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We have designed a PCB with ultra low power Bluetooth module with the objective to run it on batteries for about 2 years. We have measured the power consumption, it is 400uA on average with 20ms peaks at 3mA every 500ms.

We choose to put 3 C alkaline batteries rated for 7'200mAh in series. Our working voltage range is between 5V to 2V. Therefore we are expecting 18'000 hours of autonomy, i.e 25 months.

The problem: after 5 months being powered on, the system stopped to work. We found out that one out of the three batteries is completely dead while the other two still have some power (1.3V for alive ones and -0.6V for the dead one). We measured the impedances between batteries connectors, nothing below MOhm.

The question is: why are these batteries dying after only 5 months? Are we just unlucky with these batteries? not very probable because it happens on about 5 devices. It doesn't look like there is any unwanted power consumption, we measured it either with an amp-meter in series with an external power-supply, and also by adding a 330 Ohm resistor in series of the actual batteries to be able to see the peaks using an oscilloscope, results were consistent. What are we missing?

schematic

simulate this circuit – Schematic created using CircuitLab

Edit: More details on the current measurement

A first measure has been done replacing the 3 cells with a power supply and an amp-meter in series. with this, we were able to easily change the supply voltage to see how the current was changing. It was as expected i.e. about 400uA with 4.5V and about 600uA with 3.3V which should represent most of the time with actual cells.

A second measure has been done by adding a 330 Ohm resistor in series of the cells and we observed the voltage on the resistor using an oscilloscope. We did the measurement either with actual cells and a power supply. Same averages were found than with previous method. and we were able to see the 3mA peaks. We observed the signal for about 15 minutes without any irregularities. See oscilloscope screenshot below for details.

schematic

simulate this circuit

enter image description here

The device is running in the exact same state and environment during these measurement than actual usage in which we discovered the short battery life.

Puck
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    how sure are you of the long term accuracy of that 400uA measurement? For example, maybe it's going into some error condition every minute where it takes a lot more power. Which bluetooth module is this? – BeB00 Nov 07 '17 at 16:21
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    Environmental factors (i.e. temperature, barometric pressure, humidity, etc) will affect the long-term behavior and reliability of batteries. I very rarely rely on the spec'd mAh rating. Besides, what you really should be going from is the mWh rating. mAh is not enough to calculate how long something can be expected to run on the battery because it is not constant over the life of the battery – DerStrom8 Nov 07 '17 at 16:23
  • I really don't think that alkaline batteries are good for long time service to begin with. I would have chosen NiMh or Li. – Claudio Avi Chami Nov 07 '17 at 16:33
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    @ClaudioAviChami Alkaline batteries actually tend to last longer than NiMh or Li due to the fact that their "drop-off" at their EOL is much slower. The only benefit NiMh or Li batteries have is that (assuming they're Secondaries) they can be recharged. They won't last as long as alkalines, though – DerStrom8 Nov 07 '17 at 16:46
  • @BeB00 The measurements were done for about 15 minutes using an oscilloscope to detect any irregularities, none occurred. The chip is a uBlox NINA-B1. – Puck Nov 07 '17 at 16:50
  • @DerStrom8 It is intended and has been used only in room conditions. Definitely, the mAh ratings is not optimal, but I definitely expected a better approximate than 25 months instead of 5. – Puck Nov 07 '17 at 16:52
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    Actually I am not surprised by the results. As the voltage of the battery declines, the current required to keep the device running will increase (to maintain constant power). This is why the mWh rating is so important. Changing by a factor of 5 is not very surprising to me. What batteries are you using? It's not a matter of "using the mAh not being optimal", it's a matter of "using the mAh rating not being remotely accurate or reliable". At the very beginning of the battery's life its mAh rating may be accurate, but it will very quickly drop – DerStrom8 Nov 07 '17 at 16:59
  • @ClaudioAviChami We choose alkaline batteries because they fit well the limited current we need, the shelf life is good (announce to 10 years) and they're price is really interesting compared to Lithium which seems to be another viable option. – Puck Nov 07 '17 at 17:00
  • Here is a chart showing how battery capacity (mAh) decreases over time: http://www.mpoweruk.com/images/auto%20reliability.gif – DerStrom8 Nov 07 '17 at 17:09
  • @DerStrom8 Interesting input! We are using Energizer EN93 (http://data.energizer.com/pdfs/en93.pdf). Definitely the current will increase, but as most of the energy is between 1.5 V and 1.2 V (what we observed on the long term logs), I expect an increase of the current of a factor of 1.2 (measurement done at 1.6 V) at worst during this interval. Which seems inconsistent with the factor 5 on the capacity. What do you think? – Puck Nov 07 '17 at 17:09
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    @Puck Unfortunately the datasheet doesn't show the charts or data I was hoping for. Anyway, your factor of 1.2 might make sense if the capacity remained constant over the life of the battery, but that's not how the batteries work. Unfortunately I'm approaching the limits of what I know about battery life, all I can speak from is experience. In cases like this you really need to trust experimental data because there are so many factors affecting the performance of the battery. Theoretical data is practically useless at this point. – DerStrom8 Nov 07 '17 at 17:16
  • The harder you "slam" your batteries with the current demand (as the voltage goes down over time) the faster their capacity will be reduced and you effectively get the avalanche effect much sooner than you would normally expect. – DerStrom8 Nov 07 '17 at 17:16
  • Any chance moisture or condensation got into the device? That would add a conductive path to drain the battery. – τεκ Nov 07 '17 at 17:20
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    3mA for a C tell is a tiny current. They should manage that no trouble. One thing to check - are all the cells the right way round? What you're seeing would be the effect if one of the cells was backwards. – Simon B Nov 07 '17 at 17:21
  • @τεκ Hardly, this is inside a fully waterproof casing. – Puck Nov 07 '17 at 17:21
  • @Puck Condensation perhaps? Was the case sealed in dry air or with a desiccant? – τεκ Nov 07 '17 at 17:23
  • @SimonB Exactly what I think too, 0.0004C doesn't seems too much. I can assure that all cells are in the right way. – Puck Nov 07 '17 at 17:24
  • @τεκ Maybe condensation... The case was sealed at room condition without a desiccant. I'll try to investigate in this direction but might be hard to prove anything. – Puck Nov 07 '17 at 17:26
  • Cheap/fake cells? – Passerby Nov 07 '17 at 23:49
  • @Passerby I hope not, they are industrial Energizer EN93 that we got from digikey. – Puck Nov 08 '17 at 07:17
  • @Puck can you elaborate on how you measured the currents you mentioned? Was the bluetooth module talking to whoever it was supposed to? Could it be that the 330ohm resistor is smoothing out the peaks? Also, did you test at several possible input voltages? – FrancoVS Nov 08 '17 at 13:28
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    @FrancoVS Certainly. Please, see my edit. – Puck Nov 08 '17 at 14:22

1 Answers1

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A couple of observations:

  • Perhaps one of the cells was faulty or with less-than-spec capacity, still it had to deliver the same current as the others and got depleted faster. After the voltage dropped to a lower value, the regulator started asking more current accelerating the drain.

  • For ultra low power applications, 400 uA is a bit high as a baseline consumption. The Bluetooth module and the microcontroller should have sub-uA or few-uA level sleep mode consumption, maybe you can look into optimization. Once I was diagnosing a low-power wireless node, to find that floating input pins on the microcontroller were causing most of the sleep-mode current drain (about 100 uA); after the fix, it got to 0.1 uA.

clabacchio
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