user63316: I have a problem, I am using 56V DC combined batteries system. I get ADC values comes to Arduino like 211, 217 etc... I have to find out when the DC volt drop to 42V. How do i know the digital value for the ADC from Aruino. 

user50183: I'll assume you're using Li-Ion batteries in 14s configuration. You'll want to measure voltages up to around 60 V while charging. With a bit of safety margin, I'd set it up to measure up to 72V without damaging the Arduino - which can measure voltages up to 5 V, but the input on that pin should never exceed 5.5 V.
To bring your battery voltage down to something the Arduino can use, you'll need to use a voltage divider, as chrisl suggested.

With R1 = 10 kOhm and R2 = 680 Ohm, you'd get readings up to an input voltage of 78 Volts.
With analogRead() returning values between 0 and 1023, your 42 V cutoff would be around 546.

63316: <p>I have a problem, I am using 56V DC combined batteries system. I get ADC values comes to Arduino like 211, 217 etc... I have to find out when the DC volt drop to 42V. How do i know the digital value for the ADC from Aruino. </p>
 50183: <p>I'll assume you're using Li-Ion batteries in 14s configuration. You'll want to measure voltages up to around 60 V while charging. With a bit of safety margin, I'd set it up to measure up to 72V without damaging the Arduino - which can measure voltages up to 5 V, but the input on that pin should never exceed 5.5 V.</p>

<p>To bring your battery voltage down to something the Arduino can use, you'll need to use a voltage divider, as <a href="https://arduino.stackexchange.com/questions/72739/arduino-analog-to-digital-conversion-from-56v-dc-batteries#comment159022_72739">chrisl suggested</a>.</p>

<p><a href="https://i.stack.imgur.com/ZqJpf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZqJpf.png" alt="Schematic, allaboutcircuits.com"></a></p>

<p>With R1 = 10 kOhm and R2 = 680 Ohm, you'd get readings up to an input voltage of 78 Volts.</p>

<p>With <code>analogRead()</code> returning values between 0 and 1023, your 42 V cutoff would be around 546.</p>
