The 970-HA-JOKES Payphone Project
Nate 
Power
Seen on the left is a LiFePO4 lithium iron phosphate battery. This new battery chemistry is a bit less energy dense than a lithium ion battery, but LiFePO4s cannot achieve thermal runaway. Said differently, these batteries don’t go kablooey if something goes wrong. I’ve had a few LiPos puff on me before and given the harsh outside, public environment, LiFePO4s were a great choice. These new type of 12.8V batteries are dropping quickly in price. I picked up a couple 20Ahr sized batteries for $70 each.
The LiFePO4 12V batteries fit perfectly within the phone pedestal base.
The average standby current for the PayPhone Project was ~250mA with everything on: the VOIP Obihai device, the WiFi bridge, the overhead LEDs, relays, etc. With 20Ah we had a run time of about 80 hours. I set a calendar reminder and replaced the battery every 3 days and never had a power outage. In the future we could run two batteries in parallel (one stacked on the other inside the base) and get perhaps 10 days of run time with two 30Ah batteries.