BIOS program code
BIOS is a piece of complex program code produced by specialized manufacturers like Phoenix, AMI Megatrends and Award. BIOS software operates at the lower dungeons of a laptop and sits between the operating system and the chips at the motherboard.
It’s unknown to what extend BIOS program code can actually control the laptop battery charging process. Though based on the observation that battery charging control remains active when a laptop is powered off – seems to indicate BIOS program code is not involved.
Most likely it’s POST program code containing the functions to start and stop laptop battery charging based on the presence of a valid AC power adapter and matching laptop battery.
POST = Power On Self Test.
POST program code runs on an embedded micro-controller next to the main CPU.
Upgrading BIOS and battery charging behavior
However POST program code might be part of a BIOS program upgrade binary / executable as it's available on the DELL support website. With this there might be indirect control of battery charging behavior through BIOS upgrades.
If so BIOS program code must contain 2 separate machine code sections. One section for the main CPU (BIOS functions) and one for the POST controller.
If POST program code is not upgradeable through BIOS upgrades - how did it get there in the first place?
Back to the laptop battery charging diagram.