Backup power for the real time clock

Official Hardware Nano Family Nano ESP32
1

With older Arduinos I am accustomed to using a DS3231 for clock functions, and these have a VBAT pin to maintain time when the Arduino is powered off. We just run a clock setter sketch when changing time zones or at the beginning of a project and the clock is accurate enough over multiple weeks.

Since the Nano ESP32 has a built-in clock, I hope to get the same functionality without buying an external clock for each project. How can I create the equivalent situation of a battery connected to the VBAT pin?

I am picturing a coin cell battery connected to VIN with a diode to prevent reverse current flow. Do I also have to put the system in deep sleep before the external power is removed? Can that be automated during the moments between when a power cable is unplugged and everything stops? I'm just guessing here, hoping someone will set me straight.

Or should I keep buying external clocks and spend my energy elsewhere?

Please don't tell me "just connect to the internet on boot". The projects I build are used by students in remote locations with no internet or cell service, and their puny grant funding won't cover a satellite solution. Power may be coming from an unreliable wall outlet or a boat's generator, so needing clock backup power is a real issue.

Thanks.

2

Put the ESP32 into deep sleep, in which case the built in RTC keeps running. The ESP32 needs to be powered at all times, but consumes very little current.

I have not seen a successful battery backed solution, but if you find one, post a link.

3

Use STM32 chips. Most of them have a built in RTC, with battery back up connections available. Some hobby boards even have the battery holder on board.