Measuring battery Vcc

[Baruch Even]

Hi,

I found an explanation on how to measure the battery Vcc and would have liked to use it in the future to measure the battery status of my battery shield equipped minions. You can find the info at http://provideyourown.com/2012/secret-arduino-voltmeter-measure-battery-voltage/

I tried it and the usb input shows 3.3 volts which is probably the regulated supply, when I use it on the batteries (3xAAA rechargeables of 1.2V each) I get a reading of 0.05V which sounds a little implausible. I'm wondering if someone can shed some light on this and explain it out.

Baruch

[gpcra...@volksdata.com]

Hi Baruch,

I'm not particularly good at reading schematics, but it seems to me that the Vcc lines are connected to the output of the 3.3V regulator and thus you should always read 3.3V for Vcc.  I am not sure why you are reading 0.05V for Vcc when connected to batteries.  There is a comment in the post you reference (from Oct 16) where someone is noting he is getting strangely low readings. 

I am referring to the schematic:  http://www.minioncity.com/wp-content/uploads/2013/01/Minion_V1.0_Schematic.png

Perhaps someone who is good with schematics can shed some light on this.

That is a very interesting post that you reference.  I will have to give it a try and see what results I get.

-Greg

[Evan Sakamoto]

Yes, Vcc is connected to Vout of the regulator. So as long as VIN > 3.3+Vdropout, Vcc should be 3.3v.

I can think of two things to do:

1. Connect Vbatt to an analog in, using a resistor divider to scale 4.2v to the internal reference voltage (e.g. 39k and 56k for 2,56v)
2. Measure Vcc, when it drops from 3.3v, that means the battery is below 3.3+Vdropout. If you know what Vdropout is you know what Vbatt is.

1 is more accurate, you know exactly where the battery sits, but requires extra parts and an analogue pin. 2 is free, and if all you want is to know when the battery is getting low, it's good enough.

Home this helps.

[Glory Vandesteene]

getting strangely low readings