I’ve been observing the voltage and current of a bunch of chargers, cables and phones with a handy little power meter that plugs in between the charger and the cable. I still have no idea whether that hurts the battery or not, but fluctuations certainly do occur.

For example an iPhone just loves to crank up the current every few seconds. If the power supply or cable can’t handle it, the voltage will crash below some tolerable limits set by the phone. In response, the phone will lower the current back to a level where the voltage remains high enough and then the cycle repeats. Interestingly, an iPad or an Android phone I tested don’t do that sort of aggressive current ramping. Those two just figure out a sustainable current in the beginning, and let it be, whereas the iPhone is never satisfied if the current is a bit lower than you would normally get from a good power supply and cable. Either way, I suspect that’s just the BMS doing its thing, and every device seems to have a different BMS.

Will the BMS be able to provide a stable current and voltage to the cell even when the power draws is far from stable? Sounds unlikely to me, but ask an electrical engineer to make sure.

Will an unstable charging power hurt the cell in the long run? Ask a battery scientist. I just know that extreme temperatures definitely hurt it. Also high C-rates are harmful as well.