I'm working on one of the project and its board MCU is ATMEGA328P.
When I'm run on coin cell with custom PCB then MCU will be restart again n again.
When I'm run on Li-ion battery its code and MCU working very well.
So, please tell me the issue and suggest me the test case for overcome this issue.
What's your BOD set to?
When I'm run on coin cell
As long as you don't pull more then around 25mA, then this should work if you disable BOD. But as you have NOT posted a schematic, then this is just a guess.