I have seen posts relating to using Pi without mains power and thought it might be worth listing the solution I propose to use for the portable race control system I am developing. These batteries are used extensively in burglar alarm systems and come in various capacities. They are supposed to be replaced in alarms every 5 years but their life is probably double that so if you want a cheap one visit your local security firm and see what they have availiable. The other bits have come off ebay and I paid £1.86 for the convertor and £19.69 for the monitor which is meant for use in a car rear view system. The dc to dc convertor is 68% efficient at the Pi operating current, which I have measured as around 400ma, and the monitor draws 120ma at 12 volts. This makes the current draw at 12 volts to be 400ma so the duration with a 2.1 AH battery should be 5 hours. With a 7AH battery this would increase to 17.5 hours. The photo shows the components used and the diagram shows the circuit. The cable used should be rated a 5 amps or higher.
