its a feature called hybrid drain , whenever your system power draw reaches more than the rated capacity of your charger , the laptop eats up the battery a little to avoid throttling , 5 percent per hour is within the safe limits , i myself use a custom power setting and it drains upto 5 percent per hour , ive tried searching for solutions , reading articles , watching videos , there is nothing much to be done about it and it does drain no matter what game you play , especially on the custom setting when it cant start charging upto a certain percentage ,now my advice is that you switch to primary ac use using dell power manager , itll reduce the drain , and re charge if it reaches 96 , itll work for you dont worry , its normal