>The naive approach of always providing 4.2V at as much current as the cell can take is bad for batteries. More on-point with what I think you meant, cellphone

>The naive approach of always providing 4.2V at as much current as the cell can take is bad for batteries. More on-point with what I think you meant, cellphone fast chargers are usually putting 1C into cells below 75% when thermal conditions are met, which many consider to be stressful to li-ion chemistry. I do think this is ignoring that 1. Better charge standards like VOOC have higher efficiency and so lower thermal load, and 2. cell chemistries have improved iteratively to reduce internal resistance, again reducing heat. Where manufacturers differ is how they deliver and convert that voltage inside the phone/device. Some of the designs (qualcomm quickcharge) dump a lot of heat stepping down a higher voltage while the earlier mentioned VOOC tries to match everything inside to meet a desired rate of charge. Temperature is the big enemy followed by dwell time in the saturation zone. (over 80% state of charge, getting worse over 90%).