Bypass bluetooth speaker not working without a connected battary

Hello,

I have a very nice looking bluetooth speaker from Teufel called Bamster (no model number, only SN available at the back).
Because the speaker of my monitor are deteriorating I wanted to use the bluetooth speaker with two wires: Micro USB for power and an AUX cable.
To my surprise there was no battery inside, a bigger surprise was that there are no aftermarket batteries in sale ALSO no disassembly videos for this speaker. I guess it wasn't sold for a long time.

Anyway, the battery connector has 3 Pins. I connected it to my power supply, middle pin was ignored and I got it working, I could pair it with my phone and play music. However, no sign of life, if I connect it to the power socket.

How can I bypass the battery is not connected issue? I thought about connecting the 5V micro USB with the + battery connector pin and same with ground. This way I could close the unit and keep the aesthetics. Other way is of course to make a hole and connect straight to the battery connector pins. I guess I have to remove the micro USB to make space for the wires and this way I wouldn't need to drill holes.

I am curious though:
Would you be able to solve this issue, if you would have schematics for the board? I think is not possible, right? There must be some IC which checks the battery. Maybe some kind of a hack would help.

Thank you.

Edit:
I salvaged a small LiPo battery with 3 pins from a 2016 JBL wireless headset with 650 mA and soldered it to the battery connector. With AUX I had a very loud noise but with Bluetooth the speaker works fine. Great sound, great design and I don't need to fix my monitors speakers.

Had to use a Bluetooth dongle to connect to PC. I am worried that the battery will be charging all the time but luckily the battery has a small PCB, in case something goes wrong, I hope it will shut the battery. The led on the device indicates charing, it charges all the time. I hoped it would get the current from the micro USB.

I have to keep testing, because I noticed few things:

-In one of two restarts, I had to reconnect the speaker via the Bluetooth settings in Windows

-I lost audio in game but Windows had sound, had to restart the game, didn't happen again

-The PC is near my bed, the speakers goes in safe mod after 10 and it turns off after 20min. according to manual, I hope it makes no sounds while doing so

-I think I will turn off ErP (USB power after PC shutdown), so the speaker don't draw current.