Why won’t my car start without a jump?

Hello, the battery is the best place to start. Think of it this way: the battery provides the power to operate the vehicle at all times. The charging system - with the major component being the alternator - serves to recharge the battery so that it can do its job. The reference voltage from the battery keeps everything operating evenly, not driven by revolutions like an alternator. If the vehicle is dead, check the battery for state of charge and the ability to sustain a load first. I think you'll find it's not sufficient in this case. Once you have a known "good" battery installed, you can continue by checking the charging system to ensure it is also working properly.