I'm designing a small telemetry system that will take measurements every few hours and send the data through 3G at the end of the day. The target battery life is 1 year.
The GSM modules is an XBee 3G. The module draws about 700mA at 3.3V when transmitting. That is the averaged current, I don't know how is current waveform composed.
My first idea was to use 4 AA alkaline batteries and two 50F supercapacitors with a current limited charging circuit, so I won't draw more than 150mA from the batteries and allow the GSM modules to run just from the energy stored in the supercapacitors. The problem with this is the cost of the supercapacitors and circuit, the large space it ocuppies and the time is takes to load the caps (about half an hour if I'm not wrong).
My second idea was to use 4 AA Energizer Lithium batteries. This batteries allow me to draw up to 1A without significant capacity reduction and they are well known so you can buy them almost everywhere.
My third idea was to use a ER34615 battery. The energy density is really good and I could easily achieve more than one year of battery life. The problem is they are also rated for currents up to around 300mA which is very low. I have seen some brands like Ultralife which offers this ER34615 with currents up to 2A which is perfect for me but they seem to be rare and I can't find them in my country.
I really like the idea to use a ER34615 cell. Is there any way I could use it without destroying the battery capacity due to the large currents? I though about using a smaller supercapacitor in parallel with the battery but if the average current is 700mA there's not much a parallel capacitor can do. How are GSM modules usually powered in this applications where battery current capability is so limited?
Ok, to properly test the current drawn from the XBee 3G I connected to a 3G network and covered the antenna in aluminium foil and powered the module with 3.5V to get a -90dBm signal strength.
I sent 1000 of data (plus HTTP header overhead) with a POST request to an echo server, so I receive 1000 bytes (plus HTTP header overhead) of data back.
During the data transfer the current was around 320mA, no big current spikes as in 2G connections. The 320mA were pretty much constant during a few seconds, after that, the module sits in idle at around 80mA also with no current spikes. I don't really know what to think, because Digi has this power consumption table:
I don't have a way to know the transmission power but having such a low signal level and being at 320mA makes me think how they measured those 700mA.
I dropped the voltage to the absolute minimum of 3V and covered the antenna even more so I get -95dBm now. The current stays about the same but only sometimes the current goes to about 800mA during the transmission (a seconds or less). So this 700mA advertised by Digi seems to be the worst case scenario. With relatively good signal levels it's hard to go above 400mA, at least in my network conditions.
The ER34615 are rated for 300mA maximum while dropping the capacity to 9Ah, although this is rated at a continuous 300mA during the battery lifespan. In worst cases a 800mA current drawn from the battery for a about a second can lead to significant voltage drops unless I add some beefy capacitor? Is it possible to calculate how much energy I would be wasting from the battery due to the internal resistance when draining say 800mA during a second?
I think the Energizer Lithium may end up being a better choice because as the ER34615 discharges I may be using more than 300mA due to the boost converter and this will reduce even more the battery lifespan.