My Behringer TD-3 mod worked nicely, so it was time to proceed to the monologue. It already is battery powered, but using 6 batteries and recharging done outside of the device. In my opinion a built-in single battery is more appropriate.
To see the schematic and soldering have a look at the original TD-3 post. The monologue has an even better preparation for the mod since it is built to be operated on battery and 9V, so I assume there is effective and efficient power ORing included.
On the other hand, there is less space in the device, so careful placement is key.
As soon as I have any numbers regarding battery life I will post an update. If you are interested in the knobs: https://ekartco.com/2020/03/korg-monologue-and-minilogue-xd-color-coding-using-chroma-caps/
The device charges and connects via USB on my iMac, so no issues here. The only thing that I need to check is power draw in standby. I assumed that batteries will get protected by KORG anyway, but the “soft” power switch might be draining too much.
My assumption in the above post was that the minilogue pulls less current than the TD-3 and the power surge at startup is not as strong. However, when the KORG is powered on and the battery is not full anymore, the same issue shows up as on the TD-3. Therefore, I will need to short the TP4056 BAT- and Vout- in just the same way as in the TD-3.