Sure, directly connecting from external to the battery pins while being in the phone also connected to the phone pins almost asks for serious trouble. (do you know the phone´s internal resistance and the charge control circuitry?) Maybe you will learn about it…
How about temperatur supervision of the battery while being charged?
I would advise taking another approach, have you heared of those external chargers?
Anyway…
Look out for some of these matching your needs:
https://www.reichelt.de/rueckstellende-sicherungen-max-40a-60v-2-2s-pfra-020-p35203.html?&trstct=pos_14
(they are resetable)
or
SMD chip fuses (one time protection)
There are also so called pico-fuses but on my quick search I could only find them at this dealer.
