Battery draining so alarm goes off

I keep charging the battery of my 370z Nismo nearly every night but after I have driven for a short while, then park up and lock, the alarm goes off every ten minutes.
There is an odd thing happening, if I put the car in reverse and the reverse camera comes on, the in-car battery volt metre reads 14, when I take it out of reverse, it goes down to 13. Please help, is this an electrical fault, how can I fix this- problems always seem to happen just after the warranty runs out.
Thanks,
Jo