It boils down to applying a carefully controlled charging voltage (higher than the current battery voltage) to the battery. I would assume that the voltage is a

It boils down to applying a carefully controlled charging voltage (higher than the current battery voltage) to the battery. I would assume that the voltage is adjusted to achieve a specific charging current. With PPS (programmable power supply), AFAIK the phone will typically ask the charger for a voltage twice as high as it wants to send to the battery, possibly a bit more to compensate for losses in the cable, then halve that with a highly efficient charge pump (https://en.wikipedia.org/wiki/Charge_pump https://en.wikipedia.org/wiki/Charge_pump). The best (most "gentle") way of charging a battery would likely be a phone that is intentionally not trying to fast charge, connected to a PPS-capable power supply. This would minimize losses and thus heat within the phone without charging the battery faster than necessary. I suspect that the difference to charging off a non-PPS charger is negligible in practice. Wireless charging creates a lot of waste heat, which isn't great for the battery.