converting LED holiday lights to battery power

This is probably on old question: Is there a simple trick to repowering a 120V chain of LEDs from Xmas or novelty lights to run on a small battery pack (without using an inverter)? I bought a set of 10 x 5 mm (?) red LEDs with plastic skulls attached which I'd like to run draped on my body or my bike.

They are wired with a 2-strand wire. Is there likely to be just a big resistor (and maybe a more robust diode?) in the plug body to knock the voltage at the LEDs down? I was assuming there's not a resistor in each LED socket, since the box has a "test me" button that lights up one of the skulls, but it turns out that's a totally independent little circuit just tucked into one corner of the box. Maybe I'll slit open the insulation on that one and one of the main lights and stick my meter probes in to see if they're running at a similar voltage drop. If so, the whole chain should just require a pack of ~ 10 times that voltage, right? Or I can cut it up into 2 or 3 chains in parallel to get match the voltage of the pack, since the current shouldn't be enough to challenge the AA cells...

I could tell you Ohm's law from memory, on a good day, but I don't have much practical understanding of electronics, so try to use small words!

It's highly likely that those LEDs are wired in series and there's no way around the problem of obtaining mains voltage to get them to light. There are some holiday light strings that are wired in parallel with a step-down power supply to run them. That's the kind of set that you could adapt to battery power without rewiring the whole string.

I don't know where you are, but in the US there are several craft store chains that sell small battery powered LED holiday light strings. They're usually built around the tiny 3mm (or smaller) LEDs as they're meant to be used to decorate miniature holiday village sculptures. The wiring is pretty fragile but they do come with a battery pack.

I'm interested in this too...

my guess is the manufacturer used a bridge rectifier somewhere that converts AC into High voltage DC and simply added enough LEDs in series until the voltage load is near the supplied voltage and a resistor is used to limit the current value of the assembly

most incandescent light strings for simplicity is wired in series, to check whether its parallel look for thicker wire strands or [if in package] look for lamps being "hot interchangeable." but LED strings are usually.... :sigh:

Another way around this is to locate candelabra-based C-7 sized LEDs and use them in a reguilar C-7 Christmas light string, then chop off the AC power plug and connect the battery supply in it's place. There are non-resistored C-7 LED lamps out there. Craig has a string of them that he reviewed on The LED Museum awhile back.

I'm interested as well. I've worn (weak) incan light strings that were battery powered (2 D alkiline) in the past, and have been thinking about upgrading to LED.

My Phillips-brand holiday tree light strings pulse at about 60hz, so my guess is that those are in series. I've been thinking of sacrificing a string in the name of science and cutting them short to see if running a few off of ~12V will work. I'd like to know the VF and max current for a single LED so I can build accordingly without having to resort to trial and error.

Any input?

I read somewhere about running two banks of LEDs in series but with opposite polarity allows them to run on AC without the need of a driver....if thats the case I don't know if "cutting it short" will work unless you find the section where polarity reverses

I believe the process involves two separate circuits of LEDs wired to the same plug but in opposite polarity. The positive excursion of the AC cycle lights one string and the negative excursion lights the other one. If you untwisted all of the wires I suspect you'd wind up with two separate loops of LEDs. Each loop has enough voltage drop to cover 120VAC.

Hooboy, there are quite a few landmines in here to consider.

First, is that a few of these may use a step-down transformer that takes AC120V down to 12 or so. I have a set of Halloween "plants" from Philips that are set up like this. The gotcha is that they are assembled without regard to phase, so applying DC means that one or two plants will want the opposite DC polarity from the other to light. A rewiring should fix that.

In the meantime, most of the ones I have, including Home Depot strings and the rest of the Philips lineup (both Halloween and Christmas, and including all of their ropelights) simply run in series on the AC voltage, so you'd need to source 80V or greater DC to make them run, 100V+ if you like them bright. Several of the Philips also have the phase problem between 35-light segments, such that I've had to rewire them to get them in phase for use with 120V AC rectified. The Home Depot lights from last year (2006) came with rectifiers built-in, so are indifferent to polarity, bless 'em. Their current Halloween lights, however, are not rectified and have the 60Hz flicker.

Target has the battery-operated chains, 20 lights each, which also IMO happen to look better for lack of flicker, and they are much more portable.

Illum_the_nation said:

I read somewhere about running two banks of LEDs in series but with opposite polarity allows them to run on AC without the need of a driver....if thats the case I don't know if "cutting it short" will work unless you find the section where polarity reverses

Click to expand...

