Most Lithium battery packs have built in limiter circuits. For discharge and charging. It keeps them from over heating. That's why lithium tools just stop when the battery voltage gets too low. Not like nicads that just get slower and slower. You did not mention the brand of drill but you may want to check with the manufacturer. Maybe the internal limiter circuit is set to low on that batch of batteries. I don't think that is a user serviceable item.