Sounds like the switch is bad or the wiring from the high charge terminal has a resistance in it (bad connection, broken strands, etc.) that is not allowing full current flow. I would see if the switch can be disassembled to check the contacts for burning or corrosion. If the switch is ok, I think I would check for hot spots in the wiring, indicating a bad spot. Replace whatever is getting hot to eliminate the resistance, and recover the lost charging current. Remember, you can't violate Ohm's Law. The higher the resistance, the lower the current at the same voltage.