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I am working on a project that checks a sensor every 12 hours and reports the status via cellular modem to our server ..
Power consumption is less than 1mA during sleep mode and spikes to 900mA While transmitting for 30 or 45 seconds every 12 hours .
My question is what battery do i need to use to get the longest run time with the smallest form factor ?
Battery needs to be able to handle the following temperature range -20C to 55C
The answers thus posted about using mAh = (hours of current draw) * (current draw) are for ideal batteries and will not be good for a calculation like this.
Do you know of any batteries that you can recover all of the energy that they hold? I don't. And at a certain voltage level the battery becomes unusable for you. A 9V battery will become useless after 3V if that is what your circuit needs. You have to 'derate' the equation for these cases, digikey uses a factor of 0.7 in their calculator. This especially holds for rechargeable batteries as there is a voltage point of no return on the discharge cycle that will kill the electrodes. The current draw will also make a difference because a battery is not an ideal voltage source, it has resistance and it is dependent on the current draw.
At the uA level, you also have to start to worry about leakage current, the battery has uA-nA's of leakage current. You'll have to factor that in also. If you need a ball park number, the ideal case is a great place to start but its the maxium ammount of time that your battery will last in the ideal case.
I don't know if its still applicable.
SIM908 board does consume that much average current during normal operation. It includes the current drawn by the total circuit shown in this image.
SIM908 does take a minute or two for getting GPS satellite fix after power on, using a active antenna, Depending on how well exposed the antenna is to the sky. If power saving is the concern then, you should put the module in sleep mode and wake up when needed. The module has the capability to stay in low power mode with average current consumption <1mA. And it can automatically wake up when there is a event like SMS arrived or call arrived. Also there is a catch, We cannot enter the GSM engine to sleep mode unless the GPS engine is powered off.
So, we cant expect to get power saving at a time with faster TTF.
The ideal way would be to use separate GSM/GPS modules like this,
Here I am using the SIM800C GSM and SIM39EA GPS module. This gives me freedom to save power as I can put the individual modules into sleep mode or not. The SMS based tracking of this product is able to run for more than 72 hours on 1100maH Li Ion battery. The device is always on during this time and listening to incoming SMS commands to reply. The TTF of SIM39EA is very good. It is able to acquire GPS signal in less than 30s after waking up from sleep mode.
P.S: I am the author of the blog listed above.
Best Answer
Lithium. Cells made by the Israeli company Tadiran are popular for this sort of application.
If I did the math right, it's about 41Ah for the cellular, assuming it runs directly from 3.7V. Not much you can do about that. 1mA for the standby is very high and you should try to reduce that. That's another 43Ah. Try to reduce that to uA. The biggest I see on their website is 19Ah (max 500mA output current) so you'd need several of them.