E39 won't start

[The Stiglet]

Haven't used the car in two weeks, which shouldn't really be a problem but it won't start. It just clicks instead of cranking. There is power as the wheel moves up and down when turning the key, interior lights, fan and tv etc but I just get the feeling that there just isn't enough power to start it.

What do you guys think? Get a new battery? And is it a straight forward swap or are there things to worry about with the ECU etc?

Cheers

[E30M3SE]

Battery is flat, it has some power but not enough to crank the engine over.

[Sump]

Straightforward swap.

It's an E39!

[The Stiglet]

Thanks guys!