# Voltage divider to measure 12v battery with arduino low consumption

I want to measure 12v. Car battery with arduino. Three questions:

1. Is the R1 and R2 the best values? The car battery can go from +10v. To +15v.
2. How and what can I use to save power between the measure reads? Like a transistor to switch on and off or something like.
3. Must I add something to protect arduino pin from car battery overvoltage?

I have this:

———————————————— +10v. To 15v. Car battery
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Something here to switch off and save power?
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R1 = 150kOmh
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|——————————- to analog arduino pin to read voltage
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R2 = 470kOmh
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——————————————————- GND negative battery


Solved!

———————————————— +10v. To 18v. Car battery
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R1 = 36kOmh
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|———————————————- to analog arduino pin to read voltage
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R2 = 10kOmh  5.1v Zener diode to protect arduino pin
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—————————————————————————— GND negative battery and arduino

• The divider value is too high, which will affect the accuracy. A few kOhms is ok. – Long Pham Apr 9 '18 at 16:41
• if you want protection, maybe you should add a 5V zener diode – Long Pham Apr 9 '18 at 16:42
• A few KOhms of resistance won't drain much your battery – Long Pham Apr 9 '18 at 16:43
• How is the Arduino itself powered? – Claudio Avi Chami Apr 9 '18 at 16:45
• you can connect R2 to a GPIO instead of GND. To save power, you inputMode(x, INPUT_PULLUP); digitalWrite(x, HIGH) and to read you use inputMode(x, OUTPUT); digitalWrite(x, LOW), which connects the divider to the GND with a low-impedance path, whereas the INPUT_PULLUP mode throws it behind dozens of more kohms... – dandavis Apr 9 '18 at 20:46