IPod Issues!

Hello, I recently traded in my 2005 550i for a 2007 M5, awesome car! Before trading in I removed the integrated I Pod ( came with the car) from the glove box and replaced the 6 CD auto changer. Since picking up my M5 I did the opposite, removed the auto changer and popped in the iPod unit. Seemed straight forward and worked just as it had in the 550i. However, after fitting the car stood for a day and when I started it I got the " high rate of discharge" message. Car started no problem. Popped the auto changer back and no warning? For info the car had been sat idle at the dealer for a few weeks and needed a jump first time I test drove it. Could it be that I need to give the battery a good charge? Not had much time to give the car a good run out yet. Any ideas!