It was compounded by a hot day and being inside a backpack. The problem I was having at the time was a hard lockup of the system on sleep maybe 30-40% of the ti

It was compounded by a hot day and being inside a backpack. The problem I was having at the time was a hard lockup of the system on sleep maybe 30-40% of the time due to the ATI graphics driver. My best guess is once the machine hard locked ACPI was unavailable to auto-shutdown the machine once the CPU passed its threshold so it ran until the battery died instead.