I had planned on converting my porch lights to run from the battery, however when I came to do it I remembered these lights were controlled by a timer which required 240V. I did some searching for a 12V timer without much luck. CPC sell various DIY timer circuits but they tend to time for only a few minutes whereas I wanted something I could easily adjust allowing the time to be set over a 24 hour period.
The answer was surprisingly simple - a digital thermostat! These can be picked up easily and I got mine off an online auction site. It runs on two AAA batteries and is designed to turn a central heating system on or off at specified time intervals or when the room temperature drops to a certain level. These timers may also have several programs (mine has six) so it can be set to switch on/off at different intervals. I can also specify different timings for weekends or even separate timings for both Saturday *and* Sunday if I wish.
My timer, wall mounted and wired in (but with front cover removed).
To force the timer to always switch the porch lights on, the evening program is set to activate so long as the temperature is under 35 Celcius (which it should always be in my house unless global warming really kicks in!) Simarly, to turn the lights off, the next program is set to only keep the lights on if the house temperature is under 5 Celcius (which won't happen unless my central heating breaks down in the depths of winter). I also set a morning program with the temperature set at about 19C which, in theory, should mean the lights should kick in during the winter mornings when the postman needs to find my letterbox in the dark while they're likely to stay off during the warmer summer months when the mornings are lighter and I don't need them to be on.
The important thing to look out for when buying one of these is that the relay contacts are voltage-free. That is to say, you want one where the relay is simply a switch whose outputs are isolated from the thermostat circuit allowing you to hang whatever you want off it. You can see from the relay diagram on the timer in my photo that pin 5 is common, pin 3 is normally closed and pin 1 is normally open. I haven't connected anything to pin 3 so the lights will come on only when the relay is operated according to the timer and temperature settings.
Of course, an alternative method would have been to use a PIR sensor to turn the lights on when a visitor actually approached the house but I prefer the timer method as it means the entrance to the house and the house number are always visible from the road. It's something to bear in mind though if you're undertaking a similar project.