Battery-to-battery (B2B) wireless charging can take place in many scenarios, such as using a mobile phone to charge another mobile phone, wearable devices, or low-power sensor nodes. To facilitate this wireless power transfer (WPT) function with the minimum additional cost, we propose a monolithic reconfigurable bidirectional WPT transceiver designed for the first time in CMOS, which can be reconfigured between a differential class-D power amplifier (PA) and a full-wave rectifier. Meanwhile, we employed a maximum current charging mode to maximize the B2B charging efficiency, by directly charging the loading battery with the rectifier, and by powering the PA with the sourcing battery. Then, we reduced the number of cascaded WPT stages from five in the conventional design to three. This bidirectional WPT transceiver fabricated in 0.35 μm CMOS occupies 3.9 mm of silicon area. The bidirectional WPT function, verified at 6.78 MHz with only one off-chip capacitor, exhibits peak efficiencies of 91.5% and 58.6% for the receiver and the overall system, respectively, when the output power is 1.55 W.