Battery and the Battery Ground
"Your first goal is to determine the state of charge of your battery. Turn your headlights on and see how bright they are. If they are of normal brightness then you are probably OK. If you have a voltmeter, put it across the terminals of the battery. A fully charged battery will read around 12.5 volts, a dead battery will be less than say 11.8 volts. If your battery is dead, charge it and then proceed to the next step.
The battery earth
=Ground could also be bad or marginal. This will mean it tests correctly if it is not engaged, but experiences a steep drop (
often only when the engine is hot) as soon as you engage the starter. That will make the starter turn slowly
or not at all or reduce the voltage remaning to the point where the coil cannot prodcue a spark. A braided earth strap could look great on one side, but have many broken stands on the other. Here is an example in the image."
The above from the below site
Starter Troubleshooting