Obviously there is something wrong.
The FIRST thing to do is use voltmeter across the battery to make some initial assessments.
Check the battery voltage:
*) BEFORE starting engine.
*) With engine idling.
*) With engine at full throttle.
Then turn on the lights and other electrical loads and check the battery voltage with engine running.
Without going into technical details - expect at least 13.8v at full throttle.