Replacement battery charging slow

Let’s start with the basics just in case you missed something. Give this Apple T/N a good read and walk through all of the points: Apple Portables: Troubleshooting MagSafe adapters

Do make sure your using the correct charger: Find the right power adapter and cable for your Mac notebook Do be careful on the source of the charger as there are quite a few fakes which are not safe for you or your system: Lacking safety features, cheap MacBook chargers create big sparks & Counterfeit Macbook charger teardown: convincing outside but dangerous inside

If you are still having problems after going through these steps you’ll need to work up the chain you may need to replace the DC-In board following this IFIXIT guide: MacBook Pro 15" Unibody Early 2011 MagSafe DC-In Board Replacement if the contacts look melted or stained.

Beyond that the voltage comparator logic on the main logic board may need to be checked. This will need a deeper set of skills and tools to diagnose & repair.