I've searched but cannot find whether Mountain Mode requires a minimum battery charge level in order to recharge properly. I accidentally discharged to 0 miles electric range over the weekend. After placing my 2013 model into Mountain Mode, the engine RPM increased as expected and the gray battery segments built to the red horizontal line. It took around 20 minutes. When I returned to Normal Mode, the battery showed 0 miles and the engine continued to run in CS mode as if nothing had changed. Is this designed behavior or do I have a problem?