How to determine what kept the macbook pro awake until the battery drained

What happened could’ve been because of different reasons. Certain old Macbook Pros had troubles with sleeping (hence why zneak wants to know about the model).

However I believe that your computer woke up (wake on lan? hung process?) during the night and it burned through your battery. What are your Energy Settings?

Also, get some sort of CPU meter like MenuMeters or iStats Menu (€) and make sure your CPU/Cores are not 100%, that indicates a process that’s using all your CPU power (and a lot of energy!)

To disable deep sleep, run the following command in Terminal:

sudo pmset -a standby 0

This is a description of "standby" (deep sleep) from the pmset man page:

 standby causes kernel power management to automatically hibernate a
 machine after it has slept for a specified time period. This saves power
 while asleep. This setting defaults to ON for supported hardware. The
 setting standby will be visible in pmset -g if the feature is supported
 on this machine.

 standby only works if hibernation is turned on to hibernatemode 3 or 25.

 standbydelay specifies the delay, in seconds, before writing the hiberna-
 tion image to disk and powering off memory for Standby.

Battery savings depend on how much time your Mac spends in standby. While in standby mode, your Mac consumes 0 power, it's as if it is shut down (note: not taking Power Nap into consideration).

My personal recommendation would be to set the "standbydelay" value to something bigger. 2 hours, for example. This means that if you open the lid back in less than 2 hours, your Mac would be in regular sleep and wake up instantly. If longer than 2 hours, your Mac would enter standby mode and consume no power, and waking up will take a bit longer.

Example for 2 hours:

sudo pmset -a standbydelay 7200