Monitoring a LiPo battery while powering the Arduino UNO with it

Using Arduino Project Guidance
21

"So the current is less. This means the battery will discharge slower?"
Yes.

22

Marciokoko:
But they do reduce current flow, right?

Yes.

I think a good analogy is replacing a 4 inch water pipe with (say) a 1 inch water pipe; Current is equivalent to litres per minute (deliberate mix of systems) and voltage is equivalent to water pressure.

...R

23

Right! So what you meant was for me not to say it slows electricity but rather current flow, because current flow is the net movement of the particles from one side of the "pipe" to the other but electricity is the transfer of energy from one particle to the other...

24

Ok so I did some reading up on voltage dividers and found out that if I have a 7.4V lipo, I could use a 1k and then a 2k resistor to get about 4.93V from the middle like this:

Screenshot 2016-01-20 17.23.42.png

25

Yes, Vout = 7.4V * 2000/(1000+2000) = 4.93V.
Continuous current flow will be 7.4V/3000 ohm = 2.47mA.
What is the mAH rating of the battery?

Might want to use higher value resistor:
7.4V * 5000/(2500+5000) = 4.93V
7.4V/7500 = 9.8 * 10^-4 A, or 0.98mA
Could go 50K and 25K even as it's a slowly changing voltage.

26

If that was my project I would choose resistors to give a voltage of about 4v so I do not go over 5v (and damage the Arduino) if the LiPo goes above 7.2v.

...R

27

A 7.2V LiPo is 2 3.6V "nominal" cells.

A fully charged LiPo of 2 cells nominal 7.2V is more like 4.6V per cell...9.2V when charged to 4.6V per cell.

28

Ok so I should tap it like 40k and 35k, such that its around 54% which would be 4/7.4V.

So that gives me a current of 7.4V/75,000ohms = 0.098mA. Thats kinda low isnt it? What did CrossRoads mean by "its a slowly changing voltage"? The mAH rating is 4,000mAH.

29

Marciokoko:
Ok so I should tap it like 40k and 35k, such that its around 54% which would be 4/7.4V.

I think those resistor values are a bit too high. The Arduino ADC prefers about 10k so stick to values close to those recommended by @CrossRoads, but adjusted to give the right ratio.

...R

30

Well he was the one who said I could go as high as 50k and 20k. Why would it affect the Uno?

31

Marciokoko:
Well he was the one who said I could go as high as 50k and 20k. Why would it affect the Uno?

I had missed that. It's just that the ADC takes longer to charge up if the external resistance is high. However as @CrossRoads noted, your values only change slowly so it should be OK.

...R