What size battery is recommended for a RYOBI Ratchet Wrench?

It's recommended to use a 4.0ah battery or higher for optimum runtime and performance like the:

    18V ONE+ Compact 4.0ah Battery (RB1840C) 
    18V ONE+ 5.0ah HP Battery (RB1850X)
    18V ONE+ 8.0ah Lithium+ HP Battery (RB1880T)

Smaller batteries can be used, but often they will have reduced runtimes and performance levels.