New library to read Arduino VCC supply level without resistors for battery powered sensor nodes that do not use a voltage regulator but connect directly to the batteries ;-)


  • Hero Member

    OK, now we are recapping the investigation JeeLabs did a year ago.

    I recommend these articles:
    http://jeelabs.org/2013/05/15/what-if-we-want-to-know-the-battery-state/
    http://jeelabs.org/2013/05/16/measuring-the-battery-without-draining-it/
    http://jeelabs.org/2013/05/17/zero-powe-battery-measurement/

    He did test the various circuits and values. The conclusion is that using a FET you can get the current drain even lower, but using 10 Mohm resistors and a cap he got the drain to sub-microamp.

    Once you get the drain down that low, the uC sleep current and battery self-discharge and capacitor leakage and any other sensors are going to dominate anyway. An open pin spec'd at +/- 1 uA, so even a sensor pin which you think is disabled may be drawing more current than that.

    Still, it can be fun to really go for the minimum possible.



  • @ ZEPH Good links. Pretty much says it all - including that in the cct posted above the divider could have leakage into the ADC I/P when the FET is off.

    Something else that's probably important is to measure the Voltage just before going into sleep, rather than when the CPU powers up. This gives a better indication of what the battery condition, because the measurement occurs a little while after the load has been applied to the battery. If you measure when the CPU powers up, the battery has the whole sleep time to "recover" from its last wake up. I'm pretty sure this could be demonstrated by making a start and end measurement and seeing what happens over time.


  • Mod

    @Zeph Nice, thank you for those links!


  • Contest Winner

    @Zeph Good. The third "conclusion" is pretty much what I had in mind as well. I am going to simulate it tonight, and see if it is possible to increase the resistor values. I think they are a bit small in my opinion, since it makes little use to reduce current consumption when doing the sampling, if the total consumption will be greater than using a low current divider.
    Regarding when to do the sampling, I agree with @a-lurker that this should not be done during boot. Rather, it could be done on "every hundred sensor sample transmission" or something like that as a first action (i.e. not when radio is running), or as a last action before sleeping.
    Ideally, a lot of samples could be taken and an average calculated, but that would probably defeat the purpose; to reduce current consumption when determining battery level.

    I'll get back with what I find.

    By the way, if we want to go fancy, there are a few commercial alternatives that provide a lot of probably over-the-top features, but they might be quite power saving:
    Texas Instruments: bq2010, bq2018, bq2019, bq2023
    Maxim: MAX1660, MAX1780
    Nat Semi: LM3822, LM3824
    Dallas Semiconductor: DS2438, DS2760


  • Mod

    So, this whole thing turned into something else 🙂
    Maybe the thread should be split so that it is clear where to look later...

    The question I have is this, and it concerns wiring.

    From other discussions I understand it is better to wire the radio separately with the cap as close to the vcc and gnd as possible.

    Now, when doing the supply level resistors etc. is it preferred/better to wire these separately as well or can I wire the board and sensor(s) direct after/to the setup.

    I hope my question is clear...


  • Admin

    I can split, but what should the topic be?


  • Mod

    @hek said:

    I can split, but what should the topic be?

    Something like 'improved wired battery check' or 'better (power saving) battery check'...


  • Contest Winner

    I have run some simulations on a low-power solution using this setup:
    sim_schem.png
    Sorry for the somewhat messy schematic. For those of you unfimiliar with LTspice, this is what the above is:
    V1 is the battery to be measured. It's nominal voltage in this case is set to 3.3V. The voltage is stepped from 3.3V to 0 in 0.5 increments in the simulation.
    For some noise suppression, it is decoupled with C2 but this makes little difference for the simulation I have made.
    The battery voltage is fed through a PMOS FET (M1) before it enters the voltage divider. Below, I will use the term open for the FET when it is not conducting current and closed when it is.
    The FET is by default tied in the open state using R3 as pull up to Vin. M1 is closed by grounding it's gate, or in this example by pulling Enable low from the MCU. V2 in this example serves as a crude simulation of the MCU grounding the Enable signal after 0.05s in the simulation time domain (simulation is executed for 0.1s).
    The neat feature here is C1, which isolates the MCU output pin from the FET, thus preventing leakage through the MCU. When Enable goes low, M1 is closed momentarily, before C1 regains the charge from the pull from R3 and is opened once again. The value of C1 and R3 can be picked to suit the speed of the MCU (the MCU needs to take the sample before M1 opens).
    When Enable is left floating, any charge pushed into C1 is fed back to Vin, and is thus "free".
    This also allows the voltage divider to span the entire ADC range (depending on what ADC is used, internal references and such).

