Say when you're using your generator or when there's shore power. And especially when there's no sun out.<br><br>Supposedly at 40 amps, it can charge a dead battery in 2 hours. Would heat it up real bad and shorten it's life as compared to 2 amps (in 8-12 hours), but an average car's stock alternator is at least 65 amps already, so that's even worse.<br><br><br>http://www.harborfreight.com/heavy-duty-612-volt-battery-chargerstarter-69368.html<br><br>