Mesh Devices - Battery for real time clock

Hardware
1

I am currently using the Particle Photon, which has the VBAT input for battery (we are using a C1225 battery) to maintain the clock when power gets disconnected. It is not a guarantee in our industry that there will be internet available 100% of the time, and part of our firmware is logging events to an SD card (everything gets timestamped). If there is no internet available the device would not be able to synchronize with the cloud to get the real time.

Do the new mesh devices have a way to support maintaining the real time clock when main power is disconnected? Including a LiPo battery in our design is not really feasible.

2

Without power and a battery I do not think the RTC function will work.

You can add a RTC with a small coincell battery that would last for years to hold time.

This has proven to be a very accurate RTC from my testing.

3

The datasheet does not mention a power supply for the RTC that I can see [wait to be told otherwise]!

We have a similar issue with the SD log timestamps.

You could use another type of battery chemistry if Lithium is not possible. As we know, shipping and storing Lithium batteries has some challenges. We have gone for NiMh 9V PP3 with an AC supply taken down to 12V DC and using a dedicated charger IC. There is also some other circuitry to avoid the battery becoming over discharged and then irrecoverable by the charger. This provides enough power to shout-out when the AC supply is off and put the photon into sleep and after a further period into deep sleep mode. With other power management circuitry around the photon this battery could survive unpowered for 2.5 months.