Tracking down hard shutdown cause—software? hardware? overheat? I'm running Mac OS X 10.7.4 on a 15-inch, early 2011 MacBook Pro.

I've had the Mac crash a few times in the last couple months, all attributable to letting it discharge (careless me!) except for the most recent time, today.  Today the Mac still had 12% battery or so, and it switched itself off—with the same "power down" physical switching noise that accompanies a switch off if you hold down the power button.  No warning message of any kind was displayed prior to the switchoff.

When I then moved it over to my desk and plugged in the power, I noticed that the bottom of the laptop was hot enough to actually hurt my hand, which made me wonder if the shutdown was due to overheating.  (?)

When plugged in (without me touching any buttons, the lid still open), the computer made a couple of physical "reset" noises followed by switch-off noises, then succeeded in booting itself to the grey apple logo.  On login the computer restored windows normally, except I was then prompted me to re-enter my App Store password to enable one of my apps to be run on this "new" computer.  This is the only Mac I've ever had, though.

In the "Console" application, I have 10 different "help_(date)_(computer).crash" reports from within the past month.  The last is from today.  I can't tell if any of the info in it is relevant to diagnosing the shutdown, but I can't find any other diagnostic data that seems relevant, except the laconic message:

2/15/16 4:52:59.000 PM kernel: Previous Shutdown Cause: -128


What I really want to know is: Why did my computer shut itself down with no warning?  Was this from overheating?  Is there a way to prevent this from happening again (or does it even matter)?
As it turned out, my battery had reached and surpassed the end of its life.  It was rated good for 1000 full charge cycles; it had gone through nearly 1200 (in roughly a five-year time span.)

I got the battery replaced and all was well.