I acquired a couple of used deep cycle batteries to put on my project boat and am having trouble charging them. I dont have a larger 2-3 bank charger yet so i decided to charge them on my single bank minn kota portable charger. Its a 3 stage charger but for some reason, when I try to charge these 2 batteries it just boils them and doesnt change amperage. Am I doing something wrong, or is there some kind of fault in either battery or charger?

The charger charges my small 24cell battery just fine

-thanks in advance

Check to see if the chargers output voltage is higher then 14.6volts on start up then it should drop to about 13.3volts in maintenance mode. If the voltage is to high the batteries will boil the water out all so check the batteries with a hydrometer to make sure they are still good after over charging you may have damaged the plates. Good Luck    

If your charger is otherwise working OK and these batteries boil when on charge, they are DEAD.  I hope you didn't pay $$ for them or you got ripped off.

Seriously doubt the charger is the problem, most likely the batteries were junk before you started trying to charge them. A brand new battery left in a deep discharged state for as little as thirty days is pretty much junk.