LED lights update

I'm continuing my quest for the best way to convert the Poundland LED Camping Lights for nominal 12V operation as encountered on a boat. As my test voltage source I'm using a small lead-acid 12V battery from a burglar alarm control unit. I'm keeping it charged up with a 5W solar panel which I bought from Maplin a few years ago. I found that 3.5V across the LEDs gave sufficient light output and should prolong their life (the unmodified unit connects 6V of batteries across them: the actual voltage of even brand new batteries drops to 4.5V as soon as it's switched on, owing to the whopping current drain and the internal resistance of the batteries).

So I modified a unit to give three sets of eight LEDs wired in series-parallel (very conveniently the units each have 24 LEDs) such that applying 10.5V would put 3.5V across each LED. Now for the power supply. Here I described my experiments with an LM317T voltage regulator. I adjusted the resistors to give the required 10.5V output and connected everything to the 12V battery. It worked fine, except for when the battery's voltage started dropping. In fact, even at 12V, the regulator output barely managed 10.5V: by the time the battery had dropped to 10.35V the regulator was giving only 8.79V. Not good regulation!

The 10.5V regulator would work fine with fully charged, and charging, batteries, but the lights would start getting dim quite soon after stopping the engine. No, I'm going to have to regulate to a lower voltage, one which would have enough "headroom" to cope with slightly discharged batteries (and with the length of cable, potentially 120 feet there and back in a narrowboat). Shame really, as a higher voltage means a lower current for the same power; and less heat generated in the voltage regulator.

I know I've gone into far too much detail, but maybe someone out there appreciates it!

Tomorrow morning, then, modify another unit for two sets of 12 LEDs in series-parallel, and wire up a regulator to give 7V. One thing: I'm intending that the voltage regulator will be in the LED unit, so the supply will be the standard (unregulated) feed from the battery bank.

Time for bed.