duty cycle may be a poor choice of words.. however.. many electronic devices are much happier running at about 80% of their rated load, if you are running them continously. It's more of a rule of thumb for happy, health electronics. If it's rated for 40, it will do 40.. however I'd lay some green down that if you took 2 identical units and ran one at 40 and one at 32 continously, the 32a one would have a longer service life..
references? I could send you a copy of my engineering degree
naw.. just a rule of thumb I like to follow.
Using this rule, I've yet to ever burn up a switch, wire, genny, alternator.. or other load device.. and I know many, MANY people who have exceeded their devices rated specs and HAVE burnt them up.. repeatedly

For some devices it's a thermal mass vs cooling issue. Think of a 600hp auto engine and the lil inch thick radiator. sure, you can stomp the gas pedal and hit the passing lane making 600 hp for 35 seconds to pass.. and the cooling system can handle it. however on a endurance track, going round and round at 600hp.. that same radiator will max out . I can also full field a 120w rated 6v genny and get 180w out of her for a few minutes on the bench before solder starts flingin out of her...
Tell you what.. i probably don't have a clue what i'm talking about..so go and load her up to a full 40a rating.. that's 576w to play with... that's 10 - 55w halogens, plus a few watts left over for the blinker, fuel shutoff solenoid, and dash lamps...

as for charging the bat? who needs it once yer running right?
soundguy