The basic reason devices warm up while charging is that some electrical energy is inevitably lost as heat while converting into chemical energy. That is a normal, expected physical process, but heat spiraling out of control becomes a serious issue for both battery health and safety.
One of the most common causes of overheating is charging a phone inside a thick or insulating case. The case traps heat that would otherwise escape, pushing the temperature well above normal. Removing the case during charging is a simple but effective fix.
Uncertified or low-quality chargers often cannot keep voltage stable, which can generate more heat than necessary. Quality products include OCP (overcurrent protection), OVP (overvoltage protection) and OTP (overheat protection) circuits that substantially reduce this risk.
Ambient temperature matters directly too; charging a phone in a hot car or under direct sunlight adds extra load on top of a device's already elevated internal temperature. Charging in a cool, well-ventilated spot whenever possible is the better call.
Charjett's chargers include multi-layer protection systems as standard, aimed at protecting both the user and the device from these risks, but users still need to take simple precautions like removing cases and being mindful of ambient temperature.