Regular battery voltage

I haven't been able to find too much information on the circuit between the large and small batteries but I was looking at my app this morning and was very surprised to see only 11.8V. A good 12V car battery, in my experience, should read 12.3 to 12.5V when not being used. Is this a sign of bad things to come? Does anyone have any experience with this? Could you look at your app and tell me what you see?
TIA

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Super helpful
I’ll do a voltage measurement this weekend

 

Ok so standard disclaimers apply but I did get multiple MSc degrees in Electrical Engineering a LONG time ago. It’s mathematically impossible to provide 12V if you are below 12.5V with no load… so if the battery is truly at 11.8, it’s DEAD. So either my battery is dead and the big one is making up for it (which would explain why I only get 15 miles vs 16 miles on a full charge) OR the app is wrong… I’ll find out. You should never measure a DC battery that isn’t under load but if it’s truly 11.8 when I measure it with no load of the weekend (my trusted FLUKE multimeter will tell us) then we know it’s bad… I’m too busy now bit will find out soon and will report back.