So, from the SportIdent website:
"The small internal battery in your SIAC is guaranteed for 2 years and should last 3 years."
They also have a form for checking the age of your card and estimating its battery condition: https://www.sportident.co.uk/equipment/siac_battery_status.php
Its easy to turn a spare SI unit into a battery test box, we'd also just need to print off a sign with it to explain what the various beeps (or lack thereof) mean. Maybe with the preference nowadays to run every event as SIAC, we should consider having one at our events?
Also - I think a SIAC should operate as a normal punching SI card, even if its battery is flat - it shouldnt just stop working completely.