If one AirPod regularly reaches low battery before the other, it does not always mean the battery is failing. In many cases, one side is simply doing more work.
AirPods can use either earbud as the microphone, and that can affect battery use during calls, voice messages and Siri requests. If your AirPods are set to use a specific side for the microphone, that earbud may drain more quickly than the other. You can check this on iPhone by going to Settings, tapping your AirPods, then opening Microphone and selecting Automatically Switch AirPods.
The difference can also come from how you wear them. If you often use a single AirPod while the other stays in the case, or remove the same side more frequently, the charge cycles between the two earbuds can become uneven over time.
Apple also uses separate batteries in each AirPod, so small differences in age and capacity can become more noticeable after years of use. A minor gap is normal, but a large and consistent difference may point to a worn battery or charging problem.
Before assuming the battery is at fault, clean the charging contacts in the case and make sure both AirPods are properly seated when charging. If one side is not making reliable contact, it can start each listening session with less charge than the other.
It is also worth checking whether Automatic Ear Detection is behaving properly. If one AirPod incorrectly thinks it is still being worn, it may remain active longer than expected.
For most people, a small difference between left and right AirPod battery levels is nothing to worry about. If one side is repeatedly dying much earlier, changing the microphone setting, checking the case contacts and resetting the AirPods are good first steps before considering service or replacement.