Car Wont Start But Starts When Jumped. Most modern cars use the starter solenoid to move the starter pinion into a position of engagement with the engine’s ring gear. When the jumper cables are attached, the tension from them forces a connection with the battery and the starter engages.
If the starter solenoid is not working correctly, your car won’t start, then starts the next day. In that case every time you shut your car off you will not have enough juice to start it again. Car wasn't starting after sitting overnight unless i jumped it so i took the battery in to be tested and it tested bad.