your psu is too small. At only 12v and 7.5a it can only handle 90 watts output and that doeant even include efficiency. Here is a tool to select a proper psu that can charge your lipo at the max 4a.
http://scriptasylum.com/rc_speed/_charger.html if it doesnt show your charger right away just select it from the drop down and it will fill in every thing then hit calculate. Look at the charts to select a psu to either handle the max of the charger itself or just the max to charge the battery at 4a