I’m having the same problem. My mac is from 2011, but still runs fine otherwise. I replaced the battery at the end of last June (2020) and the battery is giving the service battery warning and has very low battery life, even on integrated graphics only. I contacted iFixit support about a warranty and think you should do the same.
They responded and told me to run the following steps:
Calibrate the battery: https://help.ifixit.com/article/265-batt...
Repair and verify Disk Permissions: How to verify and repair Mac OS X Disk Permissions you are running MacOS El Capitan or newer (MacOS 10.11+), please follow this guide: How to Run First Aid with Disk Utility
Reset the NVRAM/PRAM: How to reset NVRAM on a Mac
Reset the SMC: How to reset the SMC on a Mac
Run CoconutBattery and send us a screenshot of the including the Battery Info page.
Provide a screenshot of the Activity Monitor after following this guide: Apple Laptop Battery Troubleshooting
After that, I guess we’ll see what they do.
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