For daytime timelapse operations I also found the
rawopint script which tries to minimize aperture flickering in timelapse photos intended for movies.
For the external power supply I made a small cut-out in the side of the casing of the Xiaomi powerbank, near the front panel.
I soldered two wires to the terminals of the battery cells in the power bank; these wires end in the
JST plug (female plug but male contacts) that I glued to the PCB near the 'power' switch.
After sliding the electronics and the batteries of the powerbank back in the aluminium casing I can reach this plug with it's JST counterpart.
The counterpart JST plug is attached to the cable that I cut off the ACK-DC40 after verifying the polarity.
When I connect the Xiaomi to the SX260HS using the cable and fake battery I can power the SX260hs from the powerbank at 'normal' voltages for the camera. Filled up a 16GB card while timelapsing to test the powerbank in this setting.
Pictures show the fake battery with the cable from the ACK-DC40, now terminated with JST plug.
The other picture shows the side of the Xaomi powerbank where I cut a niche in the casing, revealing the JST plug glued to the PCB.