How does Home UPS Charge battery?
Since I'm curious about this and nobody posted an answer, I'll post this attempt to explain, to propose a discussion.

If all MOSFETs are off, they work as a bridge rectifier, using the body diodes. But since the secondary voltage is too low to charge the batteries, if both MOSFETs at the bottom are turned on briefly and quickly at the same time, they could implement a kind of boost converter, using the secondary of the transformer.

As an attempt to explain what I mean, please check this concept. \$V_O\$ is \$20 V_{peak}\$ when the circuit is opened. The switch is controlled by a simple \$50 kHz\$ square wave (no control loop, obviously). The diodes function as the bridge rectifier:



The rectified voltage, for the conditions simulated here, is higher then the open circuit secondary peak: