Gas station without pumps

2016 December 25

Coin cell batteries are pressure sensitive

Filed under: Data acquisition — gasstationwithoutpumps @ 18:03
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I found out from Zohar that one could squeeze more power out of almost dead coin cells, literally—squeezing CR1225 lithium/manganese-dioxide (Li/MnO2) batteries when they are nearly dead appears to revive the batteries. So I decided to test this with my oscilloscope.  First, I made a test jig for holding batteries that I could apply pressure to:

The contacts are made with 22-gauge copper wire. I can squeeze the battery by pressing down on it with an aluminum rod.

The contacts are made with 22-gauge copper wire. I can squeeze the battery by pressing down on it with an aluminum rod.

I still have to work out a way to provide a measurable force to the battery (probably attaching a tray to a short length of aluminum rod, so that I can put known weights on the tray), but I’ve been doing some testing with “low force” (just enough to make contact) and “high force” (about 70N).

I provided a 100Hz triangle wave from 0.4 V to 3.4 V to the gate of the nFET, to produce a variable load on the battery, hoping to trace out a nice load line.

I provided a 100Hz triangle wave from 0.4 V to 3.4 V to the gate of the nFET, to produce a variable load on the battery, hoping to trace out a nice load line.

I tested three CR1225 batteries, each of which had been used a fair amount. The most dramatic effect was from the deadest of the batteries:

I-vs-V plot for battery 2 with low force

I-vs-V plot for battery 2 with low force

Battery 2 I-vs-V with high force

Battery 2 I-vs-V with high force

The plots above are directly from the Analog Discovery 2 oscilloscope, using the XY plot and a “math” channel that scales Channel 2 by the 200Ω sense resistor to get current. I also exported the data and plotted it with gnuplot, so that I could hand-fit the internal resistance:

The battery has an internal resistance of 500Ω, but squeezing it hard brings the resistance down to about 140Ω.

The battery has an internal resistance of 500Ω, but squeezing it hard brings the resistance down to about 140Ω.

A new Energizer CR1225 battery should have an internal resistance of 30Ω, increasing to about 50Ω when the battery is “dead” (from the datasheet). This battery is clearly well beyond the point that Energizer would consider it dead, though it is still capable of delivering almost 3.5mW (2.5mA@1.4V). Squeezing the battery hard lets it deliver 12.3mW (7.5mA@1.64V). Actually, those are only instantaneous power levels. The voltage drops quickly to a lower level, where it holds steady for a while, so the sustained power is more like 11.1mW (7.4mA@1.5V). The 200Ω load is a pretty good match to the internal resistance of the battery here, so this is about as much power as we can squeeze out of the dead battery.

I tried two other batteries that were not quite as dead. Battery 3 had 190Ω internal resistance dropping to 84Ω when squeezed, and Battery 5 had 220Ω dropping to 65Ω—all of these would have been considered dead batteries by Energizer (they weren’t Energizer batteries, but a no-name brand from China):

Battery 3 was a bit better than battery 2, getting up to 7mA without squeezing.

Battery 3 was a bit better than battery 2, getting up to 7mA without squeezing.

Battery 5 was the best battery tested, at least at high force, delivering 24mW (10.8mA@2.2V) to a 200Ω resistor.

Battery 5 was the best battery tested, at least at high force, delivering 24mW (10.8mA@2.2V) to a 200Ω resistor.

I’m pretty sure that the resistance differences I’m seeing are due to squeezing the battery, and not changes in the battery-wire contact resistance, but it would be good to devise a way to squeeze the battery without squeezing the contacts.

Things still to do:

  • Modify the test jig to provide measured forces squeezing the battery.
  • Modify the test jig so that the contact force and area is independent of the amount of squeezing.
  • Test some new coin cells, to see if squeezing to reduce internal resistance is just a phenomenon of nearly dead batteries or applies to all lithium/manganese-dioxide cells.  A new battery should be able to deliver 11mA@2.6V (34mW) to a 200Ω load.

It would also be good to have a theory about why increasing the pressure reversibly decreases the internal resistance.  TO come up with such a theory, though, I’d need to have a better understanding of the mechanical and chemical properties of the coin cells: what is the chemical reaction that increases the internal resistance? What moves when pressure is applied to the coin cell?  So far, the only guess I have is a change to the bulk resistivity of the electrolyte, with squeezing reducing the distance between the electrodes.  But the coin cell is not getting skinnier by a factor of 3, so I don’t think that the electrodes are getting that much closer together.

Last, it would be good to have a better understanding of the hysteresis in the I-vs-V plot.  Why does the voltage drop then hold steady at constant current, and why does the voltage recover when the current is no longer drawn?

