Actually, something as simple as a PRAM battery can be startup related. It could cause the Mac not to start up at all or even power on.

Remember, the battery powers the PRAM, Power Management Unit (PMU/CUDA) and detection for power button. When that battery goes dead, the PMU/CUDA gets corrupted and loses its sync with the rest of the Mac. The PMU/CUDA isn't just a chip; it's an entire computer on a chip.

73s 8)