How to Use Arduino and INA219 to Log Current Draw for Telit Module Battery Life Estimation

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#1 21660597 05 Jan 2012 11:17

James Russo James Russo

Anonymous

Post #1
21660597 05 Jan 2012 11:17

Hello,

We have a product which uses a telit module w/ cellular and GPS and we are trying to tweak the software to get longer battery life out of it. However, before we can really do anything we must be able to accurately measure the current so that we can see how our changes affect things.

The simple way is to jut charge up some batteries and put them in use and see when we get the low battery alert. This just takes too longer. I'm thinking about creating a test "jig" where I could supply the power to the devices and then measure the current using a micro controller. This would be logged to a computer so I can calculate the current drawn from the battery over say a 1 hour period and then determine how long the device would last with those settings. We could also correlate these logs with the system logs to determine current measurements in different modes of operation.

Was thinking about using an arduino and then a TI chip which measures current and voltage via i2c, the INA219... It uses a shunt resistor to measure the current. I was going to use a .1 ohm 1% resistor.

Any suggestion or alternatives you can think of to help me accomplish this?
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#2 21660598 05 Jan 2012 11:33

Scott Wohler Scott Wohler

Anonymous

Post #2
21660598 05 Jan 2012 11:33

Might I recommend simply using an instrumentation / differential amp with gain, such as the TI INA196, along with the proper sense resistor? Then you can just use an ADC input on whatever platform you're using (Arduino, etc.) It might take less effort than working with the I2C interface.

As for the resistor, you want to determine what your full scale current range to measure will be. For instance, if you will never measure more than 100mA, and you know that the gain of the INA amp is x20, you would configure the sense resistor close to 1.65ohm so that at full scale measurement of 3.3V, the current being read is 100mA.

1.65ohm * 0.1A = 0.165V x 20 = 3.3V

Or maybe you use a reference of 2.5V, etc. in which case, the resistor needs to be closer to 1.25ohm for full scale at 2.5V.

Through some combination of sense resistor, reference voltage, and range, you can get the results you need. Then in software it's just a matter of simple math to convert back to floating point voltage.

Does this help?
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#3 21660599 05 Jan 2012 11:44

James Russo James Russo

Anonymous

Post #3
21660599 05 Jan 2012 11:44

Scott,

Thanks for your reply. The op-amp route was actually my first thought, but then I found that chip and thought it might be the better way to go. It will do sample averaging for you, has calibration registers to take care of the scaling and then you just read current (and voltage) out of it. This removes the need for accurate reference voltages, etc. I figured since it was just some bench equipment (not production) I could likely get away with the $3 a chip.

I do need to think more about the shunt resistance.. at .1Ohm @ 2A will only give me 20mv... I might want a bit more then that. I'll go back and read the data sheet I think they said try and set it up for between 40 and 60mV at full scale.

-jr
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#4 21660600 05 Jan 2012 12:07

Scott Wohler Scott Wohler

Anonymous

Post #4
21660600 05 Jan 2012 12:07

For the resistor it is a trade-off between what you can sacrifice in terms of voltage drop to the system and range / precision of the measurement.

As for keeping the voltage drop low, and aside from using the INA219, some MCUs have the ability to introduce gain into ADC inputs. I know Cypress PSoC devices will do this, and some Atmels (which the Arduino is based on, don't know if you have that flexibility on the platform.)

The best advice I can give is to experiment with it. You can always change the sense resistor if you don't get the results you desire.
#5 21660601 05 Jan 2012 12:16

James Russo James Russo

Anonymous

Post #5
21660601 05 Jan 2012 12:16

Thanks. I'm going to stick with the .1 right now which gives me 20mV @ 2A full scale.. I can just series .1 resistors to play with it and zero in on a value which works for the device and range I need. I think the only thing I will loose is some accuracy since each resistor is 1% +/-.. Once I dial it in I'll source the correct precision resistor..

I may actually want to do something higher like .5 ohm yielding 50mA @ 1amp and just let it clip. 99% of the time is going to likely be spent
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#6 21660602 05 Jan 2012 12:53

Steve Lawson Steve Lawson

Anonymous

Post #6
21660602 05 Jan 2012 12:53

How about a Hall Effect Current Sensor, such as:

http://www.gmw.com/magnetic_sensors/sentron/csa/CSA-1.html
#7 21660603 05 Jan 2012 17:38

Pieter Kruger Pieter Kruger

Anonymous

Post #7
21660603 05 Jan 2012 17:38

Why use a microcontroller? Why not just use a multimeter with a RS232 link to a PC to record at regular intervals? Maybe I have missed something?
#8 21660604 05 Jan 2012 17:45

James Russo James Russo

Anonymous

Post #8
21660604 05 Jan 2012 17:45

That is a good point, but the bench meter I would want is much more expensive then the parts and a few hours to put it together and we would want to put a few of these together for some of our testers. My time and salary is already paid for.. capital expenses come hard in a small startup.

Any reasonably priced meter suggestions with RS232?

-jr
#9 21660605 05 Jan 2012 20:28

Steve Lawson Steve Lawson

Anonymous

Post #9
21660605 05 Jan 2012 20:28

How about:

http://www.circuitspecialists.com/csi345.html

I can't vouch for quality of the meter or accompanying software, but the price is nice.
#10 21660606 05 Jan 2012 20:47

Pieter Kruger Pieter Kruger

Anonymous

Post #10
21660606 05 Jan 2012 20:47

I use a Brymen TBM815, it's not wonderful, but more than enough for my needs and it has an Infrared
coupled RS232 interface. If you managed to get one, it is a good idea to have a good look at the PC logging software before you buy just to make it meets your requirements. The meter that Steve suggested does not look bad at all, especially for that price, but I haven't used it.

Equipment cost versus engineer's salaries is interesting, but off topic and would probably start
a very long thread. I won't go there... (Still it feels like you will be developing your tools rather than
spending the time on improving the product.)
#11 21660607 05 Jan 2012 20:49

Pieter Kruger Pieter Kruger

Anonymous

Post #11
21660607 05 Jan 2012 20:49

Apologies for the formatting in my above reply - looks like I will be a good candidate to test the new preview feature...
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Topic summary

The discussion focuses on measuring current draw to estimate battery life for a product using a Telit cellular and GPS module. The original poster plans to build a test setup using an Arduino microcontroller and the Texas Instruments INA219 current and voltage sensor via I2C to log current consumption over time. Alternatives suggested include using instrumentation amplifiers like the TI INA196 with a sense resistor and ADC input, which may simplify the design by avoiding I2C complexity. The choice of shunt resistor value is critical to balance voltage drop and measurement range, with recommendations to target a voltage drop of 40-60mV at full scale for accuracy. Some suggest experimenting with resistor values and using precision resistors for final calibration. A Hall Effect current sensor was also proposed as an alternative measurement method. The use of multimeters with RS232 PC logging was discussed but considered less cost-effective for multiple test setups. Specific multimeter models with PC interfaces were recommended for budget-conscious solutions. Overall, the conversation covers practical approaches to current measurement for battery life profiling, emphasizing trade-offs in sensor selection, resistor sizing, and data logging methods.
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How to Use Arduino and INA219 to Log Current Draw for Telit Module Battery Life Estimation

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