OrientExpress wrote:I'm not sure that any vehicle has a low 12v battery warning, but it could be something that would add to the drain on the battery, especially if it was added to the telematics that would message a smartphone app about battery health.
Not quite how I would engineer it.
What you don't want is the car at the airport at midnight dead. In the rain. At 1C or 34F. With a stiff wind.
What you do want is a "check EV system" light turned on with a code indicating that the 12V battery is getting weak, and may fail soon. Ideally this would provide a few months of warning.
Note that other access to the 12V health might be provided to smartphone app, head unit or something else, but that's not the important thing. The warning light before the failure is. How you determine this might be to look at the 12V charging voltage and current. Simplistically: When the car is powered on, before turning the 12V charger on, measure the voltage. Then measured the Ah delivered until the battery reaches "full", remembering to subtract estimated load currents. Use this to determine remaining Ah capacity of the battery. If below a threshold, flag a code. Probably some other clues might be used...
While this, or something similar might not catch all failures in advance, it might catch most.