SKIL BY519601 20V 4.0Ah PWR Core 20™ Lithium Battery. The Power Is In Your Hands. The PWR Core 20™ 4.0Ah Lithium Battery is backed by an innovative temperature management system that results in longer run time and battery life. Each cell is surrounded by revolutionary phase change materials to keep the battery cool, and powering through any project. You can use this lithium battery to power up any SKIL 20V power tool!