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If I'm going to measure an AA battery (about 1.5 v) with a USB-powered Arduino, I will connect the battery's +ve terminal to the A0 pin. Shall I connect the -ve terminal to the gnd pin too?

  • Is the AA battery under any kind of load, or just connected directly to the Arduino and nothing else? – Majenko Jan 8 '18 at 12:22
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Yes you could although this means you're measuring in relation to the Arduino's logic voltage, which is slightly below 5v (due to a diode's voltage drop) or the voltage of vref.

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  • Ok, after connecting arduino reads 234 (1.143 v), and multimeter reads 1.116 v. Is that because of arduino logic? how to fix that? – Megamind Saiko Jan 8 '18 at 5:45
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    Try measuring the 5V pin on the Arduino. The USB on a computer is rarely exactly 5V. You might have to do some calibration. – Gerben Jan 8 '18 at 10:47
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    @jdwolf USB-VCC goes through a mosfet (T2 on the schematic), not a diode. Rds(on) is about 70 mOhm, so only a 35mV drop if the board uses the maximum 500mA that USB can supply. – Gerben Jan 8 '18 at 10:51
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    I would use the internal voltage reference of 2.56V, and a protection resistor of about 1k or 4k7 to A0. – Jot Jan 8 '18 at 10:54
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    @Majenko, for minimal protecting of the internal clamping diodes. A mistake with a 6, 9 or 12V battery is easily made. Even with a 1.5V battery there could be a problem when the Arduino is turned off and the battery is still connected. Or when the battery is connected in reverse. You know, better be safe than sorry, especially for a beginner. – Jot Jan 8 '18 at 14:09

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