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If your battery has been left discharged for any time then the plates may have sulphated. Or if it's a poor quality battery the plates can lose material which falls to the bottom of the case and effectively shorts some cells out. Get a good battery and find out why the sat nav is draining it. Rewire it to an ignition on circuit.
The battery I have is a Halfords Premium yoke, cost a fortune (I was ripped off imo after being told off for buying it on another forum) and it comes with a 5year warranty. I really don't want to pay out for a new battery when this one is still in warranty but unless I can prove the battery is at faulty it doesn't seem like i have much choice.