Before you go to a lot of trouble, hook up jumper cables from another automobile or charge the battery overnight and see if the extra boost allows some of the systems to come back online. If they do, then you know for sure it's your battery. Replace it. This is a common problem with the Cayenne. The battery has to be at its highest performance or this auto will let you know it with all kinds of crazy electrical gremlins and error/fault messages. This happens well before the battery is even close to dead. A strong start means nothing. If you have short commutes, hook up a battery charger on the weekend a couple times a month. Otherwise be prepared to purchase a new battery every 2 years or less.