I use a 12V 16Ah battery from ebay - approx $100
I then use a 12V to USB PWM (that also connects to a solar panel) - you may have better luck using a 12V to 5USB device if you don’t want to use solar charging.
Battery > http://www.ebay.com.au/itm/12V-33AH-AGM-Sealed-Lead-Acid-DEEP-CYCLE-Rechargeable-Battery-4-Electric-Bike/271571742387
PWM > http://www.ebay.com.au/itm/12V-24V-AUTO-10A-20A-LCD-PWM-Solar-Panel-Charge-Controller-With-Dual-USB-5V-DE-/252883999892
Seems to work really well.
Just remember to set the battery type on the PWM to AGM Lead-Acid battery (b1)… probably not going to make a difference though if you are not charging it.
PS: The battery size is probably overkill - but at least you’ve got a starting point.
That battery can support an Electron for about 7days being on all the time and transmitting a lot.
Quick maths -
2ah x 12v = 24w/h usage per day
33ah x 12v = 396w/h supply before flat
396/24 = 16.5 days
I measured the 2ah using a USB power measurement device - it cost $5 on ebay…so let’s say it’s roughly accurate… round those numbers down and best to test yourself. When I tested, I also didn’t have it transmitting, but had it continually searching for a network - but leaving the antenna unplugged. Next test we will connect it with the antenna plugged in