It depends what type of winch you have, the current it draws (which depends a lot on the gearing it has inside it) and what you use it for.
What i was trying to say was that there were better ways of pulling a landy up a 100ft hill than sitting at the top winding away with your winch.
Obviously if your doing 'winch chalenges' then you probably have a decently geared winch, proper cables, and a system designed for it.
A 100 amp alt will only output 100amps into a flat battery, and is not designed for constant use supply 100amps for hours on end.
As i only use things like this for getting myself out of difficut situations (e.g. out of a load of snow up the side of a cumbrian mountain) and not on some made up challenges, i obviously have diffferent kit, and i guess the author of the thread has the same.
Either way if you are pulling hard for a long time, the alt will never keep up with the winch, and eventualy whatever you are doing will run the battery down. I bet yor chalenges dont call for CONSTANT winching like pulling up a 100ft slope, and i bet you switch it off for a few seconds between use, giving it time to do some charging?????
Either way - i bet i was right about the solenoid!