workbook in SharePoint as well as admin rights on the PowerPivot-AS server.