Calculating power consumption through OBD port

Question

I need to calculate the power consumption of my device. The device is composed of an esp32 board with a cellular module, powered though an OBD port. I want to have to connected to the OBD port of a car at all time and therefore I need to know its power consumption in order to prevent it from draining the car battery.

The device sends data back to a server at regular intervals. When the car is turned off, the device does enter a low power mode where the intervals are bigger. Knowing the power consumption of the device when in this mode would allow me to determine how long I have to make the intervals in order prevent battery drainage while still getting data sent regularly.

To calculate the power consumption the amplitude is needed. I believe that I can get it by cutting a OBD extension cable and connecting the cables to a multi-meter to measure the amp (I would be using a car OBD port emulator).

Edit: completed question and added details to the problem

Answer 1

This question is incomplete. Generally, if the device is designed correctly, it will draw almost nothing unless the device is active even when the vehicle is not running, ie, the device should be able to determine whether or not the engine is running and all of this depends on the function being performed. If the purpose of the device is not engine related, then the amount of battery power needed is irrelevant since it could be powered by its own power source, not ODB-II.

If that is not sufficient, consider that the drain should be roughly equivalent to that of the cellular module which I assume would probably be the largest load. The cellular module will have a wattage value and will have to be translated into terms of 12 vdc in order to determine battery drain. If you are going to need an amp of 12 vdc power to run a transmitter, the battery will not last a long time: 600 - 700 hours to drain the normal battery and a lot less time to make it so the engine won't start. If possible have the unit periodically send a signal or answer a prompt from the cell phone. Then the power usage would be minimal except while sending a response to a request from the incoming call.

These can only be swags because no one knows what you have in mind for it and you understandably don't want to give your idea away.