If you find that your Galaxy Note 9 smartphone connects to your car’s Bluetooth function (for audio and phone use) but that after a while the connection between the Note 9 and the car stops working.
The problem is caused by the battery optimisation setting for Bluetooth on the Note 9. The factory setting is for the Note 9 to turn Bluetooth off after a while to reduce battery usage. You can change this behaviour as follows:
On your Note 9 go to Settings
Click on Apps
Click on any app you see there
Click on Battery
Click on Optimise battery usage
Change the option at the top from “Apps not optimised” to “All apps”
Scroll down to the app called Bluetooth and turn the slider off (it should go from Blue to Greyed out)