What size battery is recommended for a RYOBI Angle Grinder?

Cordless Angle Grinders are powerful, energy-hungry tools and, hence, need lots of battery power.  
 
Grinding tasks generally use more battery than cutting tasks, so as a minimum, 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+ 6.0Ah HP Battery (RB1860X)
• 18V ONE+ HP™ 5.0Ah Battery (RB1850X)

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