I just got a new 25W charger for my new Samsung Galaxy S21 Ultra. However, I realise that at 60% charge, the fast charging keeps switching off and on. I changed to the original cable yet the problem still persists. I'm wondering if the charger is faulty. (I don't have another charger to try)
Hi. If you mean that the message is changed from "fast charging" to "charging", it is not good and I would recomend to contact support. It the speed of charging only decreased, it is OK. The speed depends on temperature and the more is the battery charged the slower the charging is.
I made some measurements 2 years ago with S20+, which is similar and I got this:
There is almost no difference between 15W and 25W charger.
Here is the coleration of charging speed and temperature (blue is percent of battery, red is the temperature):
It looks like damaged charger, cable or UBC port.
did you update your phone with the last security patch the august one? if your problem started after that i assume you're one of the users facing the same issue. I'm one of them, since the update my phone can not be fast charging even if im using a fast charge plug a new one i tried many plugs it's the same issue. At the beginning my phone could be fast charge for few minutes but now it's goes directly on the slow... we hope that the september update could fix the issue.