Depending on where you source your batteries, it could very well be poor quality batteries. However, a defective charge circuit could also damage the batteries although I’m a bit skeptical that it could cause as much degradation as your seeing.
If this is happening on your own device, it will be hard to troubleshoot because Ideally you need to have either a known-good battery or known-good device to properly troubleshoot this issue. An independent repair shop may be able to help you.