Problem with Battery/Alarm on Cayenne - Help Please

[majordad]

Came back from 3 weeks away recently and the battery had gone flat on my 2004 Cayenne S. Only fitted a new Halfords Bosche Silver last September. Charged it up two weeks ago and car is behaving odd. Normally when I lock it with the key the indicators flash twice, not its four times ! Also the alarm sometimes sounds when I open it. Is it the battery or any ideas ? I have the Porsche Extended Warranty til December when it finally runs out after 10 years so cant bring it to OPC with an electrical pproblem and a non Porsche Battery.