It's more likely the 7.2V batteries can't supply the current needed or the buck converter needs more than 7.2V. We can't tell which because you have not supplied the information needed to decide between the two
That is probably the reason
This is not really a Pi question.
Most conventional computers have a BIOS which is in a ROM (containing 32 MB of code). Later models use a different, but similar technique.
The BIOS contains all the code required to mount images, initiate hardware etc. but even there require a boot loader on the install medium.
The install medium contains code to run a "...
You CAN power external components; indeed for some this is recommended.
It is (generally) essential to share Gnd connections (opto-isolated devices are a possible exception).
There is actually no need in this case, as the Pi 5V can easily supply the current needed by 2 relay coils.
HOWEVER if you are using a relay module similar to that pictured they are ...
Yes - you can definitely do that. Two things you must keep in mind:
The RPi and the external battery-powered hardware must operate at a common reference potential. This is easily accomplished by connecting the two GROUND points together.
RPi GPIO pins are biased at 3.3V, and they are rather fragile. You must properly interface them to your external ...
In general, yes this is possible.
You must connect the grounds of both power supplies together at some point.
You must also make sure that no 5 V logic signals that come from your 5 V logic are connected to pins on the Pi, as this can permanently damage the Pi. There is no danger of damage if GPIO outputs from the Pi are connected to logic inputs in the 5 V ...
This is an old question, but there are new answers:
The original question will soon be 10 years old, and the "official documentation" quoted in the currently accepted answer has been revised. The RPi FAQ is in the same location, but the relevant FAQ is now here: What is its operating temperature? Does it need a heatsink? But this FAQ is not ...