Chasbag

I have 2005 V8 and remote fob only works occasionally.. I have to enter using key which sets off alarm until I put key in ignition. Remote will lock car after I’ve driven car and also works if I return within 30-60 minutes. Otherwise it means using key with alarm sounding etc. I was told there is a small auxiliary battery somewhere under fuse box that needs replacing. Any help would be gratefully accepted.