How to fix battery connection issue?

There are several hardware faults that can cause this behavior, none of which are simple to fix, though you can find lots of such solutions by doing a search on YouTube for How to fix Battery connection issue on Oppo A3s" The problem would be diagnosing which problem it is and the varying degrees of difficulty and equipment needed to fix them once diagnosed. However, it might not be your hardware at fault so go to Oppo's support page where they tell you the steps you can take short of physical repair HERE. I could have cut and pasted their instructions but this seems a bit more honest than taking credit for their advice...

Good Luck, and if you can't fix it using their advice, come back and tell us everything you've tried and the results, perhaps someone will be able to give you more assistance.