Measuring Current

I have a new project in mind for an Arduino. I would like to measure current and voltage from a LiPo type battery. I would like to measure the unloaded voltage of the battery which should be pretty simple. I would then switch in a load and measure the battery voltage and the current to the load. I have a current shunt which I could use but I am concerned about the input impedance of the analog input to the chip because I will be measuring about 8 amps thru the shunt. I don't want that much current trying to find a path thru the chip.

Any thoughts on my idea and if it is safe to do?

RWW

RogerW: I have a new project in mind for an Arduino. I would like to measure current and voltage from a LiPo type battery. I would like to measure the unloaded voltage of the battery which should be pretty simple. I would then switch in a load and measure the battery voltage and the current to the load. I have a current shunt which I could use but I am concerned about the input impedance of the analog input to the chip because I will be measuring about 8 amps thru the shunt. I don't want that much current trying to find a path thru the chip.

Any thoughts on my idea and if it is safe to do?

RWW

Just because of the very high DC input impedance of a analog input pin you will have no problems about having to worry about current finding a path through the analog input pin. The only requirement is that the shunt must be 'ground based', that is one side of the shunt must be directly wired to both the power source ground being measured and wired to a arduino ground pin. Then the 'high side' of the DC shunt can be wired directly to a analog input pin. Just be sure the ohm value of the shunt resistor is such that it won't develop more the +5vdc at the maximum current value, as that is the electrical voltage limit for an arduino analog input pin.

Lefty

You neglected to mention some basic information, like # of cells, expected load current or the reason and method you propose to do this little task. Without those "details" answers tend to be sparse and random in their content... It is unreasonable to demand an answer to a question without any input as to what the question applies to.

Bob

Suggest you get yourself a hall sensor with a 5 volt output range. Total isolation between measurement and output so you can stick into any part of the circuit. Yes they cost more than shunts but resolve all the problems associated with shunts.

Docedison: You neglected to mention some basic information, like # of cells, expected load current or the reason and method you propose to do this little task. Without those "details" answers tend to be sparse and random in their content... It is unreasonable to demand an answer to a question without any input as to what the question applies to.

Bob

Wow, thanks for the nice response. Did you read my first post? I clearly stated that I wanted to read unloaded voltage from the battery and loaded current and voltage from the battery. I also stated that expected current was going to be about 8 amps. I did not mention the number of cells because my question was about concern of that much current finding a path through the Arduino. Further, where did I DEMAND an answer. I am very insulted by that sentence in your response.

RWW

retrolefty:

RogerW:
I have a new project in mind for an Arduino. I would like to measure current and voltage from a LiPo type battery. I would like to measure the unloaded voltage of the battery which should be pretty simple. I would then switch in a load and measure the battery voltage and the current to the load. I have a current shunt which I could use but I am concerned about the input impedance of the analog input to the chip because I will be measuring about 8 amps thru the shunt. I don’t want that much current trying to find a path thru the chip.

Any thoughts on my idea and if it is safe to do?

RWW

Just because of the very high DC input impedance of a analog input pin you will have no problems about having to worry about current finding a path through the analog input pin. The only requirement is that the shunt must be ‘ground based’, that is one side of the shunt must be directly wired to both the power source ground being measured and wired to a arduino ground pin. Then the ‘high side’ of the DC shunt can be wired directly to a analog input pin. Just be sure the ohm value of the shunt resistor is such that it won’t develop more the +5vdc at the maximum current value, as that is the electrical voltage limit for an arduino analog input pin.

Lefty

Thanks Lefty. I had concerns about the input impedance. I also thank you for your suggestion on how it needs to be hooked up. Your answer reinforces what I was thinking needed to be done.

RWW

jackrae: Suggest you get yourself a hall sensor with a 5 volt output range. Total isolation between measurement and output so you can stick into any part of the circuit. Yes they cost more than shunts but resolve all the problems associated with shunts.

I am not familiar with a Hall effect sensor that measures current. Can you suggest a part or a link to further reading. I do like the thought of isolation from such a high current.

RWW

http://www.hacktronics.com/Sensors/Current-Sensor-30-Amp/flypage.tpl.html

The above link details a complete shield to match the arduino. Whilst it is rated for 30 amps it should meet your needs.

If you search for "hall sensor" in your favourite component supplier's web-site I'm sure you will turn up something that might meet your immediate requirements (but at the price, the above seems to be good value)

That device is just what the doctor ordered. I have placed an order for one. There is even a tutorial with some code to hack. How great is that!!

Thanks for the link!! :) :)

RWW

30A Range ACS712T ELC-30A Module Current Sensor Module Special Price For Sale

search for this one on ebay, cheaper and its got terminal connectors fitted to the board

I have a current shunt which I could use but I am concerned about the input impedance of the analog input to the chip because I will be measuring about 8 amps thru the shunt.

The adc pin's input impedance ahs nothing to do with how you are going to measure the current.

Think if it needs to be done on the highside or lowside instead.

dHenry,

I was going to put the shunt in the return to ground. One side of the shunt grounded. The high side would then go to the ADC pin of the chip. My concern about the input impedance is valid as I did not want 8 amps trying to find a path through the chip rather than the shunt. Can you imagine what 8 amps would do to it. Magic smoke and fire for sure. :astonished: :astonished:

The point is now moot as I have one of the Hall Effect sensors from Pololu on order to try. I like the isolation this device will provide.

I am going to need to get creative on the voltage measurement side of the project as I have two different battery cell counts. One is a 3s battery which provides 11.1 volts and the other is a 6s battery which provides 22.2 volts. I am going to need two different voltage dividers depending on which battery I am measuring. Right now I am thinking of using the balancing ports as a place to measure the battery voltage.

The 3s balancing port has a smaller connector on it vs the 6s battery so when I plug in the 3s I would connect into voltage divider A and when I plug in the 6s Battery I would connect into voltage divider B.

I am also going to have to have two different load cells for the two different cell count batteries. I plan on using 12v 50 watt halogen bulbs for my load. Two in parallel for the 3s and 4 in series/parallel for the 6s cells.

RWW

The adc’s input impedance has nothing to do with what you are trying to do. Electrons are like water (and people) - they go through the path of least resistance. So the current shunt would have taken (almost) all of the current.

As a matter of fact, you want the adc’s impedance to be as high as possible.

dHenry,

Yep, I understand that and know that MOST of the current would go thru the shunt. My concern was how much would go thru the chip and that was answered by another poster that told me that the input impedance was high enough to not concern myself with it.

So, on with the project when the Pololu device arrives. I plan on hooking the Pololu device up with my battery charger and using the Arduino to measure the charge current of a battery. That will give me a number to compare the Arduino results to as the battery charger tells me instantaneous current throughout the charge cycle. That should be a pretty good test of my circuit and code. Then I can move on and write some code that will allow me to measure unloaded voltage and loaded voltage and current so I can determine the internal resistance of the battery under test.

RWW

how much would go thru the chip

Practically nothing will go through the chip.