Voltage Dividers and Arduino

For my WiFiTank I wanted to get a readout of the battery voltage.

But the battery is a 9.6V and the charger for it charges with up to 14V, while the arduino only can handle 5V.

To get around that problem I made a voltage divider. I based it on 20V, just to be on the safe side.

The voltage divider is a 3 to 1, meaning that between netative on the battery, and the point where the arduino’s analog pin is connected, there are a 10k ohm resistor, and a 30k ohm resistor between the same point and positive on the battery. It could also be something else than the 30 and 10k, but that is what I had.

The resistors are connected like this

Where R1 is the 30k ohm resistor, and R2 is the 10k ohm resistor.

The arduino’s analoge pin is a 0-1023 range, so it had to be calculated a bit to get the right voltage.

My first attempt was

map(analogRead(sensorPin), 0, 1023, 0, 20)

• 0, 1023 – The range of the analog pin
• 0, 20 – The voltage being measured

but it didn’t have any decimals on.

Next was like this

map(nowVolt, 0, 1023, 0, 200) / 10.0

and it got it’s decimals, but was around 0,5v off and was only jumping in 0,5V steps.
To get another decimal I added some extra zero’s, and got this

map(nowVolt, 0, 1023, 0, 2000) / 100.0

and both decimals was there and working, but the voltage was still a bit off.

To get the voltage right, I tweaked the values in the map function a bit, to narrow the range a bit down to fit the resistor tolerance in my voltage divider.

map(nowVolt, 0, 1023, 0, 1933) / 100.0

and this is as close as it can be, and is actually closer than I would have dreamed about.

The finished voltage divider, made of 4x10k ohm resistors