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CR2032 battery voltage with BLE
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jmaja:
I have made a sensor prototype using cypress cyble-022001 BLE module. It's power from a CR2032 battery. While measuring it draws on average about 110 uA with about 2 uA sleep and then 27 uAs during each measument/BLE RX/TX cycle. The interval is 250 ms and it last about 4 ms wiht maximum current around 25 mA (measured without external to module capasitance, now I have 47 uF MLCC, which takes most of the peak).

I tested the sensor last night connected to my IPad recording data with my app (IPad was locked). I got a bit worried about the battery voltage curve (attached). It is measured by the CPU while it is active, thus it's about 0.1-0.2 V lower than what I get with a multimeter.

What could cause the drop and new level at around 5 hours? Is that typical to CR2032 (Varta) or is it more likely my sensor is doing something stupid. It should disconnect automatically, if for some reason IPad would shorten the BLE connection interval and thus increase the power consumption. I don't have the means to measure the power consumption, but I can use an oscilloscope to measure how much the voltage drops at BLE RX/TX.

One possibility is also that I damaged the battery while I soldered it. I couldn't find a CR2032 with correct solder tabs for the proto, thus I used just one without. It did still show 3.2 V after soldering. The battery recovers to 2.95 V (with multimeter) when the sensor stops measuring.

jmaja:
I checked with the oscilloscope that the voltage is 2.80 between the events and drops down to 2.44 V at the peak. The app shows now 2.65 V and multimeter 2.82 V. Based on what the app does and the oscilloscope shows the sensor is not doing anything abnormal. The voltage peaks are shown at 250 ms intervals and the peak looks the same as it did when I measured to power consumption with a shunt resistor.

I also have a test point to show the power state of the CPU and it does stay in deep sleep mode outside the event.

Based on how adding capacitance changed the voltage curve I have estimated the internal resistance of the battery to be about 15 ohm (that was a few days a go with almost fresh battery). So 0.36 V drop would equal to 24 mA. The oscilloscope shows 2.70 V average for 10 ms window. Thus 0.1 V drop. That would equal to 6.6 mA average current and 66 uAs/event, which is more than double what it should be. Has the IR already gone up to more than 30 ohm?

I guess I have to go back to lab power + shunt resistor to really know is there something strange going on with the power consumption. But I would appreciate some info about what can happen with a CR2032. I have read a few articles: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=2ahUKEwiAsKaZmIvhAhXw_CoKHXOrCk8QFjAAegQICRAC&url=https%3A%2F%2Fe2echina.ti.com%2Fcfs-file%2F__key%2Fcommunityserver-discussions-components-files%2F104%2F7510.swra349-Coin-cells-and-peak-current-draw.pdf&usg=AOvVaw1YV8JY83r1_Hx8-c7xCakn and
https://www.dmcinfo.com/Portals/0/Blog%20Files/High%20pulse%20drain%20impact%20on%20CR2032%20coin%20cell%20battery%20capacity.pdf
Rerouter:
That sounds about correct for a CR2032, if your using one of these and want to really get everything you can out of a battery you need to build to deal with at least 50 ohms of ESR,

I've built low power sensors using them in the past, a key thing is, learn what your cutout voltage is, e.g. most transceivers tend to be about 1.8V, so you need to make sure that at no point in the transmission it can fall under that,

The other thing is the ESR gets a lot worst at temperatures under 5C, My approach was internally buffer until the temps rose, but that may not work for your application,

Pretty easy to set up in something like falstads circuit simulator, a voltage source, a resistor, your capacitor and a switched load, Its what I used to plan mine, but I was only running 12mA for 5ms, and ended up at 47uF, so I suspect your math might not be quite right,

The only other point I can make is look at the leakage spec of the capacitors, the best SMD 47uF capacitor I could buy still ended up being half of my sleep current.

jmaja:
I had already done a LTSpice simulation when I first noticed that the voltage dropped much more with CR2032 than with my test setup with 3 V lab power + 10 ohm shunt resistor. That's why I went for the 47 uF cap, which is quite big (1210 and the thickest component).

I attached the simple LTSpice model (sorry about the messy layout). There are three resistors 100 ohm for BLE RX/TX, 1k for some CPU activity and 6k for sensor left measuring in between (just from 0.35 to 0.6 s). With this model I get no problems with 2.8 V battery voltage and 100 ohm ESR. The lowest peak goes to 2.4 V with this. With 15 ohm it goes to 2.55 V.

But I'm not quite sure how much capacitance should I actually use in LTSpice to model a 47 uF MLCC. In the simulator by Kemet this capacitor (C1210C476M4PACTU) should have minimal DC bias loss (-3% at 3 V). When I measure it (just a separate chip) I get 38 uF at 100 Hz and 35 uF at 1 kHz. Doesn't also small AC amplitude make real capacitance smaller?

Would a tantalum capacitor be a better choice? You can get higher capacitance and ESR of 1-2 ohm should still be OK. Was your leakage problem with MLCC? How much was it? During normal operation deep sleep is about 2 uA, but the device can also be put to stop when it consumes well below 1 uA. I haven't measured those after I put the 47 uF MLCC.

The BLE module can operate down to 1.9 V with radio on and 1.8 without radio. The sensor will be a problem before that, I have to test it.


 Do you think the voltage drop at 5 hours is normal? There was no change in temperature (indoors) and there should not have been any change in sensor current consumption.  The sensor is still on and the voltage doesn't seem to drop more. I was a few hours outside with it at 5 C and then the voltage dropped to 2.49 V (measured by CPU), but now it has been inside for an hour and it has risen back to 2.6 V.

The device will be used outside and should function down to 0 C. It must send BLE data all the time it is used.
jmaja:
Interesting that I found quite different values for insulation resistance of C1210C476M4PACTU. I first found this from Farnell: http://www.farnell.com/datasheets/652158.pdf It says 21.28 Mohm that would be just 0.14 uA at 3 V. Then at Kemet home site: https://api.kemet.com/component-edge/download/specsheet/C1210C476M4PACTU.pdf it says only 2 Mohm, this 1.5 uA. Which is still almost OK, but much more than stop current and about the same as deep sleep. The latter is inline with 100 Mohm * uF specified for bigger MLCC: http://www.kemet.com/Lists/ProductCatalog/Attachments/19/KEM_C1006_X5R_SMD.pdf

Are the real leakage currents close to the specs or is that the worst case?
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