I have same issue, using 1600 battery purchased off ebay. I even re-installed leaving a 1/8" gap along bottom end of battery for heat dissipation. NO Luck. I originally suspected battery was too thin for charging components on battery to feel cool. Now, after reading customer reviews on Amazon and sellers' notes, I see that these batterys' heat sensors are incompatible with iOS 4.0 and above. Look for OEM replacement batteries for your 3G iphones... I haven't found any, just yet, but I'm still looking.