Car starting issues intermittently

There are several ways to determine whether the starter or battery failed. Try jump starting with another car battery with heavy duty jumper cables. Cheap jumper cables will heat up almost immediately as large current is drawn from the booster battery. Larger gauge jumper cables, 4 or 2 gauge are the best. If the starter turns over the engine, suspect; battery, battery cables, their connections, alternator and drive belt. Ensure the main battery cable connections are not corroded, preventing the largest current draw for starters and testing battery capacity at any auto store selling car batteries usually having battery testing equipment. If battery and battery cables are fine, the last test is a temporary wire jumper across the two starter terminals, the small terminal to the large terminal with the main battery (red) cable connection. This requires caution under a jacked up vehicle with ignition OFF. The momentary connection of the two terminals on the starter should result in the starter immediately powering up to crank the engine. Worn out starters may or may not power up and intermittent as a sign of a well worn starter. This test presumes battery, battery cables and their connections are clean and free of corrosion. There will be sparks as the starter solenoid draws a few amps. Most experienced mechanics are well aware of this troubleshooting method.