Bench Testing vs. Real world power consumption of our Arduino dataloggers

I finally have a moment to look at the data from the recently retrieved flow meters, and the results are not what I was expecting. This was a five month deployment, with the units operating from March 22, 2014 to August 25, 2014. The three pendulum units had identical data logging hardware consisting of a the Tinyduino, Sd shield, HMC5883L compass & BMA250 accelerometer, with a DS18B20 temp sensor and a 3-color 5050 indicator LED.  These units recorded Vcc using the 328’s internal 1.1 vref trick, and even with the usual caveat about 10% variability there, the flow sensors did not deliver anything like the low power consumption suggested by my dry test runs*:

Units3&4_compare_640

(*One of the pendulums had a NCP1402 voltage regulator on the power module, so I will be ignoring that unit for now although the remaining AA cell voltages from that unit were surprisingly similar to the batteries from other the power modules?)

One key observation is that power consumption was similar in both units although Unit4 was creating three times as much sensor, EEprom & Sd card traffic as Unit3. This becomes more obvious if I project these curves down to 2800 mV, which is my cutoff to prevent unsafe Sd card operations:

Unit4_projection

The vertical lines here represent one month of operation, and Unit 4 (above in orange), which gathered almost 45000 samples in that time,  projects out to another two months of operation.

Unit3_projection

Despite a boost from slightly newer batteries and a longer 15 minute sample interval, the Unit 3 projection (in yellow) has almost exactly the same amount of time left on it’s 6x AA power pack. This would seem to imply to me that the quiescent current draw of my data loggers is far more important than the power used to drive the sensors. If I just do a quick ‘back of the envelope’ here, and I assume that the six AA’s deliver about 2000mA each,  we are burning through 12000 mAh in about 5200 hours (est.at 7 months)  =  2.3 mA average current.  Ouch!

So I went back and looked at the results from my longest bench top test. This was done with a configuration matching units 3&4 above, but racing along with a sensor read every ten seconds for most of the test. I plotted that voltage curve again with one month time indicators added:

BenchTest_against time

Even with ~400 000 sensor read/record cycles it operated for almost four months – more than half of my lifespan projections for unit 3&4 and it was running on only 3 AA batteries. I had assumed that much bus traffic was the biggest load on the system by far, but perhaps it is time for me to re-consider things? The Tinyduino sensors each have a regulator, as does their Sd shield…have I been ignoring the forest for the trees?

We also had a stationary sensor unit on that last deployment, with a single MS5803-02 sensor, recording both temperature and pressure (barometric & water level). I hacked into a TinyCircuits light sensor board to provide the regulation and I2C level shifting needed for the 5803, so that system had one regulator there, and the one for it’s Sd card.  With only two voltage regs, and no power being used by the DS18B20 temp sensor (which draws  for almost 800 ms to load its 12 bit registers), the stationary unit projects out like this:

MS5803_projection

At 11 months this just squeaks back in my  design target of one complete year of operation.

These curves leave me with a couple of impressions:

Any regulator, even one with relatively low quiescent current, will draw at least as much juice over time as any of the bits you are actually powering with it in a long term application like a data logger. I might also need to take another look at the losses on the Shottky diodes isolating the battery banks, because even with all those regulators in play we are no where near spec sheet predictions here.

I needed better acclerometer sensitivity,so the new Cave Pearl builds have the sensors moved away from the main stack, and all of the power for the I2C bus is now runs through the single hacked light sensor board, similar to the stationery unit above.  While power was not my primary reason for doing this, I suspect this was the right way to go for power managment in the overall design.

And finally, the drip sensors have only one single regulator in their build, with the Sd card hanging right off of the pins.  And now I am playing with BOD fuses, probably exposing my precious data to even more hazard.  But it just might be that the humble three component logging platform actually surpasses the TinyDuino logger units, in terms of power use over time.  After some months of waiting, my eevblog µCurrent is finally on it’s way, so I will have more power test results to report soon. In future, I will do my bench tests with two identical units,  one of which will do sensor readings, etc., as normal, while the other one simply sleeps the whole time, so that I can isolate where the power is being used.

Addendum 2015-04-12

There have been quite a few field trials since this test was done, and the short version of the results is: TinyDuinio based loggers draw about 0.065 mA sleep current, which on a 6x AA battery pack will get you between 6-9 months of run time. Loggers built with generic pro-mini style boards draw 0.33 mA which will deliver the same 6-9 months of run time on 3x AA batteries. Having good low sleep current SD card is critical to the success of your data logger and you really need to test to make sure your sensors are going into low current sleep modes as well. Cheap eBay sensors from China often fail this test. 

This entry was posted in Test results & Data analysis, The quest for low power. Bookmark the permalink.

2 Responses to Bench Testing vs. Real world power consumption of our Arduino dataloggers

  1. Hello Edward,

    What does your circuit do in between measurements? Does it sleep the whole time through and/or are you using the RTC’s alarm signal to wake the microcontroller up?

    I used to do the latter but changed to using the RTC’s INT pin to pull down a P-channel mosfet which then powers my complete circuit.

    From my breaboard (full of fluff) and my el-cheapo mulimeter, I’d get around 83uA of quiescent current from the MCP1700 powering the mosfet and RTC so to speak. I’ll do some measurements later with a better multimeter.

    I am powering my datalogger using a 3.7V Li-Po batterypack with 4400mAh. I have my logger outside, measuring 9 DS18B20 every minute and as fas as my calculus go, it should be doing this for another 69 days, giving me a total of anywhere between 103500 and 126000 units of data before the MCP1700 can no longer support the 3.3V needed to power everything..

    All of the above may vary ofcourse, I’ll have to wait quite some time to see where this ends 🙂

  2. edmallon says:

    Yep, the MCU sleeps on the flow meters till the RTC interrupt line gives it a kick to start a sample cycle. I use a second interrupt line on the drip sensors, triggered by the accelerometer tap sensing function. My problem with pulling the whole unit down is that all the sensors, and most especially the SD card have a large power hit on initialization. I still have testing to do here, but 500ms at max current (80-150mA?) is not unusual for many cards. And although it’s not in the spec sheets, I find something similar with many of my sensors – when you first power them up they seem to need do a couple of readings before they give you stable output. This does not seem to be the case when you wake them from “sleeping”. So once you start hanging allot of stuff off of your MCU, you need a pretty long sleep interval to make up for the power you spend initializing everything if you use a mosfet. It would still probably work in my case…I just have not got that far in the development yet.

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