clicking/knocking sound when trying to start car

If you're hearing the rapid clicks I think you're hearing, something is definitely wrong with your battery or starting system... but you've obviously already deduced this.

• With the car's headlights on, when you try to start it, do the headlights dim? If they do not, there is likely a problem with the wiring or connection to your starter motor. What you hear clicking is the starter solenoid, which is what makes the starter connect to your engine's flywheel to start the car.

• Do you have a multimeter? If so, when the multimeter is set to "20 VDC" or just "VDC," touching the negative and positive battery terminals with the leads should read anywhere from 12.0 to 12.9 volts. Anything less is not sufficient.

• Inspect your battery terminals, and ensure that they are not loose or dirty. If they are loose, tighten them. If they are dirty, clean them with a wire brush or abrasive pad and reinstall them tightly.

• Make sure the jumper cables are connected in a secure fashion, with plenty of contact onto the battery post or terminal.
• Follow the battery cables to where they end. The negative wire will likely attach to the engine block, the car's body, or both. Where this connection is, make sure there is not a buildup of corrosion, or that the connector is loose.
• Finally, if possible, connect a battery that you KNOW to be good. You can use any 12 Volt car battery, as long as it has the same or higher CCA (Cold Cranking Amp) rating than the one already in the vehicle. If it starts with that battery, you know your old battery is discharged to the point that it likely needs replaced or tested.

If none of these produce results, a mechanic will need to do some tests to see if your car's starter motor is functioning.

Additionally, if you wish to try it, you can do a voltage drop test to verify that your battery terminals are functioning correctly. To do a voltage drop test, hold one lead of a multimeter, in volts DC, to the battery post. Hold the other end to the battery terminal connector itself, and any exposed copper cable coming out of it. You should see 0.00V, as it is measuring the difference in voltage between the two points.