The SprkCharger Slim stainless-steel case may get warm to the touch during charging and discharging. This an intentional design feature to manage the device heat. Discharging the Slim converts the chemical energy in your power bank to electricity. Charging the Slim converts electricity into storage energy in the Lithium-Ion battery cell. Waste heat is created in the cell and charge electronics for each step. The SprkCharger Slim has been engineered to intentionally conduct this heat out to the stainless-steel case which acts as a heatsink to increase the life of the cell and the electronics. The design has been Safety Tested by an independent third-party lab to UL1642 and UL2056. Stop using the powerbank if the case gets to hot to handle or if it remains hot and does not naturally cool down for an extended time after the charging/discharging is completed.