I Have A 2 Lead Pacemaker.Have Been Told The Last

I have a 2 lead pacemaker.Have been told the last 2 times that I have less then 3 months left on the battery.I am100% paced on my lower chamber and 65% on my upper.Last week or so have been feeling tired and run down.Nobody seems to be able to tell me when its time to replace the battery.The pacemaker is a Boston Scietific s603.It is 7yrs old.How do you know exacaly when to replace this life saving device?