Optima xm6 alarm fault

I installed the alarm myself with advice from friend who installed his own alarm,it has lasted around 16 years trouble free ( same battery). Then alarm went off early hours,zone 4 highlighted which is conservatory doors. Everything was ok so I bought new magnetic contacts and replaced internal battery, switched off mains to test external battery ( can't remember what I did now ) as suggested on another forum which indicates ok.
Everything seemed OK for a month then same thing happened again, always zone 4 !.
No tamper light just zone 4 light. I'm thinking ext battery MUST be faulty after 16 years so that maybe problem. Any suggestions, thanks.