Fair point. There were no such instructions.

I tried desoldering U1, disconnecting the piezo, and powering it with 3V from the batteries side. It blinks rhythmically, so I assume the MCU (or ASIC) is fine. But if I connect the piezo, power consumption drops to < 1mA and nothing else happens. So yeah, that component’s purpose is probably not to charge the batteries.

I will try to read the markings again with a magnifying glass later today. Unless you or someone else have better suggestions.