How to connect battery to PYNQ-Z2?

I want to connect Li-ion battery and make charging circuit for pynq z2 board. In user manual it is mentioned at page 7:

A battery can also be used to power the PYNQ-Z2 by attaching the positive terminal
to the “VIN” pin on the Arduino J7 connector (with jumper J9 set to REG).

Also, at schematics page 15:

At Arduino connector description I see that Vin pin is also called “VU_CK” and I think there is some circuit that automatically switches to battery if external power supply drops and vice versa. But there is no acronyms dictionary or any explanation.

What does Q13, Q14 and Q15 do? Are they for voltage control? Are there any voltage sensor/charging control on board? Or should I implement charging as completely separate circuit?

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