The #6 wire, if your battery needs the current, will get very hot before it won't pass current at those lengths.

If you measure the voltage on the battery at rest, with no charging after you use the windlass, what's the voltage of the battery when it "rebounds"?

You can see "voltage" without carrying capacity in a small wire (until it, or the connections, get "hot" from resistance).

I don't think that an extended 100 amp at a nominal 12 V DC load should be going 100' (round trip) on just #2 wire? Maybe #2/0 battery cable.