Hi @steve274 ,
There is no current drawn from the battery if the circuit on the carrier board is correctly done. The module itself requires the VCC_BATT also while the module is running. The consumption is even slightly higher than in the off state. Beside the on-module RTC, the VCC_BATT is also used for the SoC power management state machine. This state machine for example has an output signal to the PMIC for starting the power up sequence. This is the reason why the module also requires the VCC_BATT rail, even if the on-module RTC is not in use.
In order to make sure there is no current draw from the battery while the module is running, we recommend adding two diodes. You see these diodes in my drawing or in the schematics of the evaluation board. Since the battery voltage is normally lower than 3.3V, the VCC_BATT will be sourced from the 3.3V rail as soon as this rail is available.
Theoretically, it would be possible to use one of these CR123 batteries. The maximum input voltage rating for the VCC_BATT is 3.6V. The drawback with this battery could be that a fresh battery can have up to 3.25V. This means if the 3.3V rail is slightly lower (3.135V is still in the tolerance), the two diode circuit is not working fine anymore. The current for the VCC_BATT will be drawn from the CR123 battery instead of the 3.3V rail. However, this should not be a big issue since it anyway only happens when the CR123 is very fresh.
You maybe also need to consider self discharging of the CR123. I do not know how much that is, but when we assume you have only the external RTC with a consumption of 0.9uA, this would give a theoretical battery lifespan of 190 years. I doubt that CR123 will be able to do that. It would be interesting to know what the self discharge of such a battery is. Maybe you do not get a much longer battery lifespan than with a regular CR2032. But of curse, if the consumption is higher than 0.9uA, the CR123 would be beneficial whit its seven times higher capacity.