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By Dachs
#45195 I want to read car battery voltage from the OBD port (~12v, 4A) via ADC pin. The current plan is to use a voltage divider (R1 100k - 1W, R2 5k - 1W). A calculator says that this input would be 48 watts. Do I need different resistors, or a different plan? Do I need an voltage sensor board? I really don't know what I'm doing, and could use some advice.

Thanks for any help.
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By xtal
#45215 If you don't know what you are doing, the ODB port is not the place to start, you could possibly damage your on board computer.....use a commercial product....
or Buy a cheap Harbor Freight Voltmeter [on sale about, $3.00] , an measure the battery....
As for resistors , if you want to just monitor your 12 volts [ instead of using the idiot light],
Voltage Divider === > +12 ---small fuse------------resistor1----|A|---resistor 2--------gnd
then point A = (12 * R2) / ( R1 + R2) as for wattage ( 12 * 12 ) / (R1 +R2) Power = IE I= E/R so Power = E*E/R
For monitoring only I would try to use 1 milliamp or less 12 / 10000 = 1.2 ma then 1/4 to 1/2 watt R.......
Remember the max battery Volts will be greater than 12 I would guess about 15...
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By GregryCM
#45225 Hi Dachs,

The vehicle battery voltage has a range of about 9V to 16V. With all of the motors, solenoids, etc on that bus turning on and off, it is not a clean supply. When the loads turn on and off, voltage transients will occur.

The following circuit can be used to monitor the vehicle battery voltage with an ADC input. R1 and R2 divide the voltage from the 9V to 16V range to the 3.3V range of the ADC input. D1 and D2 clamp transients to the supply rails to limit the transient peaks to -0.6V and +3.9V. C1 provides a low pass filter and slows the transients. R3 limits the input current to the ADC input during a transient.

Vbatt ADC Input.JPG
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By trackerj
#45228
Dachs wrote:I want to read car battery voltage from the OBD port (~12v, 4A) via ADC pin. The current plan is to use a voltage divider (R1 100k - 1W, R2 5k - 1W). A calculator says that this input would be 48 watts. Do I need different resistors, or a different plan? Do I need an voltage sensor board? I really don't know what I'm doing, and could use some advice.

Thanks for any help.


it might be a good idea to read my post about Internal ADC and Voltage Divider calculations