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Topic: Measure Ebike Current across 0.15 mOhm Shunt Resistor (Read 123 times) previous topic - next topic

leozhi

Hi
We make electric motorcycles and are looking to design an arduino based display and telemetry circuit for the vehicle.

My problem is in measuring battery current. To measure this current, we've used a 0.15 mOhm series shunt resistor and plan to measure the potential drop across it. 75 mV equals 600 A. For this particular bike we don't need to measure more than 200A of current, implying a 0-25 mV range.

What I want to do:
Measure current with an accuracy of atleast 1 A
Range of current to be measured (-50 A to 200 A)  -- (So 0.125 mV as a least count)
Frequency of Measurement needed : 5 readings/second are more than good


What I've tried:
Used an LM358N to amplify voltage with R2 = 150k ohm, R1 = 1k ohm
A schematic diagram is attached. (LM358N schematic 20150114.jpg)

Output:
Attached (LM358N Schematic 20150114.png). I get this output irrespective of whether the battery is supplying current.

Where I need help:
I've checked and double checked the LM 358N's data sheet. The code is a simple analogRead(0) - pin where output from the opamp is plugged in. What am I missing!?

Thanks a lot for reading this far :)


weedpharma

I am not an electronics engineer.

I was just wondering if the problem is due to the input floating as there does not appear to be anything to develop the voltage across.

Weedpharma

Peter_n

I don't understand the schematic. Sorry. Is the shunt at the high-side (plus battery) or low side (minus battery) ?

leozhi

First, I'm sorry about the late reply - noob to the forum and thought notifications would come to my inbox! :(

@weedpharma: I'm sorry I forgot to mention that I've made the ground common between the Arduino and the "low" side of the shunt resistor.

@Peter_n: The shunt resistor is on the negative side. Also, I was able to look at the TI link - this might sound stupid, but I don't know what difference it makes if I connect the circuitry to the negative side, or the positive side. Finally, from what I could understand from the other thread and TI's sheet, the INAxxx devices are more disciplined versions of what I'm attempting, translating to lesser noise. That shouldn't make my circuit not run properly, no?

Something else I tried:
I made a ~1 mV input (dropped Arduino sourced 3.3V across 150k ohm and 110 ohm). The LM358 successfully amplified it to the region of the expected value (gain was of about 101). However, when I connected the motorcycle and gave it a spin (while it was on its paddock; current was at about 5-7 A, equal to about 1 mV), the analogRead value refused to budge from zero.

Any idea what I could have done wrong?

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