With some long strings (70+), I have found that the one link where there are only two wires instead of three is the switchover point. Cut, reverse and resplice puts them in phase. These usually occur at intervals of 35 LED's.

35 LED intervals...:thinking:
at 3.5vs that equates to 122.5, sounds about right

The strands I remember as a kid were all series strings, you could use like a 9V battery to test the bulbs.

I thought the ones that kept going if one bulb burned out had some kind of funky shorting mechanism kick in when the filament broke, but I could be mistaken.

But you're missing the question- 3mm/5mm LEDs are focused. Their light pattern is inappropriate for the application, they have an intense light within a narrow forward angle and almost nothing to the side. Their are certainly wider angle ones, the panel display ones are wide but still under 180 deg. Anyhow loads of cheap white ones probably aren't going to be panel display, that would be unusual for whites since everyone wants them for narrow beam flashlights.

Here's my take on LED light strings so far this year. Many stores I did not mention had LED strings last year, but I have not seen any of their offerings yet this year, and so did not include them in this post:

LED light string beam angles:
Most LED strings (except the "Dome" style) have diffusers over the LEDs which do an OK job of spreading out the tight beam LEDs typically produce. Also, some LED strings are made with inverted-dome 5mm LEDs that have roughly 180 degree output angles.

12 VDC strings:
Foreverbright 20-light 12VDC strings are available online. They use regular wire, not the thin, cheap wire typical of most battery-powered light sets.
They can be made by cutting down a 120VAC set, although you have to be wary of resistors. Some bases have resistors molded in; other strings use a resistor in every other or every third base, sometimes soldered to the LED in a removable base/socket arrangement. If you know about the resistors, you can work them to your advantage, otherwise you will wonder why you're getting less current than expected! You can often find the resistors by running the string on AC for awhile and marking the sockets that feel warmer than the others.
When making your own strings, assume about 3.3 V for blue, green, or white and 1.8~2.1 for red, orange, and amber, unless you know otherwise. If you put more than three white, blue, or green LEDs or more than five red, orange, or amber LEDs in series on 12 VDC, the string brightness will be highly dependent on battery voltage. Also, if you try to use such a string in a car, the surges will kill the LEDs. If it is to be used in auto applications, use a max. of 3 or 5 as mentioned above, but design it for 14.4 Volts or maybe even a little more. A 240 Ohm resistor per series segment would be a good choice. If strictly battery-powered, not auto, design it for 12.6 Volts. A 130 or 150 Ohm resistor would work well.

Wal-Mart:
Last year, they had a bunch of offerings using inverted-cone "180 degree" LEDs. They also had red and red/amber/lime-green battery-powered strings that ran off two C cells and had FIVE circuits which chased in various selectable patterns! This year, the one I checked doesn't seem to have these, at least not yet. They very well may - it's still too early.
This year they have a very nice Multi 60-light teardrop set that's rectified AND filtered!! It's a pretty small capacitor they're using, so there's still a tiny bit of noticeable ripple, but only if you move the lights very fast in front of your eyes and look for it. Compared to rectified, unfiltered sets, they're LOADS better! $10.00.

Home Depot:
Last year, they had a wide variety of rectified LED strings, with various shapes. This year, they have the same selection, but have REGRESSED back to half-wave on ALL of them! Go figure!! Same prices, $7.00 to $9.00 or so. They also have an orange string for Halloween. Overpriced.

Lowe's:
Last year, they had a decent selection of Foreverbrights. I don't know if they will this year. I saw the starting of their display, and all they had were GE 50-light sets in multi, blue, and a VERY nice, slightly yellowish warm white!! (I do also like daylight, BTW!) These sets have the unusual feature of having 25 series sets of two in parallel, so that if one LED falls out of its socket, the other in parallel with it will take all the current and the set will continue to light!! These are rectified. $10.

Costco: Last year, Foreverbright 100s in neutral white (~3700K) and multi with lavender and fuchsia for $12. This year, only multi w/o the lav and fuchsia for $9.40.


Series/ Parallel:
Most AC power sources, including inverters, will not care if a small half-wave load is placed on them. Most light string makers don't particularly care that the load be symmetrical on the AC line. Some make two-segment strings with both 30-LED segments having the same polarity. They just assume that if you're using a huge enough number to matter, they would be randomly half on one polarity, half on the other anyway. Foreverbrights seem to be, as a rule, inverted-polarity parallel, which I find annoying when I later wish to rectify these strings.

