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I need to make a car tracker powered from a car battery. The problem is the SIM800L needs to be powered with 3.8v to 4.2v, and at 2A preferably. I want to connect the car battery to my Mega first then the Mega powers the SIM800L.

So:

Car battery > Mega > SIM800L

What do I need to achieve this?

This is the SIM800L model:

This is the Sim800l

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With this setup you are overstressing the onboard regulator, while wasting a lot of power (not suggested on a battery setup, even if the battery is quite big).

First of all, the power. You have a MEGA, which runs at 5V/few mA, and a SIM800L, which runs at 4V/2A. I suggest you to have two power supplies. On ebay you can find cheap XL4015 converters rated at 5A. You connect IN+ to the +12V, IN- to the ground, then set the first one to output 5V, while the other to 4V. If you want to have the most correct powering scheme connect the 5V to the USB port, otherwise connect it to the +5V pins (note that you will exclude the fuse).

Then there is another problem: the UART levels. As a first approximation you can hope that the arduino reads the correct levels, so you'll just have to lower the TX voltage from 5V to 3.3V. You can do this by using a voltage divider with a 4.7k resistor between arduino TX and SIM RX and a 10k resistor between SIM RX and ground (see, for instance, how they did it in this image).

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I want to connect the car battery

The Mega has an NCP1117 linear voltage regulator onboard. This is qualified for automotive use, however the max. output current at 12V input voltage is only 1A, which is too less to supply your SIM800.
Next thing you should consider: The car voltage is not stable. Voltage peaks up to 40V can occur, especially when starting the engine. This will stress your components, and destroy them in worst case.
I would use two DC/DC converters with high enough input voltage and adjust one to 7V output (for the Arduino), the other one to 4V (for the SIM) and connect the ground lines together.

For voltage level matching between Arduino and SIM Uart see page 32 of SIM800 hardware manual.

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