How to update BIOS on Dell Inspiron 3721 without battery (or with a dead battery)

(Only tested with BIOS version A14 for Dell Inspiron 3721)

1. Download BIOS version A14 from the official dell website.
2. Extract the content of the 3721A14.exe file to a folder with 7-zip (there may be en extraction error: "Cannot open the file as [PE] archive", it's all right).
3. Run "InsydeFlash.exe" file in the folder, there will be an error message "The AC adapter and battery must be plugged in before the system BIOS can be flashed". Press cancel.
4. A file named "platforms.ini" will be created in the directory after you run "InsydeFlash.exe", rename it to "platform.ini".
5. Open the file and find text "BatteryCheck=1", replace it with "BatteryCheck=0", save the file.
6. Run "InsydeFlash.exe" again, no battery warnings should appear.
7. Pray that your power company does not switch off the power during the BIOS update.