Only one thing can cause your starter to just click, low voltage. Which means power from your good battery isn't getting to the starter.
Things to look for are corrosion between the terminal and the cable connection. If that is squeaky clean, look to see if either of your battery cables is showing signs of corrosion. The way to look for this is to see if there is any swelling in the cable, or "puffiness". You will normally see it close to the battery connection, but it could be further along if there is damage to the insulation that allows foreign matter into the cable.
Lastly make sure that both ends of the battery cables are properly tightened. If it is loose, it will not allow correct conductivity between the battery and the starter.
A single hard click is an indication of a failed starter. The contacts inside the built on solenoid have failed.
have you checked the cables to see if they are corroided, loose, or broken If your battery is good then it is your starter you will need to replace it.
starter
Age, overuse, bad voltage from the battery
It sounds like the starter solenoid is bad.
Probably it's a low battery, though it could be a bad starter. You are probably hearing the starter solenoid clicking.
Most likely the starter solenoid needs replacement
A single click when trying to start the engine is usually an indication of a loose battery cable/wire or a faulty starter solenoid/relay switch.
Weak battery? Loose or corroded battery terminals? Engine seized? Bad starter? Bad starter solenoid? Bad neutral switch?
battery or corroded connection on battery One click most likely is a bad starter motor or solenoid. Many clicks battery or connection problem.
It can be a bad starter, but it can be a corroded battery cable. If you know the battery and cables are good, remove the starter and have it tested at most auto parts stores.It can be a bad starter, but it can be a corroded battery cable. If you know the battery and cables are good, remove the starter and have it tested at most auto parts stores.
One of a few things depending on the click. If it makes consecutive clicks when you turn the key (click,click,click,click) then the odds are the battery is dead or in need of charging and there just isn't enough charge to fully engage the starter and turn over the engine. If it clicks just once, the selenoid is usually bad, or the starter is "locked" and will need to be replaced.
dead battery bad starter solenoid bad starter loose or corroded battery or starter cables