Is it snapped off or is it corroded? If its corroded, use this guide to help you fix that.
[guide|141908]
If the metal piece itself is broken or the cleaning doesn't work, you can use these replacement contacts and take out the bad ones.
[link|https://www.ebay.com/itm/144692625883?chn=ps&mkevt=1&mkcid=28&var=443927590050&srsltid=AYJSbAfc9mS6R8iAUDodN1-RqpuLzmoqQTbtScOsvA3aTKsB9kkB_H1OlSY|Nintendo Wii Controller Battery Contacts Genuine Replacement Part - JAPAN|new_window=true]