My xiaomi portable fast charger clearly charges my girlfriend’s iPhone faster than my high end android device. Doesn’t matter which port, which cable or which android phone. It starts by charging fast but as soon as I plug her phone the fast charging speeds go straight to hers and mine just chargers very very slowly.
I’ve got the impression that this may be on purpose or at least has some clear explanation.
Any thoughts?
Assume from the various pd fun that could be occurring, it could be voltage related. Android phones often set their charge current by the input voltage. If the voltage sags, the phone assumes it is overloading the cable and backs off. If the Iphone and Android phone are sharing a 5V rail inside the charger then this could be an issue. If the combined load of both phones causes a voltage sag, the android phone will back off. However, if the Iphone doesn’t also slow down at the same rate, you will get a bias. It might be that 90% of the power is going to the iPhone simply because the android phone is being polite, and being screwed by the deal.
The best way to test this is to install a charge monitoring app on the android phone. I personally use AccuBattery. It shows me both the voltage and current draw of my phone. The pulsing effect of a bad cable is very obvious with it. I suspect it would help you diagnose what is going on.