since you already replaced the battery and it still shows the low charge it could be the connector but it could also be related to a bad PMIC . You will have to check those things out. Ultimately I’d start with a new charging board and re-evaluate after that. This shows how to replace the board. If you have the tools and the knowledge to test, you can use this schematic for reference.
Start with replacing the charging board. It may just resolve your issue. If you replace it and it does not resolve the issue, then you will have to consider board level repair. Soldering a small board with some really small components requires some good tools and good skills. You would need to post some good pictures of your boards with your question 기존 질문에 이미지 추가하기 so that we can try and help with identifying components etc.