Rectifying a half-wave string:
I have done quite a bit of this, and it doesn't seem to cause any problems. The peak current through the LED does not go up, it just happens twice as often. Since a half-wave string is peak-current-limited, the result will not overheat the LEDs. The resistors could conceivably overheat, but probably not, unless they used just barely enough resistor power to start out with.

30 vs. 35 (or 60 vs 70) LED strings and series resistance:
If 35 LEDs are used per segment, the Vf of the total will be around 3.2 x 35 = 112 Volts (assuming a peak current of ~25 mA and fairly low Vf LEDs). This is just barely enough to work. I've made a bunch of strings. I assume the 120VAC to be 130VDC operating at 14~16 mA average (for a full-wave rectified string) when sizing resistors. This fudge factor formula seems to give approximately the same result as what commercial light string makers do. For a single segment, I find 1,600 to 2,000 Ohms total resistance to be a good ballpark figure to shoot for. Be sure not to exceed, or even come close to, the power rating of any resistor. They are usually closed up in heatshrink or epoxy, and can't get rid of heat like they were intended to.

When I design strings anymore, I try to leave at least 25 Volts to drop in the resistors. Much less, and the chance of blowing LEDs due to line surges seems to go up. Also, small changes in line voltage begin to result in noticeable changes in brightness. If you designed for lower current, you could probably get away with pushing it a bit closer to 130, since the LEDs wouldn't be as highly stressed from the forward current. But then your string wouldn't be as bright, and would still suffer from dimming with small line voltage sags.
I have a batch of old Foreverbrights with 35 lights per segment that used very high Vf LEDs (around 3.8 V) and practically zero series resistance. LEDs are always blowing out in these strings, and I have given up maintaining them. The LEDs are all damaged from surges, and it isn't even worth trying to turn them into 30-light segments with more resistance, IMHO. Although modern FB strings use much lower-Vf LEDs, some companies have opted to put only 30 or even 25 LEDs per segment, presumably to allow more resistance and hence cut down on the chance of surges blowing the LEDs.


Filtering a rectified LED string:
If you add your own capacitor to filter a rectified LED string, the chance of blowing the string goes way up. These strings are built to take a RMS voltage of 120 and not have the LEDs overheat. If you put a big enough cap across that to filter out most of the flicker, the voltage will be pushing 170 DC, and the LEDs (and resistors) will likely overheat. You'll have to add more resistors and/or LEDs to make up some of the voltage difference. Properly done, the string will be somewhat brighter.

Filtering a half-wave LED string:
If you add your own capacitor to filter a half-wave (unrectified) LED string, it is less likely to overheat, since the design was probably done more to prevent exceeding peak current limits. Properly done, the string will be much brighter.

Of course, as Flashaholics, we all know that the best source for LED light strings would be a regulated DC supply! I have one string running this way, and those LEDs are VERY happy!!

Costco: Last year, Foreverbright 100s in neutral white (~3700K) and multi with lavender and fuchsia for $12. This year, only multi w/o the lav and fuchsia for $9.40.

Great post, lyyyghtmaster, very informative!

A few notes I can add: the "fuchsia" color on the above noted Costco string has a very interesting property I've not seen in any other LED: a high-persistence phosphor. Unlike the lavender, which I suspect is just a filtered white, the fuchsia appears to be a pink/purple phosphor LED, where the phosphor glows red and continues to do so for most of the dark period between peaks, after the diode has shut off. When I get around to it I'll post a photo to show what I mean.

Thanks for the tips on finding resistors by warmth; I find it's easier to do with halfwave strings when I feed them rectified AC. One note regarding peak current and AC: while peak current does not go up, I expect the manufacturers operate on the assumption that the heat generated by pushing the peak higher than advisable with DC has time to dissipate with halfwave, time which is cut I half with rectified power. I notice a distinct extra warmth with my ropelights, for example, when using rectified and unfiltered AC -- through the highly insulating surrounding material, to boot.

Home Depot: I can confirm the regression to halfwave, at least for their Halloween lights. They also have a multicolor ropelight, 18 feet for $15, which have red, white and green. 1.5 inch spacing (never seen that before). No idea about rectification, but these look like the same manufacturing style of the odd gold-box rectified Philips I have.

Target: They still carry the Philips lights, which are mostly direct-AC halfwave lights, with occasional exceptions (such as the Halloween "plants" I have, which run off 12V AC halfwave). They have Halloween lights and ropelights as well, orange and purple; the orange are native, the "purple" are clearly filtered white, and not very purple at all, more of a blue color. They also have $5 battery-operated sets, 20 lights each; I've only seen orange so far. They goofed these, using 4 AA cells running through a quarter-watt 20ohm resistor which gets pretty toasty. I'm going to rewire mine for 3 cells.

