probably ur battery already formed dendrites, causing it to have micro shorts internally and thus spreading heat in the phone case. a phone with 10w charging causes the battery to heat, let alone ur galaxy s20 charging at 15w. i actively cool my phone battery while charging by putting it in front of a fan so the battery stays cool and doesn't form dendrites, and thus stays healthy. i also use a silicon case instead of leather else u ll be thermally suffocating ur phone: it uses metal as a case material for a reason. exynos for s20 seem to be made on the 7nm process node which usually should be efficient in term of temps. even when the cpu heats up (even using the 4g antenna, let alone a 5g one when active, the camera flash, all those heats up and causes the phone case to heat up too and then a chain cycle ensues by causing the battery to heat up sponatneously too, then it dries up faster than usual which compels the user to plug the charger and the infinite cycle of heat will never end..i never owned an exynos phone but sometimes a phone need to be underpowered so it could last.samsung phones are really overperforming but unluckely their users aren't just thermal conscious. no engineering degree is needed to have a grasp over phone thermals but sometimes having some knowledge is needed if u cherich ur phone