I fitted a stock 100amp to my vehicle. Got two batteries under the seat 950 cca's joined by a split charger. 2nd bat solely for my winch's. It didn't lack once on the last winch challenge I was on :-)
My first ever winch challenge, only driving for 3hrs of the comp I :
Broke rear half shaft
Broke rear diff
Broke steering rods
Broke front diff
Then 15mins from end. Pulled a tree out of the ground which everyone had been using all day for winching :-(
But I still came 6th lol
I run two 930 cca batts I believe there 633's. I have put a split charger between them and so use the aux batt just for my winch's. I too wasn't too sure and took this advise off another forum. I did a winch challenge last week and believe me the batts and the winch's were probably the only things that never let me down lol.
I do actually seem pleased with this set up but I dare say I will be corrected by a few of you hardened winchers !