I bought one of these

http://www.harborfreight.com/automatic-battery-float-charger-42292.html

to charge small less than 12V 30AH batteries.


Instructions say it is not for charging batteries but to maintain their charge.

I am assuming this is because a discharged battery's internal resistance is too high for the wimpy charger to overcome?

But will it still charge a discharged 12v 30 AH battery?

 

If it uses foldback current limiting then the additional current drawn by a discharged battery would cut the output voltage to something below the charging voltage (13.8+) that a lead acid battery requires to charge. Little to no charge current would flow. Looking at the sketch in the manual shows no heatsinking on what looks like the pass devices so foldback makes sense to avoid overheating.

But will it still charge a discharged 12v 30 AH battery?

Not if I am right..

 

The reason they say its not for charging is because it likely is only is capable of providing a small (unspecified) current to provide a float-charge current to the battery. Thus it probably would charge a discharged 30AH battery but it could take several days.

But if it does have foldback limiting as John states then it could provide very little charge current to a discharged battery.