Have you been driving it in eco mode? I was doing that and drove the 12V battery until it had a dead cell and had to be replaced, and the new 12V battery will not attain full charge. I started driving it in performance mode and with the engine running more, the 12V battery now gets charged. My car has been through two sets of diagnostics, so I doubt that anything is wrong with the hardware. I would get myself a 12V battery maintainer, bring your 12V battery up to full charge and start running in performance mode.
