Gas station without pumps

2016 December 25

Coin cell batteries are pressure sensitive

Filed under: Data acquisition — gasstationwithoutpumps @ 18:03
Tags: , , ,

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.

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