Get a better charger. That one is innefficient, since it provides low amperage at the voltages needed for charging. It may damage your batteries in middle-long term.
Also it does not stabilize the voltage, so peaks may appear. That may damage the batts too.
Get one who does at least 800mA for 9V (DC) from 240V (AC). The more amperage the better.
-- Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety. (Ben Franklin)