I don't think it's a bad charging circuit -- everything seems to check out fine with the battery and it charges in the expected amount of time. Also, I don't think it's the power mode -- both battery and AC settings are on Maximum Performance. I think it's just the additional heat from the battery charging and discharging. Also, I think it might be because when the battery is attached, it's harder for the heat sinks and fan to dissipate heat because you have this already warm block right next to one area of the fan assembly. Not sure. Just a theory.