I don't know if anyone has determined the maximum resistor divider value that still works on analog input, but I suspect 1M would be ok. That would give you 2M in total, so not much current would flow. But yes, it would flow all the time.
If you want to switch it, you would need a P-channel to do the switching, plus an N-channel, or bipolar NPN, to control the P-channel's gate. You can't connect an Arduino pin directly to the P-channel's gate.
Re: 4 - I've used the techniques described here to measure battery voltage. It's usually been 3x AA non-rechargeable batteries but i've also used it to monitor the voltage on a 3.7v LiPo battery connected to a CN3065 based solar charger.
My setup is Vin to 2M2 to a 560K to Gnd + a 100n cap in parallel with the 560K. The junction of the 2M2 and the 560K gets fed to one of the 328P ADC channels. There's no switching, the resistors stay in circuit all the time. This generates about 1.014V from 5V in so can be used against the internal 1.1V reference.
My 100n is unpolarised - just some axial 100n cap i've had lying around for ages.
The internal voltage reference is just that - a reference voltage that is known. In order to measure something, you have to compare it against a known reference in order to determine its value. Note that the internal reference on a 328P is 1.1V, so you need to calculate the resistor values to deliver no more than 1.1V.
A quick play with the Electronics2000 potential divider calculator shows that if you leave your R1 at 2M2, then make your R3 a 360K, then you should get around 1.041V from a 7.4V input.
Very helpful reply. My mistake was in the third resistor as I'm still learning to draw from text descriptions. I can calculate the value across a divider but thank you for saving me the futzing around This is now a 7.111 divider.
What does the capacitor in this circuit do? Does it block the ground when it reverses? Does that stop the current draw?
Issue with the AREF pin. If I've read correctly, it's available on the Uno but not the Mega. Do I have that right?
INTERNAL: a built-in reference, equal to 1.1 volts on the ATmega168 or ATmega328P and 2.56 volts on the ATmega32U4 and ATmega8 (not available on the Arduino Mega)
INTERNAL1V1: a built-in 1.1V reference (Arduino Mega only)
Re the capacitor - have a read of the link I gave you in reply #4.
As far as I know, the AREF signal is brought out to a pin on the actual micro package. Whether that signal is routed to an accessible connector is down to the board maker. On an UNO, its available on the header near the USB connector. Same for the MEGA.
I think the confusion may be simply down to the names given: INTERNAL, INTERNAL1V1 and INTERNAL2V56. I've not coded on a MEGA board so i'll hazard a guess that selecting the internal 1.1V reference on a MEGA is:
But if you're writing code for an UNO, then the same 1.1V reference is selected by:
The way it all works is that the ADC (Analogue to Digital Converter) compares the voltage you want to measure against a reference - say 1.1V. On the 328P (used on the UNO), the ADC is 10-bit which means that it will give you values between 0x0000 and 0x03FF. In theory when you apply 0V, the ADC gives you 0x0000 and when you apply 1.1V it gives you 0x03FF, with a linear relationship between Vin and the ADC value.
In simple terms, the resolution is the voltage change required to change the value reported by the ADC by 1.