Can I ask why you are trying to flash the BIOS.
BIOS updates are released when a particular problem has been recognised and the BIOS update is only aimed at resolving that/those particular issue/s, flashing the BIOS unnecessarily is like playing russian roulette, someone will eventually come unstuck.
This may be a clue to what is causing what you have described occurring;
but when i cut off powerline, battery works smoothly for 30 to 50 minutes
As this may also be;
my computer model is dell inspiron 6400
A fully charged battery in your laptop should last at least 3+ hrs, the laptop could potentially be nearly 6yrs old, how old is the battery, just a FYI the Dell Inspiron 6400 was plagued with complaints about short overall battery life, 1 - 2yrs and poor quality AC adapters brought about by using poor quality materials in the manufacturing process.
In a nutshell if your battery or AC adapter are bad then no BIOS flash can repair them, only replacement parts, also be aware Dell are known to not work too well if at all with non propriety components.