A swollen battery is your only urgent sign.

Replace it when the battery swells or when you can’t get through a normal workday without plugging in. Everything else is just annoyance.

Apple’s battery health percentage is a rough guideline, not a rule. I’ve seen MacBooks at 80% health that still last fine for browsing and email. I’ve also seen ones at 85% that randomly shut down. The number is useful, but it’s not the whole story.

The real signs: trackpad clicking weirdly (swelling pushes it up), the case feels slightly bulged, or you need the charger with you everywhere. If you’re checking battery cycle count, you probably already know it’s time. Most Airs are good for 500-1000 cycles, but age matters more than cycles. A three-year-old battery at 200 cycles might be worse than a two-year-old one at 400.

Don’t wait for swelling to get bad. If the trackpad clicks unevenly or the bottom case won’t sit flat, stop charging it and get to a repair shop. That’s a fire risk. Otherwise, just replace it when it stops meeting your needs. You’ll know.