This paper presents a digital battery management unit (BMU) with built-in resistance (BIR) compensation, modulated frequency detection (MFD), and multi-mode protection (MMP) techniques to realize fast, efficient and safe charging. The charger incorporates the proposed BMU based on the low-dropout (LDO) regulator. With BIR compensation, a large rated current in the proposed pulsed constant current (PCC) mode charges the battery, and reduces the charging time by omitting the constant-voltage (CV) mode. The MFD technique allows continuous monitoring and obtains precise BIR compensation with accelerating sampling speed. The MMP technique can alleviate safety concerns while charging, including over-voltage protection (OVP), over-current protection (OCP), and the proposed accidental fluctuation protection (AFP). The digital BMU, implemented in 28 nm CMOS, occupies 0.014 mm2 of silicon area. Experimental results show that the proposed circuit saves 73.3% of charging time with 1.5A charging current, and 21.5% ADC dynamic power with 1A charging current. For the digital controller IC, the power consumption is 59.1μW.