Call support. If the system is still in warranty they can get usually the part, and a technician if needed, there within 24 hours (in the states at least, I don't know about other countries). If the system is out of warranty, the battery can be purchased on line either driectly from Dell or possibly through third parties. They are simple to install.
The system will still opperate with the failed battery, just with marginally lower performance. All data written to disk must be done immediately and not cached so that the RAID controller can do it in an optimized way.