the reason for battery drain while driving
To the best of my knowledge, the only reason a battery discharges while driving the vehicle is that charging current is not getting to the battery.
There could be many reasons for this, and although I can’t recall all of them, here are some possibilities.
The alternator, digital battery or generator if so equipped, is defective and not generating any charging current, or not enough.
Even if the alternator/generator is good and working, it is possible that the current does not get to the battery.
The drive belt is either slipping or missing altogether.
The wiring system has a defect which prevents current from a functioning alternator from getting to the battery.
IF the vehicle charging system has a seperate voltage regulator, it could be defective.
Another possibility is that the battery wholesale electronics is defective and will not accept a charge.
These are some of the most common causes which come to mind. Maybe others can improve this answer with other causes for the problem.