Thanks Mursal a very useful post.If you want to find a bad connection, in the solenoid cap for example:
Connect voltmeter (set on 20V DC) across the contacts of the solenoid, one meter lead on battery lead and the other on the copper strap going into the starter from the solenoid.
You will have battery voltage (whatever that is on the day) on the meter before you go for a start.
When you crank the engine the meter should go to ZERO or at least under half a volt (0V5) if you have a few volts on the meter when cranking, solenoid contacts are bad.
This can be used on any connection you are suspicious off.
For example an earth strap.