Electronic – Battery lifetime calculation approach for Arduino + GPS/GSM

The parts most likely to fail first on an electronic device are:

1. Electromechanical parts.

   In an official Arduino, these are the connectors and possibly a pushbutton. Some Arduinos have slide switches and such as well.

   I assume you aren't going to press the reset button very often, so we can ignore it.

   If there are any other buttons or switches — e.g. the settings slide switches on a Seeeduino — I assume they're going to be set and left there for the rest of the device's usage life, so they can also be ignored.

   That leaves the connectors.

   Vibration is a problem in industrial applications. If you let the DC power plug get vibrated out every month, the vibration and repeated re-plugging might put enough wear on the connector to make it unreliable. This won't kill the Arduino, but you can expect intermittent reboots, which many applications won't tolerate. I recommend hot-gluing the barrel connector in place, and strapping the DC power cable down to something solid nearby.

   Be sure to leave enough slack in the power cable that vibration and tugs on the cable don't translate to angled tugs on the connector, stressing the solder joints.

   Shields have enough contact area that they shouldn't come loose on their own, unless there's flexion involved.

   Ethernet and DB-9 serial connectors are fairly tolerant of vibration, but you should still strap the cables down to prevent their weight from dragging on the connector.

2. Electrolytic capacitors.

   Some Arduinos have these, while others get by with lower-valued solid tantalums. Tants are more reliable, particularly at high temperatures.

   As a rule of thumb, electrolytic capacitor life doubles with every 10°C drop in temperature. A really nice electrolytic cap will be rated for 5,000 hours of life at 105°C. 45°C is six such doublings, so 64 × 5,000 hours, giving an expected lifetime of ~36 years.

   Sounds fine, right? Now consider that a cheap cap might only be rated for 1,000 hours at 85°C. That's only 4 doublings, or 16 × 1,000 hours, which is only about two years of continuous use.

   The story is more nuanced than this. By "lifetime," I don't mean that electrolytic capacitors magically cease to be capacitors at the next hour past their rating. Instead, what happens is that the manufacturer has made an educated guess that the capacitor will still be within its rated limits after that many hours of use.

   After the rated lifetime, the capacitor is still going to be a capacitor, but its accumulated drift is allowed to take it out-of-spec once it's given you its rated service life. The more well-made sort of Arduino board will have some design margin that can absorb some spec drift. So, even if you get only exactly as many hours of life out of the cap as the manufacturer guessed you would, the Arduino might continue working.

3. Anything connected directly to a raw power source.

   It doesn't take a very big power surge to kill a delicate microcontroller. To some extent, the fact that you're using the on-board regulator helps. But, consider adding some outboard protection components: zener clamps, maybe a polyfuse, etc.

Wifi is probably hopeless for this application. You should use a sub-1GHz ISM band radio instead.

The Texas CC1101 and Nordic nRF905 are both quite good. The CC1101 has an edge on power consumption I think. Texas also have some newer parts, such as the CC1120, that are on-air compatible but a bit more sensitive.

In any case using sub-1GHz bands will give you much better range than 2.4GHz. Depending on where you live 433MHz, 868MHz or 915MHz should be available. Modules can be bought cheaply on eBay or Amazon.

As an example a pair of lithium AA cells can run a CC1101 and Atmel microcontroller transmitting every ~5 seconds for about 5 years. To get that kind of performance you need to do a bit of work optimizing your code and hardware. For example, run at the lowest voltage possible with an ultra low quiescent current LDO.

Data rate is important to get right too. If you only need to transmit once every few minutes then the lowest possible (1.2kbaud is reasonable for a CC1101) should be fine. Obviously the lower the bit rate the longer the radio has to stay powered up. You can avoid having to do listen-before-talk by simply randomizing the transmission time by say +/- 1 minute, since you will only be on air for a few tens or maybe hundreds of milliseconds maximum.

Then you need to make a receiver for your PC, or perhaps a bridge to wifi.