Update 31 Dec 2016:  I tried testing a couple of brand new coin cells. They had an internal resistance of about 15Ω and did not seem particularly pressure sensitive. I’d need a more careful setup to measure the small changes in internal resistance and be sure I wasn’t just seeing a change in the contact resistance.

2016 December 19

Impedance of inductors and parasitic impedance of oscilloscope

Filed under: Data acquisition — gasstationwithoutpumps @ 01:04
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Because the Analog Discovery 2 makes doing impedance spectroscopy so easy, I decided to do a quick check of my inductors to plot their impedance, checking the series resistance in the process.  This was just going to be a short interruption to my day of working on my book, but it ended up taking up most of the day, because I got interested in seeing whether I could determine the characteristics of the scope inputs that were limiting the performance at higher frequencies.

Here was the data I started with, after converting the dB scale to |Z|. I used a 20Ω resistor in order to get reasonably large voltages at both ends of the frequency sweep. With a larger resistor, the low-frequency measurement across the inductor was too noisy, because the voltages were so small.

Here was the data I started with, after converting the dB scale to |Z|. I used a 20Ω resistor in order to get reasonably large voltages at both ends of the frequency sweep. With a larger resistor, the low-frequency measurement across the inductor was too noisy, because the voltages were so small.

The data looks fine up to 1MHz, but above that is a resonant peak, probably from the capacitance of the oscilloscope and the wiring to it.

I tried modeling the oscilloscope inputs as capacitors, but that resulted in way too sharp a spike at the resonance to match the data, so I tried a resistor in series with a capacitor. Initially, I tried modeling both channels identically, but I got better fits when I used a different model for each channel:

The resistor in series with the capacitance of the scope limits the sharpness of the resonance peak. Channel 1 was measuring the voltage across the 20Ω resistor, and Channel 2 was measuring the voltage across the inductor, so the setup is more sensitive to the Channel 2 parameters than to the Channel 1 parameters. I don’t really believe that the Channel 1 parameters fit here are correct.

It might be interesting to swap which channel is connected to which device, and see whether the R+C models still fit well, but I’ve not got the time for that tonight.   I did have some earlier data (from playing with resistor sizes) and I fit the oscilloscope models to it:

The fits here suggest some  difference between the channels, but not as radical a difference as the previous plot.  The 62kΩ sense resistor, though not good for determining the DC resistance of the inductor, does give a good handle on the parasitic impedance of the oscilloscope channels.

The fits here suggest some difference between the channels, but not as radical a difference as the previous plot. The 62kΩ sense resistor, though not good for determining the DC resistance of the inductor, does give a good handle on the parasitic impedance of the oscilloscope channels.

2016 December 15

Function generator bandwidth of Analog Discovery 2

Filed under: Data acquisition — gasstationwithoutpumps @ 15:56
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The network analyzer function of the Analog Discovery 2 USB oscilloscope makes it easy to characterize the function generator’s bandwidth—just connect the function generator to the input channel (making sure that the input channel is not specified as a reference) and do a sweep.  The only choice is whether to use the wires that come with the basic unit or the optional BNC adapter board and scope probes.  I tried it both ways (and with both 1X and 10X settings of the scope probes), using 1V amplitude on the waveform generator one in all cases:

There is not much difference in the bandwidth between 10X probes and wires (both high impedance) (8.5–8.8MHz bandwidth), but the 1X scope probes provide higher bandwidth—higher than the 10MHz measurable with the network analyzer.

There is not much difference in the bandwidth between 10X probes and wires (both high impedance) (8.5–8.8MHz bandwidth), but the 1X scope probes provide higher bandwidth—higher than the 10MHz measurable with the network analyzer.

I tried loading the function generator with resistors, but this made essentially no difference in the frequency characteristics. It isn’t the 1MΩ resistance of the scope that matters, but the capacitance of the oscilloscope plus probe.

So I tried adding capacitive loads and found that I got a very clear LC resonance. With a 330pF load, I got the peak near 10MHz to approximately cancel the drop:

The resonance around 9.1MHz with a 330pF load is actually a little too strong and over-corrects for the drop in bandwidth. Adding 6.8Ω in series with the 330pF capacitor makes a load that nicely compensates for the inductance of the wires.

A resonance around 9.1MHz with a 330pF capacitor implies an inductance of about 0.93µH, which is in a reasonable ballpark of the sort of inductance one would expect for 80cm of wire (4 wires each about 20cm).

Electret mic DC characterization with Analog Discovery 2

I tried one of the standard labs for the course, producing an I-vs-V plot for an electret microphone, using the Analog Discovery 2 function generator and oscilloscope, rather than a bench function generator and a Teensy board with PteroDAQ.

