Why doesn't the app indicate 12V battery level?

I will try one last time.

Your secondary 12v battery was not isolated from the car's 12v battery because you had the cut-out voltage set higher the than the cut-in voltage. The battery-isolator did not work properly to protect the car's 12v battery.

The car's 12v battery was drained down so many times that the battery was ruined and will no longer hold a charge. No amount of charging is going to restore the energy capacity in that battery. Why can't you understand that.

If you don't believe me, you can do an experiment to prove that the battery is not good. Remove the secondary battery. Install the Bluetooth battery monitor on the car's 12v battery. Disconnect the negative cable from the battery. Fully charge the battery, then remove the charger. Let the surface charge of the battery come down for four hours. Note the battery voltage at that point. Then, monitor the battery voltage over the next three days. The battery voltage for a good battery should not drop more than 0.2 volts. I think that you will find that your battery drops much more than 0.2 volts. Are you willing to try this?

 

Do you have results to share from following the procedure suggested in #42?

 

Following advice whose connection to reality isn't apparent to you occasionally results in learning something.

 

All the same, back when you were convinced your Prius had an AGM battery, and people suggested you just go look, you seemed sure there was no reason to do that, all the way from post #66 on page 4 of a thread through post #150 on page 8 of the same thread, until finally (#153) you did look, and you learned something. It would have been possible to learn that sooner by ignoring people less.

Likewise, back when you thought the battery state sensor assembly was part of the battery itself, you went four pages of that thread, from post #42 through post #107, selectively ignoring what you had the opportunity to learn.

Those are just a couple examples of past times your selective ignoring greatly prolonged the time needed for you to learn something. It seems to be more of a pattern than a one-off.