That voltage drop is way too high.
It indicates you have a bad connection, wiring
that is too thin, or both.
Generally marine wiring
should be done so that there is less than 3% (0.36v). Some people will accept 5%(0.6v).
A good general rule
is to engineer
the system with wire that gives less than 3% voltage drop but after all the connections circuit breakers etc less than 0.5v should be measured.
Whatever standard you choose 1.8v is way too high.
You can use a multimeter to trace where the voltage drop is occurring by breaking the connections down and measuring the voltage drop in each section.
Start by measuring from the positive pole of the compressor
to the positive pole of the battery, then do the negative. The circuit can be further broken down, until you find the culprit.
Often a defective connection will reveal it self by feeling hot when the current
is flowing so feel all the connections as well.