2021-03-01 at 12:58 PM #15175Jim MooreParticipant
I have attached a document I wrote up on some battery testing I have been doing. My conclusion is that a modem cutoff voltage 3.3 Volts shouldbe used rather than the 3.55 that is in the code I used for my LEC Sensor stations. I would appreciate any comments and explanations of of some open questions.
2021-03-01 at 1:11 PM #15177Sara DamianoModerator
I’m confused by your tests and your results. The purpose of having the voltage threshold for data upload is to conserve power for the sensors so that data will still be *collected* as long as possible, even if that data isn’t reported online. Of course, decreasing the minimum threshold for powering the modem will increase the amount of time that the station will report data online after being charged. But it should *decrease* the overall time that the station can collect data – which is what I would call the run time. I would prioritize collecting data to the SD card well above reporting that data online. If you have a station with no solar charging, I would not recommend installing a modem at all.
Also, the reason that value is in the main program (rather than buried somewhere in the library) is so that it’s easy and obvious for the user to adjust it to their needs. Not everyone uses a 3.7V LiPo.
2021-03-01 at 1:12 PM #15178Shannon HicksParticipant
We typically code our stations to cutoff modem transmissions at 3.45v, and file operations at 3.4v. The Mayfly will stop operating around 3.35v, so if you set your modem cutoff too low, then you’ll stop transmitting AND stop collecting data on the memory card at the same time. By setting the modem cutoff a little above the memory card logging cutoff, then you’ll have time to visit the station to swap batteries before you start losing data on the card. But in general, the station shouldn’t be falling that low repeatedly. Use a larger solar panel and make sure the battery pack is matched to it, based on the amount of sunlight you get on a typical day. Most of our stations stay in the 3.7v-4.0v range without user intervention to replace dying batteries.
2021-03-01 at 1:19 PM #15179Sara DamianoModerator
The bottom cut-off (below which the logger will only sleep) exists so that when a near-dead or completely dead solar-powered logger suddenly sees sunshine it gives the battery some time to charge before putting any load on it. Otherwise the logger can get in a loop of repeatedly resetting when the just-barely-charged battery browns out with any load on it.
2021-03-01 at 3:07 PM #15180Matt BarneyParticipant
Thanks for sharing your findings; this is interesting and useful for me. I’ve been thinking about battery capacity and power consumption issues recently and have used a coulomb counter to measure the charge consumed by a Mayfly/sensor test configuration. I hope to share my findings soon. One question on my mind is: how many amp-hours can I pull out of a given battery (4400mAh in my case) before it drops to a given threshold voltage. Your results give me an idea how I might test that.
I’m curious: how did you measure discharge current when doing the battery test?
Sara’s and Shannon’s points about prioritizing data collection to the SD card make good sense for a sensor station in the environment. I also wonder what is a safe depth-of-discharge for a LiPo before its life expectancy or performance is adversely affected, but I haven’t done any research on that yet.
2021-03-02 at 4:44 PM #15188Jim MooreParticipant
Couple of clarifications:
- The discharge is a fixed load of 22Ω so ohm’s law works!
- My question is why is the Mayfly station drawing less and less current as it approaches 3.3 Volts? Yet the 4G modems continue to work and upload the data.
- I believe that Shannon said that some older sd cards tend to cause current draw during sleep mode. Maybe this is what is happening.
- Matt- I am interested in your coulomb counter? Where can I get one or is it home brew?
- I have learned that not all Li-ion batteries are equal in terms of stated Ah capacity. I bought some from Amazon rated at 4.5 AH yet they were less than half the size of the 2.5 Ah batteries I got from Adafruit! And the JST connector polarity was reversed!
2021-03-02 at 5:03 PM #15189Matt BarneyParticipant
Thanks Jim. Ohm’s law – copy that! 🙂
The coulomb counter I’m using is from Sparkfun. It pulses one of its pins low each time that 0.614 coulombs of charge have passed through its sense resistor. Very easy to use.
Regarding the decreasing current: Is it possible that the battery’s internal resistance is increasing as it discharges, and this causes current flow to decrease? I’m no EE, so I’m just making a guess.
2021-03-03 at 4:48 PM #15196neilhParticipant
Jim fascinating, thanks for sharing and been re-reading your document a couple of times.
(I’m redoing my previous post as I got a bit excited with a power discussion).
I use a higher threshold V for transmitting cellular information. The voltage is dependent on the battery type/capacity and the load on the battery.
I use 3.8V as a threshold to stop transmitting,.
Under 3.8V I’m reserving the power to purely take readings, and extend the length of time that readings can be taken. Objective is at least two weeks.
On a transmission attempt, if the Voltage is above 3.8V then it transmits all the outstanding readings taken under 3.8V.
See this test that shows this.
So a question, from your document, how are you measuring the LiIon battery voltage?
I don’t have good results from using the Mayfly’s mega1284 Vbat.
I put some data out and never got a response https://www.envirodiy.org/topic/battery-measurement-accuracy/
I implemented a separate monitor on ADS115 AA0 https://github.com/neilh10/ModularSensors/wiki/Hw-Mayfly-ECO-R04
The results can be seen on the two Battery Voltage measurements on https://monitormywatershed.org/sites/TUCA-Na13/
For maintenance the “EnviroDIY_Mayfly Data Logger”/”V, solar or LiIon Batt 5-3.5V” indicates there is a good solar aspect charging the Adafruit 4Ahr battery.
The AA0 sensor “All_External Voltage Divider”/”LiIon battery V” indicates how well that solar aspect is charging the battery.
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