The battery voltage started with 13 volt and dropped to 12.1 volt after driving the vehicle for a few days. I thought the battery should get charged while driving. Is there a problem with the generator so that the battery didn't get charged normally? Because of the reduced voltage, I had a difficulty to start the engine sometimes.
It could be several things:
1) The cables or cable connection might be bad. Check them all, remove any corrosion, and tighten all connections to recommended specs.
2) The battery itself is bad. Remove it and take it to a shop to have it load tested.
3) Your alternator is failed.
It could be any of the above or any combination of the above. I believe you'll find it's the alternator itself but check the other things first.