Teufel Rockster Charge Control Repair

My Teufel Rockster has developed a power-related fault: the internal 12V battery no longer charges while the unit is connected to 230V Power.

The former Owner, told me he may accidently reversed the polarity.
I dont know however if he did that on the internal 12v battery clamps or on the external 12v power supply connector, for which i dont have a cable.

Summary:
The unit is working on internal 12v Power and 230V Power, but the 12V Battery isnt charging (Battery has of course been replaced)

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I need an experienced electronics technician troubleshoot the board, replace or re-solder any failed components, and confirm safe charging and normal power-up behavior.

You will need solid bench-level skills (multimeter, oscilloscope, hot-air rework, fine SMD soldering) and a methodical approach to battery management circuitry. If you have Teufel service documentation or similar experience with lithium-ion BMS boards, that’s a plus.

Deliverables
• Diagnose the root cause
• If economical repair or replace defect components
• Evidence of success: photo or short video showing the Rockster charging to 100 %

I can ship the unit to you securely and will cover return shipping once the repair is verified. Let me know your estimated turnaround and any questions you might have.