If a secondary modular bay battery is installed, it will be used first, and then the system automatically switches to the primary battery. This is a deliberate design to allow the most amount of run time to remain in the primary battery in case the modular bay battery is removed and replaced with an optical drive.
This is a 48 WHr 6-Cell Secondary Modular Bay Battery for Dell.
Part#: Dell 4R084 (Secondary Modular Bay Battery)
Capacity: 48 Whr
Size: 5.17″ x 5.66″ x 0.57″
Dell Part Numbers: 312-0102, 312-0085, 4R084, 5P171, 0M787, 7P806, 9X001, 312-0069, 312-0122, 310-4345, 310-9124.