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Why does my ThinkPad battery discharge while in sleep mode?



I have a W510 and windows 7 OS and usually I shutdown the pc , before putting it into my laptop bag for travel.


Last night, by mistake I put my laptop in sleep mode, and put it in bag for >12 hrs.  When I opened the bag .. There wasn't any heat build up, and the processor fan was  not running, and when I  pushed power button, the pc booted within seconds.  I did notice that the battery charge had declined about 1 hour of run time from where it was when I put it to sleep.


My question is whether it is safe to store the PC in sleep mode for travel, and why if it was asleep did the battery drain?



There are two offline modes for laptops provided through power manager and / or Windows and these are often referenced by a variety of names.


1) Suspend or Sleep or Standby.  In this mode, the LCD is turned off, the hard disk stop, and the CPU reduces operations to a minimum level.  The content in RAM remains - all open programs, applications are held.   Some system power continues to be consumed in this state, which is why the battery of a system that is in "sleep" or "suspended" state will continue to discharge.   Since the power consumption and CPU activity are very low, there is no need for cooling and the the fan does not operate.   It is generally safe to transport a system in the suspend mode, even in a carry case.  It should not be stored indefinitely as the battery will discharge to zero in time.


2) Hibernate - In this mode, the system writes the content of RAM to a file on the hard disk and then powers completely off.  Upon restart, the image file is read back into memory and the system returns to the state before hibernation.  The hibernate state uses virtually no power and the system can be stored in this state for significantly extended periods of time - weeks.  While more secure, the hibernation state takes longer to enter and return from than standby / sleep.


Because there are some rare situations in which a system could unintentially rusume from the suspend / sleep mode (if the latch was damaged and lid should partially open, or some other glitch) it may be a good practice to use hibernation if a system is going to be transported in an enclosed bag. 

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