Here's an idea I've been pondering for powering LED strings with minimal voltage conversion losses: As we know, filtering rectified AC yields 170V, too much for one string. How about running two in series off that power source? 85V each DC should still give decent results, but have a *lower* peak current, surges notwithstanding.

Another idea I've had is to build a switching power supply that puts out 120V AC at a very high frequency; no flicker, and the strings work within spec.

I made some 12vDC sets out of 120v sets 2 years ago. To run them on my existing 12v low voltage light transformer Worked great. I built a little box to rectify and smooth the 12v, just a full wave bridge and a large capacitor, and then recut the strands so that they would have the correct number of LED's to use the slightly higher DC coming out of that little box. Since I was using all white and all blue strands there was no problem with resistors, at least not in the sets I had.

This year I"m really looking forward to getting some warm white ones, still exploring. My Target had a single orphaned box of the phillips ones but no display for them at all so I didn't pick them up.

Illum_the_nation said:

I'm interested in this too...

my guess is the manufacturer used a bridge rectifier somewhere that converts AC into High voltage DC and simply added enough LEDs in series until the voltage load is near the supplied voltage and a resistor is used to limit the current value of the assembly

Click to expand...

Well, I did a little testing, a little trial-and-error, and successfully blew up my first set of lights. A partial success!!

The single battery powered light shows a 2.2 V DC drop. The diode mode of my multi meter says 1.65 and lights the LED weakly, but I'm not sure what the units are. Is this Ohms? The manual isn't very informative. It's a Sperry DM-350A; if someone can tell me what it's trying to tell me, that would be great. I can't get a resistance reading in either direction in any of the regular Ohmmeter ranges (or at least the several that I tried).

In my previous description, I overlooked 3 extra sockets in the string that have a non-LED plugged into them. Still not really sure what they are. These get warm when operating, and have about a 33 V AC drop, and show 1.4 on the diode setting when unpowered or 3.4 mega ohms on the regular scale. More relevantly, the LEDs again show a 2.2 V AC drop when running on wall power, and when unpowered light up with a 1.7 reading on the diode mode, so they seem to be identical to the one on the battery.

So, I proceeded to cut the 10 LED string free from the plug and the 3 mystery modules. Attached the whole string to a 10-pack of AA eneloops, no light. Stripped the insulation between the 4th and 5th light and hooked the pack up across 6 LEDs, since 6 x 2.2 V seems pretty close to the 13.5 V of the pack. They light! Not quite as bright as before? Tried powering just 5 LEDs. Pfft. Cool, even a tiny puff of smoke. Tried the other half of the string touching just one strand of the wire to the alligator clip. Same result! Gee, I guess the resistance of that little single strand wasn't very significant.

So, back to the store for another set of lights. Seems like I'm on the right track, but I need adult supervision. Not sure if the few seconds of light from the 6 LEDs is a sign of safe, stable operation, or did I just get lucky?

Would it be safer to put a resistor in series with 5 LEDs, and if so, do I have enough info to calculate how much resistance? That would seem ideal, since then I assume I could have 2 x 5 LED circuits running in parallel, and get a run of 5 lights down each arm from just one 10 LED set, rather than having 4 unused.

Please help me fill in the blanks, if you can!

lyyyghtmaster said:

When making your own strings, assume about 3.3 V for blue, green, or white and 1.8~2.1 for red, orange, and amber, unless you know otherwise. If you put more than three white, blue, or green LEDs or more than five red, orange, or amber LEDs in series on 12 VDC, the string brightness will be highly dependent on battery voltage. Also, if you try to use such a string in a car, the surges will kill the LEDs. If it is to be used in auto applications, use a max. of 3 or 5 as mentioned above, but design it for 14.4 Volts or maybe even a little more. A 240 Ohm resistor per series segment would be a good choice. If strictly battery-powered, not auto, design it for 12.6 Volts. A 130 or 150 Ohm resistor would work well.

Click to expand...

:candle: So, 5 red LEDs and a 150 Ohm resistor for a 10 pack of eneloops should work? Can I pull 2 of these in parallel off of one pack? (See also my recent reply to Illum the nation.)

