If you're getting a bios error on bootup about your "time not being set" or something similar, then its possible that your backup battery for the motherboard's real time clock (RTC) has gone flat. In many motherboards this will be a small 3V lithium button cell. Sometimes these are soldered directly to the motherboard, sometimes they are in a cell holder and other times they are plugged into the motherboard via leads.
Here's an example of one on leads. To buy a motherboard specific battery with the leads attached can be expensive or impossible depending on the model and the connector you need. It's better to just order the appropriate cell and solder the leads from the flat cell onto it.
Make sure you wrap the hole thing in some suitable size heatshrink so it can't short out on the motherboard anywhere:
Then its a simple job to reinstall on the motherboard (in this case a tablet PC):
