Yr 20, you should be on, at least, your 5th battery.
Just because they are rechargeable, it doesn't mean they last forever.
That is only a guide, might need replacing sooner than every 5yrs (BUT, no longer)
Change it using your code, just remember there is a mains supply inside the control panel.
p.s.
Search, top right of every page would have found this question asked / answered dozens of times.
acorn, August 2011
