Most notably you're dumping it into the battery, which is already heating up itself just fine having to supply the extra current for your full CPU power burst.

Most notably you're dumping it into the battery, which is already heating up itself just fine having to supply the extra current for your full CPU power burst. And sustained high temperatures are a surefire way to rapidly diminish the capacity of LiIon batteries. So I guess it's a good thing they have external power and a fan, even though it's dubious how much that does given the battery isn't seeing any of that airflow and is in the usual protective sleeve that doesn't exactly help thermal conductivity. There is a reason Tesla watercool every individual metal 18650 cell.