A battery charging system for charging a battery of a portable device includes: a power supplier, generating a power supplier output voltage and a power supplier output current; and a charging circuit, dynamically updating an over-voltage protection threshold and/or an over-current protection threshold based on a charging current, a battery cell charging voltage and an equivalent impedance of a charge path. The power supplier output current and/or the over-voltage protection threshold and/or the over-current protection threshold are time-varying during the battery is charged. The power supplier performs over-voltage protection and/or over-current protection based on the over-voltage protection threshold and/or the over-current protection threshold from the charging circuit.