Mayfly low battery voltage readings

This topic has 5 replies, 3 voices, and was last updated 2021-03-12 at 4:38 PM by neilh20.

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- 2021-03-09 at 7:47 PM #15230
  Matt Barney
  Participant
  I believe that the Mayfly is reporting a battery voltage that’s erroneously low. I have a 4400mAh lipo that measures 4.16V fully charged, and I’m connecting it to the Mayfly on my desk and running a program with a 3-minute sampling interval. During the measurement cycle (including LTE modem operation), the lowest reading seen on my multimeter is 4.14V, while the Mayfly reports 3.533V (and I’m taking into account the fact that the Mayfly reports the battery voltage it measured during its previous sampling period). Is there some component failure that could be causing this?
  
  As additional background, my Mayfly is operating a Turner Turbidity Plus sensor (not currently using the wiper), and after 16 measurement cycles and the sleeps in between, including the LTE modem, the circuit has drawn a total of 15.87 mAh from the battery according to a coulomb counter, so it seems unlikely the battery voltage would drop from 4.16 down below 3.6V. I had noticed previously that, when I’d turn off the Mayfly and disconnect the battery, it would still measure 4.15V, immediately after the Mayfly had been reporting < 3.55V. I’ve repeated these tests with a different battery, with similar results.
  
  I think Neil may have been getting at this same issue in this thread, but there’s enough jargon in there that it’s mostly inscrutable to me (sorry @neilh 🙂 ).
  
  -Matt
  
  Trout Unlimited
- 2021-03-09 at 10:03 PM #15231
  Shannon Hicks
  Moderator
  Sounds like it might possibly be this issue: https://www.envirodiy.org/topic/innacurate-lipo-battery-voltage-data/
  
  I’ve only seen it happen on a few boards since that post a couple years ago. The only fix it to replace the bad MOSFET transistor.
- 2021-03-10 at 12:44 PM #15233
  Matt Barney
  Participant
  Aha – I think that must be it. I retested with a new Mayfly, and its reported voltage agrees with my multimeter within 0.1V. The old Mayfly (s/n is either 180080 or 190080) was probably purchased in 2018, and has been my desktop tester, almost always under USB power, so didn’t detect any problems until now.
  
  Thanks @shicks!!
  
  Matt
- 2021-03-10 at 9:32 PM #15236
  neilh20
  Participant
  Sorry Matt for the lingo – its hardware speak. 🙂 It was on at least two Mayfly 0.5b that I measured.!! I now measure the LiIon via the ADS1115 – here is what the two measurements look like in the field. A lot of work to get it there. They have the 4400mAh battery from Adafruit, and 3W solar panel.
  
  https://monitormywatershed.org/sites/TUCA-Na13/
  
  Fancy that Turner Turbidity Plus sensor!. Interesting spec, says its supply is 3V-15V, but not clear what the wiper takes at 3V. It specifies the wiper at 12V@280mW (23mA?)
  
  If you haven’t got an easy interface to the wires from the Mayfly, the “RS485 boards” could provide a reasonable interface to the wires (you wouldn’t need the separate RS485 hybrid)
  
  https://github.com/EnviroDIY/SensorModbusMaster/tree/master/hardware/Modbus-Mayfly_WingShield
  
  If it needs the 12V for the wiper there is a range of Pololu boosters that fit on the board 12V. U3V12F12
  
  If you want to switch directly from the LiIon bat, I have done this new board
  
  https://github.com/neilh10/SensorModbusMaster/blob/release1/hardware/knh002-MayflyWingShield/rev6/knh002r6_rs485schematic.pdf
  
  Also I’ve put on an STC3100 Battery monitor that can measure V and mAHrs/Coloumb counter – looking interesting so far though, still a wip https://github.com/neilh10/STC3100arduino
  Attachments:
  210310_MayflyBattery.jpg
- 2021-03-12 at 1:38 PM #15246
  Matt Barney
  Participant
  Hi Neil,
  
  I’m powering the Turner Turbidity Plus via the Mayfly’s 3.3V, and its average current draw during wiper cycles is 109mA.
  
  Thanks for sharing your wingboard. As far as wiring, I have some of the EnviroDIY protoshields that I plan to use. I’m interested in how, physically, you connected the Vbat to your ADC for measurement. Are you using the onboard ADS1115 or an external one? Any photos you can share?
  
  The STC3100 looks interesting. Have you found a breakout board for it, or are you a surface-mount-soldering ninja? 🙂
  
  Best,
  Matt
- 2021-03-12 at 4:38 PM #15249
  neilh20
  Participant
  Hi Matt, that’s great it can do the wiper at 3.3V.
  
  I’m using the onboard ADS1115 dedicating AA0 for LiIon battery measurement see photo .
  
  https://github.com/neilh10/ModularSensors/wiki/Hw-Mayfly-ECO-R04 Its using 1Mohms and 100Kohms.
  
  I did initially try 1M and 1M but the ADS1115 still had some voltage offset I couldn’t account for or get rid of with software settings. Data sheets say Analog Input impeadance Typ is 6M so seems like any leakage current should have been ok with 1M/1M. I got more accurate results with the 1M/100K .
  
  Yup soldering STC3100 with a with hope and prayer (my catholic background) to the god of solder reflow. DFN-8 0.5mm. Yeah!!! for toaster ovens. With SMD its so nice to see all the soldering reflow in 2minutes. I use stencils from OshStencil.com . Just waiting for the parts to build some more.
  Attachments:
  20210311_Knh002stc3100.jpg
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