Sorry it took me sooooooooooooooo........ long to reply, I've been extremely busy working to pay off debts, some from buying lights. :devil:

Yes, that value of resistance will be good for one series string of 3 green/blue/white (InGaN or GaN semiconductor composition) or 5 of the amber, orange, or red types, (although some yellows and lime greens can be closer to 2.2V, and 4 in series might be better.) You can easily put two or more such 5-strings and/or 3-strings in parallel across a single resistor, as long as the total forward voltages (Vf) of each string are all pretty similar once they're heated up. However, you will have to divide the resistance I gave by the number of series strings in parallel to come up with the value of resistor you use, or each LED will get progressively dimmer the more strings you add.
If you use a 150 Ohm resistor for one string, you will need a 150 / 6 = 25 Ohm resistor for six strings in parallel. You probably won't be able to find this, so use a 27 Ohm resistor.

Or you can just use a separate 150 Ohm resistor for each string. Sometimes you may want to use a higher value resistor to obtain a lower current. I have found that some cheap Hong Kong LEDs are unreliable even at 20 mA, so I run them at 10 ~ 15.

The number of strings you can run off Eneloops is vast, almost too enormous to comprehend! This is due to how well Eneloops hold up their voltage under heavy discharge. If you could accept the runtime that would result from a 2 Amp draw, you could put

2 Amp / .02 Amp = 100 of these strings on that battery pack!

Also, you'll need to calculate the power of resistor needed. If you have 3 whites in series, the forward voltage will be roughly 9.6 Volts, leaving 3 Volts to drop in the resistor (12.6 - 9.6 = 3). Recall that power is equal to current times voltage in a DC circuit. If you have a single string in series with that resistor, and it's drawing the usual 20 milliamps (.02 Amp) maximum, the power will be equal to .02 A times 3 volts, which is .06 Watt. A 1/4 Watt resistor would be more than adequate.

But let's say you put 12 of these strings in parallel, across a single resistor. 12 x .02A per string is .24 Amp. You still have 3 volts or so to drop, assuming reasonably beefy batteries, so this is .24 x 3 = .72 Watt. Clearly, this is way above the .25 Watt rating of a "regular" resistor. A 1 Watt unit would work, but I might use a 2 Watt device, especially if it's going to be enclosed in epoxy or thick heatshrink tubing, or be near something that could melt.

My resistor recommendations are based on 20 mA LEDs and an average lead-acid battery after a bit of discharge. If you connect a fresh-off-the-charger battery to this circuit, it will likely see 13 Volts or more instead of 12.6. This would now be 3.4 volts across the resistor, not the assumed 3.0 Volts. By Ohm's law, voltage divided by resistance gives current. For a 150 Ohm resistor,

3.4 / 150 = .0227 A, not much above the assumed .02 A.

This is the reason for not adding more LEDs beyond about 75% to 80% of battery voltage. You get a good compromise between reasonable efficiency (75 to 80% of the total power is getting used in the LEDs) and also good immunity to battery voltage swings. But try to push the efficiency too far and it can easily blow LEDs. How does this happen?
If you put four white 3.15 Volt LEDs in series, this would be about 12.6 V total. With a battery that was weak or had been partially discharged, the voltage could be lower than this. The string might light fairly dimly with a very small resistor or even no resistor at all. But if you connected such a string to a freshly charged battery, it would see much more current.
Let's say that the string wasn't very bright with a partially discharged battery pack, so you only used a 2.2 Ohm resistor. Let's say you now connected this string to a freshly-charged Eneloop battery pack, which we will assume to be operating at 13.5 V under this load. Let's also assume that the Vf of the LEDs rose to 3.35 Volts each under this new, higher current.


13.6 – 13.4 = .2 Volt
.2 / 2.2 = .091 Amp! Those little LEDs don't much like 91 milliamps!


In this example, there is simply not enough resistance in the circuit to result in a reasonable current with the new, higher voltage. After not too many seconds, it would be for those poor LEDs!!

In a car situation, you must accept even lower overall efficiency of the string, unless you provide a voltage regulator. This is because the voltage can swing from about 12.5 to maybe 15 or so! (not counting cranking). If the resistor was sized to deal with only 3 Volts to start out with, the current will almost double! 5mm LEDs might handle 40 mA for short times, but it will fry a lot of them in the long term. So you must use a larger resistor to get lower operating currents when the engine is on. This results in even lower currents when the engine is off. The more LEDs in series, the greater this current difference between engine on and off will be. With one LED, it will barely be noticeable, but 4 white, blue, green, etc. LEDs would be unworkable.


Taking all these things into account, I have successfully used two strings of 3 whites, each in series with a 240 Ohm resistor, on my car's license plate for over a year now, thus my recommendation.



Have fun! :shakehead