Electrical solder is way not the right stuff. Use acid flux and a harder species of solder. The best way is to melt stick solder in a wee pot (a 3/4" galvanized plumbing end cap is good), assemble the cable and terminal (with cable end splayed to keep the terminal from falling off), heat them, dip in acid flux, Then dip the end in the molten lead until you see some of it wick up the cable.