C5 Additional Battery Alarm Defect

Greetings! While I am not intimately familiar with the Citroen C5's specific architecture, I encountered a similar predicament with my previous vehicle, a 2014 Volvo V60. The symptoms you describe—specifically, the error codes coupled with the alarm system's failure—strongly suggest a deficiency in the auxiliary battery's capacity to maintain adequate charge. It is plausible that the battery's diminished performance stems from the natural degradation associated with age. Have you performed a system diagnosis to pinpoint the exact error code? A battery check might also provide valuable insights into its state of health.

Thanks for the advice. I haven't done a system diagnosis myself. Where exactly should I check for the battery's state of health?

In response to your query, I must emphasize the potential complexities involved in accurately assessing a battery's condition without specialized equipment. While visual inspection may reveal obvious signs of physical damage (e.g., bulging, corrosion), a comprehensive evaluation necessitates the use of diagnostic tools capable of measuring parameters such as voltage under load, internal resistance, and state of charge. Given the Citroen C5's sophisticated electronic systems, I would strongly recommend entrusting this task to a qualified workshop equipped with the appropriate diagnostic capabilities. They can perform a thorough system diagnosis, interpret the error codes, and provide a definitive assessment of the auxiliary battery's functionality.

Thanks for the confirmation. I went to the workshop, and it turned out the auxiliary battery was indeed discharged due to ageing, just like you said. Had to replace it; cost me 330€. At least the alarm works now.