I have a couple of portable LED emergency lights which have an internal 6v sealed lead acid battery. The unit was left connected to the mains and the battery would be trickle charged, whn the mains failed the lamp would light fo about four hours. I have a 12v suply I could use so would need a circuit to drop the voltage, trickle charge the battery and sense when the supply had failed and switch the LED unit on. Any ideas?
Do the lights work as you want now, with a 6V supply (which I presume they do)? If you just want to run two emergency lamps off a 12V supply, then you might like to try with the lamps connected in parallel, but with a suitably rated 6V Zener diode in series with the supply to each light. (i.e. each lamp has a separate supply.) Take care to see that the 12V supply will cope with the loads.
A little bit more info, such as pics, schematics, ratings, and where they are to be used would be very helpful!
If these are suitable, I would be inclined to say cut to the quick, and move on.
On the other hand, if you wanted to make a device that works as you mention, I would suggest that you start with a small relay (only one c/o contact is needed) which is kept energised when the source is on, and when the source fails the relay drops back to its non energised condition which then connects the batteries to the LEDS. You may well need a diode to isolate the charged batteries from the dead feed. You may well have to do some experimenting.
Hope this helps.