There really is a LOT of good info already on Mud about alternators. Search, search, search.
I can share my personal experience about using an electric winch and choosing a suitable alternator.
My experience has revealed that for winching (while using dual batteries), a stock 55 amp alternator is sufficient for any typical 4WD electric winch. A larger alternator won't help you winch significantly faster, if at all, because preventing winch overheating will be the limiting factor.
Using lots of big lights may be a different story, and a larger alternator can help with that.
A couple years ago in Mexico, I winched a fully loaded FJ60 up a 47º slope for a distance 160 ft. It took 8 hours to make it to the top.
- Winch: Warn M10k (with a fan mounted under it's motor)
- Alternator: Stock Toyota 55 Amp
- Batteries: 2 AGM batteries total of 1700 CCA.
- Ambient Temp: 72º
- Pulleys used: Two for a 3:1 advantage.
Winching was done in 25 ft. segments before the cable was re-rigged 25 ft. further up the line. It took a total of 6 re-rigs before the vehicle made it to the top.
The limiting factor on winching up this hill was avoiding winch overheating… not alternator charging ability. The 55 amp alternator could charge up the batteries fast enough between the short 45 second pulls on the winch before it had to shut down to cool .. (and allow for the batteries to recharge). The run time to cool-down/charge time was about 1:8 to 1:9.
A few times I got impatient and ran the winch longer (ran volts below 11.8 V) or restarted sooner.. and the winch overheated and stalled because I had not let it cool down sufficiently. This happened three times. I was lucky I did not burn out the winch.
On this slope, on a 72º day, with this winch, with two pulleys… any alternator larger than 55 amps would not have got me to the top of the hill any faster… It was the waiting for winch cool-down that was the bottleneck.
A properly set up hydraulic winch on the other hand… would have been SWEET!
