Kona Hybrid Auxiliary Battery Issues

That sounds familiar. My 2018 Volvo V60 had similar issues: error messages, weak starts, the works. Turned out to be a voltage drop related to the auxiliary battery's integration. Have you checked the voltage across the auxiliary battery terminals when the car is running? Should be around 13.5-14.5V. If it's lower, or the same as resting voltage (around 12.6V), it's not charging.

Thanks for the insight. Could you provide more details on how the voltage drop was diagnosed in your Volvo? What diagnostic procedures or tools helped pinpoint the problem?

The workshop used a multimeter to check the voltage drop across points in the auxiliary battery circuit while running. They also used a diagnostic scan tool to monitor charging current and voltage in real-time. The charging unit eventually failed completely. Given the symptoms and potential damage to electronic components, get it to a workshop ASAP. Replacing the module cost me around 290€.

Thanks for sharing your experience and the diagnostic details. I'll schedule an appointment with a trusted workshop for a comprehensive evaluation of the auxiliary battery system. It turned out to be exactly what you described. I appreciate your help.