user11802: This questions has been formulated several times with not clear answer, and I wonder if there is one.
it is well stablished from several sources that measuring the voltage of a battery alone is not good indicator of the State of Charge (SOC) of the battery because voltage does not decline proportional to the soc.
Another solution (http://fritzing.org/projects/battery-charge-level/) suggest measuring the voltage (with analogread) between two resistors connected to the gnd and vcc of the circuit (I presume this constantly draw power) and then choose what is the critical voltage of the circuit.
However, my question is how to measure the SOC of a battery using units of 0 to 100% in arduino?

user7856: To do it correctly, you need a coulomb counter. Something like this.
Even that is not fully sufficient, as the charge that can be obtained from a battery depends on various factors, such as temperature and ageing.
More recent batteries have somewhat flat discharge curve, when it comes to voltage, till they run flat, then the voltage drops significantly.
Which means that you would need a very accurate voltage meter.
However the starting point is that you must be able to count how much charge is going in and out.

11802: <p>This questions has been formulated several times with not clear answer, and I wonder if there is one.</p>

<p>it is well stablished from several sources that measuring the voltage of a battery alone is not good indicator of the State of Charge (SOC) of the battery because voltage does not decline proportional to the soc.</p>

<p>Another solution (<a href="http://fritzing.org/projects/battery-charge-level/" rel="nofollow">http://fritzing.org/projects/battery-charge-level/</a>) suggest measuring the voltage (with analogread) between two resistors connected to the gnd and vcc of the circuit (I presume this constantly draw power) and then choose what is the critical voltage of the circuit.</p>

<p>However, my question is how to measure the SOC of a battery using units of 0 to 100% in arduino?</p>
 7856: <p>To do it correctly, you need a coulomb counter. Something like <a href="http://www.maximintegrated.com/en/products/power/battery-management/DS2740.html" rel="nofollow">this</a>.</p>

<p>Even that is not fully sufficient, as the charge that can be obtained from a battery depends on various factors, such as temperature and ageing.</p>

<p>More recent batteries have somewhat flat discharge curve, when it comes to voltage, till they run flat, then the voltage drops significantly.
Which means that you would need a very accurate voltage meter.</p>

<p>However the starting point is that you must be able to count how much charge is going in and out.</p>
