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### Topic: Measuring LiFe battery pack voltage with Arduino (Read 3600 times)previous topic - next topic

#### Aduen

##### Feb 24, 2011, 12:30 pm
Hey,

I was wondering what the possibilities are of measuring the voltage of a LiFe battery pack of 13.2V. I know I can divide the voltage so its 5V at 13.2V and measure the 5V with the analogue input of the Arduino to see the drop.

Now what I am worried about (this is where my limited electronics knowledge gets me), is it safe. I am afraid that I might feed the Arduino boards analogue input a massive current and completely blow the life out of it. Maybe my previous stated idea is wrong from the start and there is a safe solution.

The battery feeds a big motor, a big servo some sensors and the Arduino board. The battery pack is from Graupner (13.2V 2000mAh) 4C LiFe.
For these kind of batteries it is not recommended to completely discharge them, that's why I want to cut it's supply before this happens.

I you guys can help me, (maybe there is a thread already about this topic but, I was fighting the new search engine and I lost)

#### Magician

#1
##### Feb 24, 2011, 01:13 pm
Don't worry, resistive divider is safe approach to decrease voltage.
Just don't exceed +5 V

#### mowcius

#2
##### Feb 24, 2011, 03:55 pm
Add in a 5.1V zener diode as protection but apart from that - should be fine

#### jackrae

#3
##### Feb 24, 2011, 05:30 pm
Put 22k resistor in series with the input to the arduino.  Even if you totally screw things up, the resistor will limit the input current to a safe value of <1ma.  (13.2/22000 = 0.6ma) and there will be no need for the external zener

jack

#### retrolefty

#4
##### Feb 24, 2011, 05:57 pm
Just be sure whatever action you take on low battery voltage, that it also removes the voltage divider from the battery. That is because even though it's a small current it will continue to discharge the li-po battery and potentially damage it. Some li-po batterys contain internal protection that will prevent over discharge and overcharging conditions. I think the ones SparkFun sells have such internal protection.

"Battery includes built-in protection against over voltage, over current, and minimum voltage."

http://www.sparkfun.com/products/339

Lefty

#### Aduen

#5
##### Feb 24, 2011, 07:13 pm
Ah, sweet dudes thanks! I'll will also take the extra 22k resistor recommendation just to be sure (not having a 5.1 zener diode lying around  ).

Thanks everybody!

This is what it's all for:

...soon it will be rolling around

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