Provided the original cables are not internally corroded, then just cleaning up the mating faces to bright metal will make all the difference. After all, they did work once.
The connections suffer from Verdigris building up and adding resistance. It comes off fairly easily with a wire brush or some wire wool.
New leads should also be connected to bright clean metal.
Be very careful with the starter motor terminal. If the threaded post turns it could be very bad. Sometimes there is a square on the end of the post to help you hold it while you try and turn the nut.
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