# Battery monitor -Voltage Divider Consumption

Hi i'm developing aproject to analyse my batteries that are charged by my solar panels.
my system is "24V" system my batteries run between 20V and 30V in full charge, i made something like this..

i'm seeing that the cable that connects to the batteries taking 0.3 amp at 28V so in s simple math 0,3*28v 8W consumption ...
So my voltage divider is consuming 8w ???
Please help in these maths and how can i have a voltage divider with lower consumptions i don't want to discharge the batteries just to measure them ..
Thanks

i'm seeing that the cable that connects to the batteries taking 0.3 amp at 28V

How are you seeing this? According to Ohm's law, there should be a 0.0025 A drop across the 11K voltage divider. That's considerably less than you are "seeing".

0.3 amp at 28V so in s simple math 0,3*28v 8W consumption .

30V across 11K will give a current of 30/11000 = 2.72 mA

So in the 10K resistor you have I2R = 0.073W
And the 1K you have 0.0073 W

Did you get the decimal point in the wrong place?

Sounds like you are someone who has some expertise which could be helpful in moving....

http://www.arduino.cc/playground/Main/IntWithHW-PwrSup

... forward. It is a new page on the Playground, with information about different ways of powering systems. And it is weak at the moment on the details of charging batteries from photo-volatic sources.

Thought driving something through a voltage divider was bad, and it only should be used to measure?

yes i'm only measuring voltage..
And what is seeing "0.3amps" is my charge controller that also as a load side (that i was not using) where i now connected the measure cables...
The charge controller has a internal meter for charge and load maybe its wrong has it normally shows 0.1amp with nothing connected... i will measure it with the amp-meter later...

but so in theory i would consume 0.0073+0.073=~0,08W that's ok now 8W as a nogo ..
Thanks to all, i will report back when i measure the consumption with the amp-meter

Thanks all.

The charge controller has a internal meter for charge and load maybe its wrong

Yes I would put money on that, those things are not only very inaccurate at low readings but also not very linear.

You are correct. But if you will read the question again, you will see that "measure" is exactly what fca is doing. He is monitoring the voltage.

Yes, but it was actually to tkbyd who mentioned a page about powering it. And that got me puzzled since the voltage dividing didn't have anything to do with powering it.

And I made a voltage divider close to this one, it is just up to 20V instead of 30V, ended up getting a pretty accurate result. (using it on the wifitank)

Hi in fact i'm not powering arduino direct from the batts, but the batts are connected to a inverter that powerup the computer where arduino usb is connected and powered from....
So we can say that these batts are providing power to arduino
what arduino and the pc soft that is talking to arduino are doing is to choose the best power supply batts/mains/generator..
one of the variable for taking this decision is the bat voltage..
Regards