How far is your battery from your pump ? 7 volt drop to the pump is not good if it even will power up .I do security stuff working with 12 volts ever day when we run long runs of wire to cameras with LEDS , the leds burn out quicker they require 1 amp .S0 with that said.when I run 1000's of feet wire normally 16 or 18 I will put a bigger transformer on 12volt 1.5 amp.A small volt drop will not hurt but low amps will .


Closer the batteries are to the pump means more current the better it is for the pump , your charger only puts in a small charge to keep your batteries up.