Amazon.com: Synergy Digital CMOS/BIOS Battery, Works with Dell Latitude E6400 CMOS/BIOS, (Li-Ion, 3V, 200 mAh) Ultra High Capacity, Compatible with Acer 23.22207.041 Battery : Electronics
FX191 - GC02000KH00 RTC CMOS Battery for Dell Latitude E6400 E6500 Vostro 1700 - CPU Medics
How to remove BIOS password from a DELL E6400 (works for some other models & brands) - YouTube