When the computer was not in use, contents of the memory were preserved as long as the main lead-acid battery remained charged. The PowerBook 100 Power Manager was an integrated circuit, usually placed on the logic board of a PowerBook, and was responsible for the power management of the computer. Identical to that of the Macintosh Portable, it controlled the display's backlight, hard drive spin-down, sleep and wake, battery charging, trackball control, and input/output (I/O). The 100 did add a new feature: 3.5 V batteries backed up permanent and expansion random access memory (RAM) when the PowerBook 100's battery was being replaced or when the 100 was otherwise temporarily removed from all power sources. This made it a perfect candidate for use with Apple's RAM disk to help increase battery life by accessing the hard disk less frequently, since the 100 was the only PowerBook that maintained the contents of RAM on shutdown in order to reduce startup time.