This guide applies to Ryobi One+18V Li-ion Battery (130501002), but should have more general application. This guide will show you how to disassembly the battery pack and check the cell balance and rebalance the cells if necessary.
The battery should normally measure about 18V across the terminals. If it reads about 12V, then it is likely the battery protection circuit has activated because of cell imbalance. (those were my symptoms)
The time estimate for this guide is for disassembly and cell balance measurement. Cell rebalance charge time is extra.
CAUTION: Battery disassembly exposes high current circuits. Be careful!