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[on:]

hello,
I need to build a simple 8 cell lipo monitor.. I was thinking of doing that using a voltage divider for each cell, and a 4051 multiplexer to save some analog pins.. I dont need this to be acurate because i am not using this for balancing, just for be sure that one individual cell dont drop under 3v more or less... If I monitor the entire pack this is not possible to know...
So the question is;
1. will this work?


For the same project I am planning to use that same lipo pack to power the arduino with a dc-dc regulator..
2. Is it better to use a 5v regulator and connect it directly to the arduino 5v pin, or use a 7-12v regulator and connect it to vin?


I am planning to use a relay to disconnect the battery from everything else..
3. I it better to disconnect the negative side, the positive side, or both?

4.How will this affect the battery, voltage divider, the 4051, and arduino, becouse the individual cells will remain connected, only the last one and the common negative end will be disconnected?
5.Will i need to disconnect each cell as well?

Thanks for your time..

[Reply #1 on:]

Hi, OK, this is 8 LiPo's in series so, um, about 30 volts?

How much capacity do the LiPos have? Like Amp-hours?  How much current drain on each cell can you afford for the monitoring?

You MIGHT put a series string of resistor, diode, diode, Optoisolator diode across each cell, and get a go/no-go for each cell when it drops below 3.x volts. Hmmm.

Look at the PC817 isolator here (scroll down) http://arduino-info.wikispaces.com/Popular-ICs

So it has a forward drop of 1.0V at 2 mA, add 3 silicon diodes at .7 that's about 3.1, probably more like 3 at 2Ma. Add a resistor so it doesn't draw more current than you want to afford at 3.7V, mm. maybe 100 ohms. 

Maybe gives you 8 isolated "answers"??

Anyone seen something like this??

...just an idea...

[Reply #2 on:]

Hi, thanks for answering..
The batteries are 8 cells in series, and 10Ah. I am not so good in electronics but how i can understand this would be just a low cell monitor, this means that i dont know what is my individual cell voltage, only if one cell drops under x volts it will let me know somehow..

If this is true, It is basically a ok solution, but i would realy like to know my cell votlage(even a rought one), and then I can set the lowest voltage in the code, and I can change it whenever i wont it, and easily use other type of batteries by changing the code...
I will never drain the batteries even near the lowest point so even if it is not accurate it will be a good solution for me..

thanks..

[Reply #3 on:]

Ok, if you want to know the voltages, you'll have to either use the 4051 multiplexer or use an Arduino Mega or compatible (which has 16 analog inputs.. might be easier) Example:
http://goo.gl/cOe3B

So your voltages will all be referenced to the Arduino/Battery ground.  You need to scale the 30 volts down to 0..5 volts (or less) with resistor voltage dividers. Since you'll handle this in software, the ratio does not have to be anything special.  Something like 8 to 1 would be good and safe. You need a separate voltage divider of 2 resistors for the "top" of each cell.

You want fairly high value resistors so the current drain is not a problem. If you used a 470K resistor on the top and a 68K resistor on the bottom of each voltage divider, the ratio would be 0.145 so the 30 volt "top" would give you about 4.3 volts into the Arduino analog input. So the actual voltage reading done in software would be like 4.3/.145 = 29.7 volts etc.

You would read the "top" of every cell, and calculate each cell's voltage.

You can get 1% tolerance resistors cheap now. Here are 20
...( Ooops.. Short Break for an Earthquake here in Italy )... ok.. Back from the street to the house..

Like I said, 20 resistors each of 20 values of 1% resistors for $3.75 here (and many other suppliers): http://goo.gl/EW055 

For your purposes, even 5% resistors would give you 10% accuracy worst case.. 

Let us know how it goes and what you end up doing.. others need to do this too..


DISCLAIMER: Mentioned stuff from my own shop...

[Reply #4 on:]

Thanks,
So separate, and different voltage dividers for every cell will work..? I am planning to use a 4051 to save some analog pins..

The question is will there be any problem if the cells remain always connected, becouse I am only going to disconnect the negative and maybe the positive end of the entire battery pack, so the cells will remain connected to the arduino thru the 4051, and thru the dividers...
Do I need to disconnect each cell as well?

[Reply #5 on:]

Hi,

I think you can leave them connected. The voltage divider resistors will draw less than 100 microamps, and you have 10 AH battery..

The 4051 should work ok on 5 volts, with applied voltages of 4.3V at most..  So 3 output pins to address the 4051 select, and one Analog input to read the 4051 output.. Right??