    Below is the resulting waveforms when using a Fairchild FDS9934 as FET (somewhat equivalent breadboard-friendly component could be this).
    sim_wave.png

    Note here the large span of V(sample) as well as the peak current at the largest voltage level (2.4uA). The current remains constant for a brief period of time (decided by C1 and R3) and then dissipates down to 0, even if Enable is kept low. The current can be reduced by manipulating R1 and R2, but the circuit becomes more sensitive to noise as the current used decreases (as can be seen for lower currents in the simulation, where capacitances in the FET start to make a difference.


  • Mod

    @Anticimex nice, thank you!


  • Admin

    Great stuff @Anticimex . But I understand just a small bit of it. 😉


  • Contest Winner

    Thanks guys,
    I'll happily elaborate more on the details, but perhaps it is best if you state some specific questions regarding parts of it that are difficult to comprehend. Imo, working with sensors should not require a degree in engineering, so nobody should "feel stupid" for asking questions 🙂


  • Mod

    @Anticimex said:

    Imo, working with sensors should not require a degree in engineering, so nobody should "feel stupid" for asking questions 🙂

    When I have questions I will ask 🙂
    But, like @hek, I need to let things 'sink in'.
    And I really want to understand.
    To be honest I have learned a lot (and spent a lot) since I stumbled upon MySensors.
    Wonderful new hobby!


  • Hero Member

    As long as you connect VCC direct to the battery I do not understand why people opt for using an external voltage divider optionally with a FET to reduce standby current. Additional components and current consumption while only a possible small improvement in accuracy.

    For the voltage divider the ADC reading is 1023 x R2 / (R1 + R2) x Vcc

    For the 1.1 Volt reference the reading is 1023 x 1.1/Vcc

    If you are using +/- 10% resistors (or the FET resistance is not measured correct or varies), the accuracy is more or less matching the (uncalibrated) bandgap reference method.


  • Mod

    @daulagari I guess the discussion depends on whether you have a software-mindset or a hardware-mindset.
    The software guys currently seem to form a minority on this board...
    I still think, after all discussions and distractions, that this way of measuring can be very usefull, when you understand the limitations.


  • Contest Winner

    I have not studied what support for measuring Vcc is built into the Arduino, but if there are support for doing that, I am sure it should be adequate. Regarding the external circuitry, my point is just that if you are going for an external solution, you might just as well design it to consume a minimum amount of current, as that is a one-shot optimization. Yes, it is a couple of extra components, but to me, that outweigh the limitations of the simple voltage divider in the long run.
    Personally, I would even consider a battery management unit. Such a thing should be able to handle both charging (if you want that feature) as well as readback (with battery health compensation).


  • Hero Member

    @Yveaux

    I guess the discussion depends on whether you have a software-mindset or a hardware-mindset.

    Being on this forum you are likely not having a hardware- or software-mindset only 😉

    @Anticimex:

    I have not studied what support for measuring Vcc is built into the Arduino

    I think a study is not needed, the ADC ref power is VCC and there is a 1.1 V bandgap in the Arduino that you can measure; that's the whole trick.

    Yes, somewhat more funky like a battery management unit can for sure make sense.


  • Contest Winner

    @daulagari I see. Then as the whole topic suggests, it should suffice to use internal functionality to determine battery level if batt level = vcc. But some form of external circuitry is required if vcc is regulated. And I don't see a big reason to put a lot of effort into making a high precision solution for monitoring battery of "our" small nodes. But the current proposal of a simple voltage divider is a bit too wasteful imo (off topic).


  • Hero Member

    @daulagari said:

    As long as you current VCC direct to the battery I do not understand why people opt for using an external voltage divider

    For the voltage divider the ADC reading is 1023 x R2 / (R1 + R2) x Vcc

    For the 1.1 Volt reference the reading is 1023 x 1.1/Vcc

    An external voltage divider is useful only if you are NOT connecting the measured battery directly to VCC (ie: useful only if you are using a regulator of some sort between VBatt and VCC - whether linear, buck or boost).

    Your first calculation doesn't take the reference in to account. The reading is really:
    1023/Vref x R2 / (R1+R2) * VBatt. If VBatt is also VCC and VRef is also VCC, then the ADC reading is based only on the constant resistor ratio, independent of battery power (VBatt and VRef cancel out if both are VCC).

    So when using the default VCC as Vref, the Vbatt voltage divider is only useful when VCC is NOT VBatt.. in that case, if VBatt is the same as VCC, using an external divider is not an alternate technique with wasted components, it's just a non-starter period.


  • Hero Member

    @Zeph: Fully agreed.

    Your first calculation doesn't take the reference in to account. The reading is really:
    1023/Vref x R2 / (R1+R2) * VBatt.

    My formula's were for the case VCC = Vbatt but yes your formula is more generic but reduces to the same when VCC = Vbatt.


  • Code Contributor

    @Yveaux The only comment i have to this lib now then I'm using it is that you should not use floats but int instead, and just have an imaginary decimal point and divide at later stage to save program memory..
    And talking about this, I should probably start a new thread talking about optimizations, There are some to be done in the mysensors-lib also..


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