Probably your CMOS battery has died, but I would have thought if the main internal battery was installed, the system would use the main battery by default.

 

Instructions on to remove it:

 

http://support.dell.com/support/edocs/systems/ins4150/en/sm_en/rsrvbatt.htm

 

I did a Google search on Dell 4150 battery and got a bunch of hits on web retailers that sell them. Unfortunately I can't recommend one to you and I would check to see if the pc board connector is already on the battery leads.