Based on the vintage, your machine has two bios batteries. One on the WS board and one on the RT Board. You most likely replaced the one on the WS board (has the hard drive mounted to it, but did not look at the RT board. The RT is the board on the far right side of the A2100 control box. It is possible this battery is dead and the BIOS settings are lost, but I've never seen this cause the control not to boot.
It sounds like you lost the WS BIOS settings, but those are not contained on any of the backups you mentioned. These have to be manually set by entering the BIOS at the initial splash screen.
If you contact Siemens, their typical answer is to send the WS board to them for repair (at about $1000). If you can tell me the exact BIOS version on your motherboard I may have the settings for you. To find this, turn on the machine, and watch the screen immediately as it powers up. The upper left of the screen will usually say somthing like:
AMIBIOS (C) 1992 American Megatrends, Inc.
BIOS Version. 1.00.07.CV2
Let me know what version you have and I will see if I can send you the BIOS settings you need.
If you have lost the RT settings, these are more difficult to restore because you don't have direct access to this board's BIOS (no video card or keyboard).