coin battery with a big capacitor?

BTW- in Intel systems, a dead coin battery will not prevent booting. A lot of manufacturing flows involve booting the system without any battery installed at all. More likely the battery was still alive but didnt have enough juice left to keep the battery backed state from getting corrupted, and when you swapped out the battery it reset the RTC state to known values that allowed it to boot. Did you try just shorting the "Clear CMOS" jumper first?