Battery for EWORK 20V/21V Max series cordless power tools High-density lithium-ion battery has passed 800 charge and discharge tests and still retains 80% of its capacity Built-in protection system to avoid overcurrent and overcharging, effectively extending the use time EWORK 4.0Ah Lithium-Ion Battery for 20V/21V Max Series Cordless Power Tools - EK-4.0G