Low cost power switchover device
Total parts cost is about $6 for the circuit with switcher supply and $20 for a gel-cel battery.
This schematic at the bottom of this page shows a simple battery floater circuit and switchover to power a USB power supply. The purpose of this is to keep the Raspberry PI up through power drop-outs and while moving the network cabinet. A 4 amp gel cell should run the raspberry PI for several hours or more. With my circuit it would take a day or more to recover the battery after a couple of hours of outage. That's ok because I don't expect to be on battery very often.
The resistor should be such that even if the battery was a dead short, it wouldn't draw enough power to hurt anything. I figured about 200 ohms was good. Anything above 200 up to 1k is probably adequate but 200 is a good lower limit. 200 ohms will get you about 70mA of current flow even if the battery were shorted. 70mA at 14v is about 1 watt of power so you need at least 1 watt of resistor rating. I took several 3.3Kohm 1/4watt resistors and used them in parallel. Larger resistor values will cause the gel cel to charge slower but will also generate less heat.
My later implementation uses a 50 ohm power-resistor with 10 watts rating.
The schematic needed 3 diodes. The nice thing about the bridge rectifier is that it gave me the diodes I needed in a single package and it had a screw mounting hole. I mounted the bridge to the wooden wall of my cabinet. If you get a bridge rectifier with 1 amp diodes or larger, this is fine.
This 12v to USB supply device is available on ebay from multiple vendors:
http://www.ebay.com/itm/181008007744 was the source of the USB power supply image.
Search for competitive sources I have bought 50 of these and they have all worked so far.
The Bridge rectifiers come from Parts-Express and are 400V 6A Bridge Rectifier -- Part # 050-030
The resistors come from Parts-Express and are 56 Ohm 10W Resistor Wire Wound 5% Tolerance -- Part # 016-56
The gel cells I've used so far run from 2 to 4.5 amp-hour. Here is my favorite model because it fits in the housings we've been using:
Parts Express has Power-Sonic PS-1220 Sealed Lead Acid Battery 12V 2.5Ah -- Part # 140-360
The 13V supply on the schematic is off image (click image to enlarge) to the left but the power from that supply comes into the picture on
the thick black and red zip-cord wire.
To the schematic below, I added a fuse in between the 13v power supply and the loads which are taken off of the terminal block. One of those loads is a feed to the charger circuit.
The USB socket for the Raspberry PI is on the little board with the red light.
The big grey and black block on the back side of the little wooden bar is a coax antenna tri-plexer and has nothing to do with the power supply.