The Panasonic EY Li-ion Battery Pack helps to protect your tool in heavy conditions. The improved design features materials that give reduced internal resistance, allowing a 250% lifespan when compared with an original Li-ion battery. Other features include: an over-discharge prevention sensor (OPS), battery overheat protection sensor, individual battery cell monitoring and heat-resistance layer (HRL) technology.The Panasonic EY9L54B 5.0Ah Li-ion Battery Pack offers the highest Li-ion battery capacity without an increase in size. This battery is the same size as the 18V 4.2Ah Li-ion battery pack.