My 2018 Avalon Hybrid looses all electricity and disable all functions.

I have requested to replace the 12v battery but dealer rejected my request since their test indicate battery is ok. Is there any warning message when battery is low rather than shutting down the system all the sudden? Any special requirements for 12v battery? Thank you for your help.

No special requirements, just a different system. Typical vehicles use an alternator to charge the battery, but your hybrid uses the an inverter/converter to charge the battery 12v battery. Sounds complicated, but it is just a transformer to convert the high voltage hybrid battery power into 12v for the service battery. If the charge on the 12v battery is dropping at random, it may be the battery, but it may not be charging as it should due to the inverter/converter, battery cables, 12v battery, etc... the list could go on. This vehicle being a 2018 model... lemon law comes to mind. They only get so many chances to fix it before they have to replace the vehicle. Look at the lemon laws in your state to see what criteria has to be met before you can invoke your rights.