Repairing a Milwaukee M18 Battery that won't fully charge

Fixing a Milwaukee M18 9.0Ah battery that will only charge up to 3 bars or 19.1v and then the charger shows it's fully charged. The most likely cause is from over-discharging the battery when I pushed my angle grinder too hard. Never continue using the tool when it shuts off due to low battery. I learned the hard way... Tools needed: T10 Security Torx Bit T8 Torx Bit (Standard or Security) Lithium Ion Smart Charger Multi-Meter