Measuring Voltage and Current possible?

Using Arduino Project Guidance
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The project involves monitoring/managing voltage and current drawn from a portable system. A couple of 12V batteries in parallel will power the Arduino (through Vin) + other devices. The negative terminal of the battery will be used as the common ground for the entire system.

Is it possible to measure the voltage of the bus (where all of the batteries' positive terminals are connected to), as well as the total current drawn from all of the batteries? all while the Arduino is being powered by the batteries. Are there any buffering devices required?
(Sorry if this question may seem obvious, but it feels like a "chicken or the egg" question.)

I have been testing Hall effect current sensors (+-5A) in measuring some test currents. However, since the current drawn by the system is predicted to be quite small (100mA-500mA), the noise from the hall effect sensors is quite large in comparison to the actual signal due to the current. (for example, the voltage output from the sensor is 2.48-2.5V when open. With 100mA~ of current, it will jump to 2.5V-2.52V; so effectively the noise signal is as strong as the 100mA signal). Is there a way to deal with this?

Thanks!

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Voltage, sure, resistor divider down into 5V range, analogRead it.
Current - may be better off with votlage measurements across a shunt:

Will need an op-amp to boost the levels - 300mA across a 50mOhm shunt will have just 15mV drop across it. If use the internal 1.1V Aref, that provides 1.07mV/bit (1.1/1024), so you'd only have reading of 5 to 25 across range of 100mA to 500mA.

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I'm currently using the Allegro ACS714 current sensor:

I'm wondering if the voltage fluctuations are mostly due to:
-noise from the current sensor itself?
-the 5V voltage output from the Arduino is not stable at exactly 5.00... V (eg varying due to load)?
or
-the analog read pins not being very accurate/precise?
The current sensor claims to have "an extremely stable output offset voltage" as a feature.

As well, how well will the resistor shunt method perform, in terms of accuracy, in varying temperatures? The system will have to operate outdoors at anywhere from between -30 to +30 degrees Celcius.

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