SIM800 L GSM module

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Joined Sep 6, 2022

i have a circuit with a gsm module powered by 18650 3.7 Li-ion battery fully charged . There is a esp32 microcontroller powered separately and between them is two communication lines RX / TX and a common ground

The circuit works fine in layout 1 where the ground of the SIM is before the ground of the Microcontroller
however when the circuit is in layout 2 and battery ground sits between the sim ground and the Microcontroller ground there are problems, i get the microcontroller browning out during transmissions. During transmissions the SIM pulls 2 amps. It seems the SIM is trying to draw current from the microcontroller and causing the brownout

My understanding of circuits is very basic and i would like to understand why the two layouts work differently.