It was fairly easy to set up a 0–5V triangle wave, running at a very low frequency (50mHz, for a 20-second period).  The maximum output from the waveform generator is 5V, so setting the amplitude higher did not get larger voltages.  The signal was applied across the microphone in series with a sense resistor, and the voltage measured across the mic and across the sense resistor.

I ended up using two different sense resistors: one for measuring the current at high voltages, and one for measuring the current at low voltages, and I had to adjust the voltage scales on the two channels of the scope for the different ranges.  The results were fairly clean:

The low-voltage behavior of the nFET in the electret mic is not quite a linear resistor, and the saturation current definitely increases with voltage.

The low-voltage behavior of the nFET in the electret mic is not quite a linear resistor, and the saturation current definitely increases with voltage.

I tried extending the voltage range by using the power supply as well as the function generator: I set the function generator to a ±5V triangle wave, and used a -5V supply for the low-voltage reference. This worked well for the higher voltages, but the differential signal for the mic had an offset of about 12mV when the common-mode was -5V, which made the low-voltage measurements very wrong.  This offset may be correctable by recalibrating the scope (I am currently using the factory default settings, because I don’t have a voltmeter at home that I trust to be better than the factory settings), but I’m not counting on it.  When I need measurements of small signals, I’ll try to make sure that the common-mode is also small.

One other minor problem with the Analog Discovery 2: the female headers on the wires seem to have looser than usual springs, so that the wires easily fall off male header pins.  Given the stiffness of the wires, this is a bit

2016 December 12

FET I-vs-V with Analog Discovery 2

Filed under: Data acquisition — gasstationwithoutpumps @ 17:41
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Yesterday, in FET Miller plateau with Analog Discovery 2, I started posting about the Analog Discovery 2 USB oscilloscope, an oscilloscope with two differential input channels, 2 arbitrary-waveform function generators, a dual regulated power supply, and a logic analyzer.

I want to modify something I said yesterday:

If I look at the square wave with nothing but the scope attached, then I see a voltage of about 4.005V.  With a 100Ω load, I see 3.44V, which gives an output impedance of 16.4Ω.

I think that what I was seeing should not really be characterized as an output impedance, but as a current limitation.  The AD8067 op-amp that is the output device for the waveform generator is specified to have a 30mA current limitation (for -60dB spurious-free dynamic range) and 105mA short-circuit current, and 3.44V/100Ω is 34.4mA.  I can test this assumption by seeing what happens with a triangle-wave signal:

The triangle wave with a 100Ω load is clipped at approximately ±3.48V, corresponding to a current limitation of ±34.8mA.

The triangle wave with a 100Ω load is clipped at approximately ±3.48V, corresponding to a current limitation of ±34.8mA.

With 100Ω, I get ±3.48V, for ±34.8mA.  With 33Ω, I get ±1.475V, for ±44.7mA.  With 18Ω, I get +1.014V, -0.8458V, for +56.3mA, -47mA.  In each case, I am getting clear clipping, not scaling of the signal, so the best model is as a 0Ω output impedance, combined with current limitation, rather than as a non-zero output impedance.  The current limitation is not quite constant—I can get more current at lower voltages.

Something else you can see in the image above is that the time axis is not limited to starting at 0—I can move the trigger point around either graphically or by typing into boxes that hold the trigger level and time position for the line in the middle of the screen.

What I really wanted to show today was not the waveform generator current limit, but Ids-vs-Vgs plots for an nFET (the same old AOI518 nFET that I was playing with yesterday). I can use the differential inputs to measure the gate-to-source voltage on one channel and voltage across a drain resistor on the second channel.  It is easy to adjust the voltage range for a slow triangle wave driving the gate, and to look at an XY plot:

Voltage across 20Ω drain resistor to 5V for AOI518 nFET for a range of gate-to-source voltages. To get the large current, an external 5V wall-wart had to be connected.

Voltage across 20Ω drain resistor to 5V for AOI518 nFET for a range of gate-to-source voltages. To get the large current, an external 5V wall-wart had to be connected.

It would be nice if there were a way to scale the voltages across the load resistor to plot currents on the XY plot, instead of just voltages.  I can, of course, do this scaling with external programs, as I have with other measurement devices. I tried changing the resistors to get different current ranges, exporting the data in tab-delimited formats, and plotting superimposed I-vs-V plots. The results were not as good as I’ve gotten in the past using PteroDAQ:

The Ids-vs-Vgs curves do not superimpose as nicely as curves I’ve measured with PteroDAQ. I don’t yet understand why not.

I’m also not sure why there seems to be a 4µA leakage current.  At the top end, I’m not hitting the current limit of the voltage regulator, which is 700mA when powered by an external power supply, as I did here.

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