Hi @greyhathacker
Usually it takes disconnecting the main battery and also the RTC battery (aka cmos battery) to reset the BIOS to default.
Also doing this won't reset the BIOS password (if there is one required) so if you don't remember it you will still be locked out.
Here’s the procedure:
a). Disconnect the charger if connected.
b). Disassemble the laptop, disconnect the main battery cable from the motherboard and disconnect/remove the RTC battery from the motherboard. Here's the owner manual for the laptop.
Go to p.71 to see how to remove the battery (you don't have to remove it, just disconnect the battery cable from the motherboard) and p.79 to disconnect the RTC battery. When the RTC battery is removed from the laptop, use a DMM (voltmeter function) to measure the battery’s voltage. If it is <2.6V DC or >4-5 years old replace it. Search online for Dell 5540 rtc/cmos battery to find suppliers that suit you best.
c). Press and hold the laptop’s Power on button for 30 seconds and then release. This will restore the BIOS back to the factory default settings.
d). Reinsert/reconnect the RTC battery (or replacement battery) and reconnect the main battery
e). Reassemble the laptop and connect the charger.
f). Switch on the laptop and check if it starts OK. There may be a message on the screen about the date and time being incorrect. This is normal as the BIOS has been reset. Once the date and time have been corrected, the message won’t appear the next time that the laptop is started. Any user definable BIOS settings that were changed by you to suit whatever operating requirements you wanted, will have to be changed back again as well
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