If you still have an older iPhone and have not yet gotten the battery replaced, you have until December 31 to get it replaced by Apple for just $29.
Last year, it was discovered that Apple was unknowingly throttling iPhone performance in order to prevent unexpected shutdowns on devices with old, degraded batteries. The fix was rather easy: just replace the battery.
As a goodwill to keep customers happy, Apple started offering discounted battery replacements for devices as old as the iPhone SE and as new as the iPhone X. Normally $69, you can get the battery replaced in your iPhone for just $29, but you only have until the end of the year. Afterward, the cost will likely jump back up to $69 for out-of-warranty service.
If you’re not sure if your iPhone needs a battery replacement, you can check the battery health by going to Settings > Battery > Battery Health. Anything over 80% is pretty good, but if the percentage is lower than that, you would likely benefit greatly from a battery replacement.
You can also replace the battery in your iPhone yourself if you feel up to the task. In older iPhones, it’s not terribly difficult—it just takes a bit of time and patience.