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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.

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    \$\begingroup\$ 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? \$\endgroup\$ – BeB00 Nov 7 '17 at 16:21
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    \$\begingroup\$ 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 \$\endgroup\$ – DerStrom8 Nov 7 '17 at 16:23
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    \$\begingroup\$ @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 \$\endgroup\$ – DerStrom8 Nov 7 '17 at 16:46
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    \$\begingroup\$ 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 \$\endgroup\$ – DerStrom8 Nov 7 '17 at 16:59
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    \$\begingroup\$ @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. \$\endgroup\$ – DerStrom8 Nov 7 '17 at 17:16
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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.

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