Hi, not 100% certain but on my gtd if the fob battery goes flat then you can remove/unclip a small part of the door handle to reveal the keyhole for the key, so after opening the door you then put the key fob against the side of the steering column and apparently the car will still be able to recognise the key and then you just press the start button…….presto.
ps, if I’m mistaken then just ignore everything I just said…Thanks.