I want an Arduino Nano to send its GPS coordinates via SMS. I'm going to use the GSM/GPS module SIM908. My problem has to do with how many SMS will I be able to send before the battery dies.
I've done some calculations but I'm not sure if I'm doing something wrong or missing something...
- I read in this question: What is the Max Power Consumption of Arduino Nano 3? that the max consumption of an Arduino Nano is 280mA.
- Then we have the SIM908 module. From this link: SIM900/SIM908 power supply circuit I learned that "The module consumes around 300mA current while under normal operating conditions, and around 2A current during transmit bursts".
The arduino nano voltage is 5V while the SIM908 Operating Voltage is 3.2V to 4.8V.
Then I've done this timeline trying to be really conservative:
- Turn on: 3 seconds @ 580mA (280mA Arduino + 300mA SIM908).
- GPS data gather: 2 seconds @ 2280mA (280mA Arduino + 2000mA SIM908).
- Data processing: 1 second @ 580mA (280 Ard + 300 SIM).
- SMS Send: 3 seconds @ 2280mA (280mA Ard + 2000 SIM).
- End process + turn off: 1 second @ 580mA.
(I totally made up the durations, I just hope they're upper bounds)
average = (5*580+5*2280)/10 = 1430mA.
That is, the process of transmitting an SMS consumes in average
e=1430mA. Then, if I have a battery with a capacity of
C=10.000mAh, I could get the battery lifetime as,
L = C/e = 10000/1430 = 7h.
Since 7h = 25200 seconds and we spend 10 seconds in each gather data - send data process, we could be able to complete the process 2.520 times with a full battery...
Now, some considerations: I know that the voltage is slightly different for the Arduino and the SIM908, and the calculations are assuming their voltage is the same, but anyway my result is an upperbound, right?
This calculations are not taking into account the time interval between executing the process. Actually this will be done automatically once every 12h, so I guess the Arduino should actually be ON all the time and fire an interruption every 12h. I don't know if there's any low-consumption mode for such things... or perhaps there are some other ways to handle this like having really low-consumption specific hardware.
Am I missing some important fact that will drop my battery? The battery is meant to last months with just one charge... the device should just send its coordinates via SMS every 12h (or maybe 8h), and perhaps this long periods of time might cause some other problems.