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I am using a CR2032 battery module to operate a BLE 4.1 module. The BLE radio for communication takes around 3.5ma to 5ma of current. But when I look at the datasheet of the battery (https://cdn-shop.adafruit.com/datasheets/maxell_cr2032_datasheet.pdf) it shows the nominal discharge current is 0.2mA.

My question is: during BLE communication, when current is about 5mA, will this battery be able to provide this current? Also for what purpose are nominal discharge current values shown?

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I did my PhD thesis on energy management in wireless sensor networks and was working with sensor nodes using CR2032 batteries. We designed the nodes ourselves (my supervisor designed the PCB and I designed the firmware and all the energy-related testing).

I can confirm what people above say that you can draw 100mA peaks from a new CR2032 cell. But as they say, to get the nominal capacity in terms of mAh you need to discharge it at the specified nominal current and temperature.

The sensor nodes I was working on drew some 27-35mA on transmission. But transmissions lasted for 110-140ms at a time, once each minute. At room temperature using a single CR2032 in parallel with a 75mF supercap from CapXX we managed to use some 87% of the rated capacity of the CR2032 (the tested nodes were in function on average 99days). We used a CR2032 from Renata. The same setup without a supercapacitor would get roughly 10-15 days less of functional time on average. However, the supercap becomes crucial if you decide to go down in operating temperature!!! (which we did in tests to -30°C)

The consequences of discharging with higher current are that you manage to get less energy than specified from the battery. The current peaks create voltage drops and at the moment when that voltage drop goes below your brown-out voltage - your circuit resets. Needless to say at that point there is still some energy left in the cell. To aleviate this problem you could add a supercapacitor to flatten out the transmission current peaks (voltage drops).

But:

  • supercapacitors are expensive
  • you need one with low leakage current and balance circuitry such as from CapXX (they have leakage currents in the range of 1-2uA)

A high leakage current supecapacitor will do more harm than good if the device needs to be powered for days or weeks.

Also do not hesitate to connect CR2032s in parallel if you need to and you have space - you basically double the current capacity.

Having said that - there is still a ton of work to be done in this world to improve energy management in such applications.

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  • \$\begingroup\$ Thanks for detailing. Here if i connect big value Tantulam Cap of 47uF across the battery, does that mean that Capacitor can be the Power Source during BLE communication as it will get discharge during BLE advertisment providing current, i have read somewhere. if so we can able to minimise the power or not ? \$\endgroup\$ – Viral Embedded May 18 '16 at 10:01
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    \$\begingroup\$ lowtech talked about supercaps in the range of millifarads, not microfarads. \$\endgroup\$ – CherryDT May 18 '16 at 16:39
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    \$\begingroup\$ 47 – 100μF should already help. SiLabs recommends it for their BGM111 Bluetooth module too. To quote the datasheet: “Coin cell batteries cannot withstand high peak currents (e.g. higher than 15 mA). If the peak current exceeds 15 mA it’s recommended to place 47 - 100 µF capacitor in parallel with the coin cell battery to improve the battery life time.” \$\endgroup\$ – Michael May 18 '16 at 18:41
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    \$\begingroup\$ I'd hesitate to sell a device which requires to connect CR2032 cells in parallel. And there's always the CR3032. \$\endgroup\$ – fgrieu May 18 '16 at 21:24
  • \$\begingroup\$ @ViralEmbedded: the current consumption profile during transmission would help you a lot. Are you sampling the consumption current at high speed? We did this at 50ksps (50'000 samples per second) using DAQ multimeter. I am surprised that transmission current is only 5mA - I would expect more than 10mA. \$\endgroup\$ – lowtech May 19 '16 at 12:39
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From the data sheet:

  • Nominal capacity indicates duration until the voltage drops down to 2.0V when discharged at a nominal discharge current at 20 deg. C

So, if you discharge at a steady 0.2mA, you will get 220mAh capacity, or 1100 hours. Discharging at a higher rate will reduce the capacity of the battery. Pulsing will also affect this, as BLE pulses to 5mA. A pulse of 5mA for a fraction of a second while there is no draw the rest of the time, could result in a equivalent draw of 0.2mA.

If you look at the graph labeled Pulse Discharge, it shows a 5 second pulse of 300 Ω, or 10mA @ 3V. A pulsed discharge like this will reduce the total capacity over the life of the battery to roughly 180mAh from the nominal 220mAh.

To answer your two direct questions:

  1. A typical CR2032 can source much more current than 5 mA. You could pull 100mA from it, for under an hour, with some caveats about it's high ESR.

  2. The nominal current is to establish a base lifetime of the battery. CR2032, and coin cells in general, are meant for low current, long life applications, like real time clocks or battery backups of data. They are not meant for powering heavy loads.

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  • \$\begingroup\$ Two small comments. First is that technically the pusled capacity would be considered 160mAh from the graph. Second is that at 100mA a typical MnO-Li coin cell is not at all unlikely to drop to 1.5V or below depending on size. \$\endgroup\$ – Asmyldof May 18 '16 at 6:50
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    \$\begingroup\$ @Asmyldof If I wasn't just displaced by a house fire, I'd test out the max draw, plenty of spares lying around. But as for the 160mAh, the battery's nominal life is defined to 2V, not 2.5V. It's more like 188mAh according to the graph. (which I rounded down) \$\endgroup\$ – Passerby May 18 '16 at 6:58
  • \$\begingroup\$ As for the current capability, consider two 2032's supplying a year 2000 quality Blue or early White LED directly in a keychain. If they wouldn't drop to 1.8V each at 30mA those LEDs would certainly burn. For the discharge that's a complex discussion in which you can be right. But that note does stress nominal indication at nominal current and nominal temperature, none of which are exactly in play in that scenario, so the difference between 2.5V and 2V in that would be 1% or such. Generally when the rate of change is past the none-linear bend any battery becomes unreliable in predictions. \$\endgroup\$ – Asmyldof May 18 '16 at 7:25
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    \$\begingroup\$ @Passerby house fire? :-o not caused by overdischarged lithium cells, we hope? \$\endgroup\$ – nekomatic May 18 '16 at 12:56
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I think the answer is in this graph of the datasheet:

enter image description here

If you draw vaguely 10mA (\$3V \div 300\Omega\$) 5 seconds at a time, you will see the voltage fall off more over time, and the overall capacity of the battery will be reduced.

(This is due to more power being burned up in internal resistances of the battery. But it seems to be just fine if you're ok with the reduced capacity)

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