I think you will find you have 3 batteries. Your motherboard has a small battery to keep the CMOS settings alive. You have a main operating battery, and apparently a second, or backup battery.
From the system's viewpoint, the 2nd backup battery may not actually be in use until the main battery craps out. Or . . . the system may see tham as a single unit bonded together. Why not check with Dell to see which is the case?