If you see the POST error in the picture on your IBM Thinkpad 600X's screen, your BIOS battery is probably dead. BIOS batteries for Thinkpad 600Xs go for $40.00 online. However, the actual battery is simply a common lithium watch battery with a special (expensive) connector. I will show you how to carefully remove the connector from your dead battery and attach it a new watch battery purchased for $5.00 at Radioshack.
Parts and tools:
1x dead Thinkpad BIOS battery
1x CR2025 watch battery
1x Roll of packing tape
1x Sharp knife
1x Needle-nose pliers
Step 1: Get a new battery and remove the old one.
Get a CR2025 lithium watch battery from RadioShack.
Remove the dead BIOS battery from the laptop. It lives in a little holder next to the RAM on the bottom of the Thinkpad.
Step 2: Remove plastic cover from dead BIOS battery.
With a sharp knife, carefully slice along the edge of the plastic to avoid damaging the contacts.
Step 3: Remove soldered contacts
Carefully unstick the soldered contacts from the BIOS battery. Work around the edges with a knife and then pull off slowly with needle-nose pliers.
Step 4: Insulate the edges of the battery.
Since the battery will be in a tight spot you need to insulate the edges with tape. Keep in mind that the original had a yellow wrapping around it.
Step 5: Attach the BIOS connector to the new battery.
Since we are not going to spot solder the contacts to the new battery, we have to make sure that they make consistent contact when taped to it. To do this make bends in the middle of each of the contacts. The bends should be made such that the points of the bends will press against the battery. Then tape the contacts to the exposed metal on each side of the replacement battery, such that the bend on each contact gets flattened out.
Step 6: Connect the new battery
Reconnect the battery by carefully plugging in the small white connector. Make sure you have it turned the correct way and do not force it. Then stuff the taped battery into the battery holder.
Step 7: Boot!
Put the cover back on the memory bay and power the machine on. You will have to reset the date one final time, but that should be it!