If you leave off after a certain point, I don't think you will get it going again.
When the battery reaches "empty", the iPod is forced into sleep mode. The battery actually holds a reserve beyond "empty", to maintain the iPod in sleep for a period of time. Once the battery is truly exhausted, the iPod is forced to shut down. At this point, any open files could be lost. Therefore, it is important that you find an electrical outlet and connect the adapter before the forced shutdown occurs.
That MAY NOT be 100% the case, but what I have read, that's what I got out of it..