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iPhone XS replacement battery is quite expensive compared to the iPhone X can I just use the iPhone c battery in the Xs

They are very different designs! The XS uses a newer single cell were as the X uses a dual cell. The charging logic is very different as the X uses a double tap where the XS uses a single tap. Your logic board is not designed to support the other battery charging needs. Please use the correct battery for your phone! iPhone XS BatteryiPhone X Battery