Check the voltage on the battery terminals (right on the posts) while cranking. This will tell you if the drop is internal to the battery or connectors or cables. The voltage at starter will be lower voltage on posts. Use same technique to determine if cables or connectors have resistance.
note:
On a 12v system, a drop to 9v on the posts while cranking indicate a battery with high internal resistance. A good battery should read 10.5-11 v.on the posts.
