Converting kmart 600 led string to run on car battery


New elf
Dec 2, 2021
I wonder if this place has anyone who could help me out.

First some background:
My house is off grid no mains connection I use solar panels and drive most stuff direct off the panels during the day like cooking and boiling water and getting my electric hot water system spitting boiling water out of the pressure release valve and at night only use an old car battery with a charge controller to run a led lamp for lighting before i go to bed early. The lighting inside the house is crap. But I bought this 600 led string warm light from kmart and run it on an inverter and click the button on the wall wart until they all stay on, and its great, put some nails in the walls and run it around the edge of the cieling and it lights the inside of the dark house up beautifully.

But inverters are crap.

Have to run it direct of DC no AC inverter for efficiency and reliability.

Now looking at the label it says the following:
600 led warm white string lights
keycode 42781868
bulb rated: 3v, 0.06w (600 non replaceable led)
light sets rated: 24v ac max 6w
adaptor aproval no saa-191243-ea
adaptor inut: 220-240v 50-60hz 0.12A
adaptor output: 24v ac 6w
adaptor model jt-el/fc24v6w-h6
made in china f004709 date code 34/21

Now the plug of the string has only 2 pins, a + and - and I've noticed that the wal wart has modes, and in some of those modes, one set of lights are on whilst the other are off, in pairs. so thinking about how to do that with only 2 wires, they must have every second led wired in parellel with its neighbor but at reverse polarity. that means to run it on DC I'd need to switch the DC polarity constantly like an artificial AC waveform which I suppose is why they say the output is 24v AC not 24v DC.

so 0.06W at 3v is 0.02A otherwise known as 20mA, a very familiar current rating for a 5mm led indicator. The controller says it is 6w, but 600 of them at 20ma is 36W not 6w, so each 20mA led is only being run at 3mA on average? Or because only half are on at a time, thats 300 at 20mA would be 18w so 6w total must mean 6mA per bulb. Now if it is 24v, and each bulb requires approximately 3v, wouldn't that mean that there must be 8 leds in series? so 300 on at any time (half the 600) and 8 in series (3v leds and 24v ac supply), 300/8=37.5 not possible. so maybe the first and last led are part of the same set, but i just checked and held the first and last next to each other and put the wall wart into alternate fading mode, and they are on and off at different times. and I don't notice any pair that are both on and off at the same time consecutively on the string but there is 600 of them so hard to be sure. so what is the topology?

I know that leds have varying strike voltages but brightness is based on current, and will burn out if too much current is allowed through them, so they way to go is with a current source power supply, but what size current source supply, a 20mA current source? only if every single led where in series with each other, which I don't think is the case or the voltage of the supply would be 900v (or 1.8kv if all in the same polarity). I think i need a 12v in, 12-36v out 850ma current source power supply unit (and let it drive whatever voltage it needs to in order to sink that 850mA of current through the string), but I'm not sure if it is 1500mA, 850mA, or 500mA that would be best suited?

Also, how would I go about switching the polarity? 3v leds means blue leds, these are white so there is probably a phosphor coating, phosphors have after glow, so if polarity is switched fast enough then it should look like all are on to the naked eye at once. But how to switch it? relays would burn out, maybe 4 igbt and an arduino or breadboard AVR to control them,
PSU + ----> igbt A -> string + | \----> igbt B -> string - PSU - -----> igbt C -> string - | \------>igbt D -> string +

And then because we don't want to short out the current source power supply (or maybe we do because if it were a true current source then voltage would drop really low and only 850mA would flow through the cable, but I don't know if these cheap $30 current source boost modules would last being shorted all the time) we disconnect the igbt before connecting the other so there is no time that both are partially connected.

pair one lit: A on, C on 333ms
switching to light pair two: A off C off 100us
pair two lit: B on, D on 333ms
switching back to light pair one: B off D off 100us

But then I don't know if it would like having its load disconnected and reconnected constantly either, considering that it is a 12v->12-36v boost module. Maybe I should stack a few batteries in series get 48v then use some transistors to drive the leds at whatever current they need. but I still don't know what that current would be because I can't figure out how many leds are in parallel, the only clue is 600 total, 2 wires and 2 groups independently lit so probably using polarity so probably only 300 on at a time. and 24v supply giving 250mA, for a total of 6w, but 6w doesn't make sense for a 600 led string unless they are being way under powered. any advice or hints?


340,000+ twinkly lights
Generous elf
Oct 16, 2016
Christchurch, New Zealand