Jacques:
Your symptoms are those of low voltage at the starter. That’s usually caused by a low battery, but it also can be caused by bad connections in the electrical path or by a defective cable. The usual way to handle this is:
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If you have a multimeter, check the voltage between the positive battery terminal and the starter, and the negative battery terminal and engine ground while someone is trying to start the car. There will probably be measurable voltages, but if most or all of the battery voltage is being dropped outside the starter, you have a connection/cable problem, not a battery or starter problem.
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Without a multimeter:
a. Remove the battery connections. Scrape them to bright metal. Reattach
b. Track the wire attached to the negative battery terminal to wherever it goes. Unbolt it. Scrape to bright metal as best you can. Reattach.
c. Look carefully at the battery cables near the battery. Sometimes battery acid finds it’s way into the cables somehow and eats them – converting the copper wire to white and/or blue salts that are lousy electrical conductors. The cure for that is to replace the cables.