What is the lowest wattage that can be used to charge a USB 3 Cellphone?
While I don't have a 100% solution, I would like to give some information that would be useful anyway. Maybe you'll solve your problem with it.
First of all, when the phone is turned on and not in standby mode, it uses some 4V (battery voltage) @ 250-1250ma depending on what it does, typically around 400-500ma. I can speak for iphones more confidently, I work in repair thing, power consumption is something I look at a lot). When the device is idle (lock screen, display off), it usually uses a few milliamps to zero, actually impressive. So you can only charge the phone as long as it's off or not in use. Second of all, it's hard to predict what happens if you limit the provided current. How do you want to limit it? If your charger output voltage drops to limit the current like lab bench power supplies do, you will probably end up with undervoltet output and the internal circuitry may stop accepting it as a valid charger. Alternatively, some charging circuits will accept even 4.4V input instead of 5. You will have to experiment to see if it works, and there's no guarantee it will work for everything. Tbh you'll have to try stuff, as I see it. Don't worry, you won't burn your phone or anything else. There is no way to break something from a short term undervolt even if it happens, it will just stop charging, no harm. If you keep experimenting with it and get some results, post them here, please. Would be interesting to see.