Battery light flickering on the dashboard, alternator or just a loose belt?

My trusty old Corolla has started showing the battery warning light flickering intermittently, mostly at idle and sometimes when I'm stopped at signals. It goes away when I rev the engine slightly or when I'm driving at normal speeds. The car starts fine and I haven't had any dead battery issues yet, but I know that flickering light is trying to tell me something and I'd rather address it now than get stranded somewhere. From my basic understanding it could be the alternator starting to fail and not charging properly at low RPM, or it could be something as simple as a loose or worn serpentine belt that's slipping at idle. I checked the belt and it looks okay to my untrained eye but I know they can wear without looking obviously cracked. Was browsing around trying to understand the difference between alternator problems and belt problems and stumbled on https://carbatteryreplacementsharjah.ae/car-alternator-dynamo/ which has some good information about diagnosing charging system issues. My question is whether a mobile battery service can test the alternator output properly or if that's something that needs a workshop visit with the car on a proper diagnostic machine. Also curious what a typical alternator replacement costs for a basic Japanese sedan, trying to mentally prepare for the worst case while hoping it's just a belt tension issue. Anyone else had a flickering battery light that turned out to be something simple rather than a full alternator replacement?

A flickering battery warning light at idle is often an early sign of a charging system issue, but it doesn't always mean the alternator needs replacing. A good mobile battery service can usually test the battery, alternator output, and charging voltage on-site, which is often enough to identify whether the alternator, voltage regulator, belt, or battery is causing the problem.

Catching it early can save you from a breakdown later. On a different note, if you're interested in modern EV technology, it's also worth learning about the driving modes in Cadillac Lyriq, as they show how advanced vehicle systems can optimize performance and efficiency compared to traditional drivetrains.