Emergency Solar Christmas Lights

It’s Christmas season at the old school, and it is time to do something with those light strands. These days, LED light strands are cheap and ubiquitous. With a bit of judicious modification, we can use them as emergency lights directly off our off-grid battery array, no inverter required.

There are many advantages to modifying Christmas LED lights as emergency lights, including:

 As a packaged light with cords, they are convenient and inexpensive.
Driving these lights from DC eliminates AC flicker.
 The overall efficient use of available power is good enough to be worthwhile.
 The low current and low voltage operation means that wiring is simple and inexpensive. Even Cat5 network cable will work.
 They will work even if inverters or generators fail or trip, making them useful as backup lights.
Converting them to 24 volt (or other DC voltage) operation can increase their useful life.
They can be controlled with low voltage devices such as a Raspberry Pi + telecom relay.

Our regular readers will recall that we have been working through an entire series of articles on solar power fundamentals and our experience using off-grid solar power during Hurricane Matthew for our freezer and refrigerators. One of the lessons learned from that experience was to not stumble in the dark into the power system, or anything else for that matter. Even in the daytime, when the power is out in our industrial building, it gets a little hard to move around in those back rooms. Plus, if the inverter trips, which they are wont to do, then the AC lights also go out. Better would be to have lighting which completely bypasses the inverter and runs directly off the batteries themselves.

So, we went to Walmart and bought some cool white (1) Christmas lights, as shown to the right. This 50 light strand only cost $3.42. There are also 100 light strands, and 300 light strands which predictably cost less per light. The only difference between the light count is that there are essentially two or six 50-light strands in parallel. There are also similarly priced colored light strands available also, but for emergency stumbling lights the white is probably best. Some stores, other than the one near us, have warm white lights available also; these typically more expensive lights have a more natural light spectrum rather than the cool white (2).

The first thing to notice about these lights is that they are effectively two 25-light strands in parallel. Look closely at one of these strands once you get some, and you will notice that the first and last light, and the two lights in the middle have three wires, while the remainder have only two. This circuit is illustrated below:

christmaslightscircuit_600x200The three-wired lights are the junctions for the AC power lines, with two of the three wires just bridged inside the socket. These strings also have two connectors, a male plug on one end, and a female socket on the other for chaining multiple strands together. The lines marked A and B in the diagram connect through a fuse in the male plug to the AC lines.

We want to pick a number of LEDs in this string that we can reliably connect across our 24 volt array (and the equivalent number needed for 12 volt or 48 volt arrays). We have some flexibility depending on what we want the lights to do for us.

Now for a little bit of helpful science and math. The first bit of science is that LEDs only operate during half of the AC cycle. This becomes obvious as soon as the strand is plugged in; the lights flicker noticeably. This effect can be mitigated somewhat by plugging another strand into the first in reverse; the orientation of the plug can be reversed by trial and error as they are not polarized. Then, if both strands are laid against each other, the flickering is filtered out somewhat. It is still annoying for trying to get anything done by that light. Fortunately, our 24 volt strand won’t have that problem.

The second bit of science is that LEDs operate over a very narrow voltage range. Lower, and they are dim or dark; higher, and they are briefly bright and then permanently dark at any voltage. However, an AC waveform varies from 0 to plus or minus about 170 volts at the peaks for a 120 volt line. To allow operation over such a wide range, these LEDs have an internal resistor to smooth out the peaks at the expense of throwing away some power as heat all the time. For a typical cool white LED (and warm white and blue, which are fundamentally all the same internally), the typical operating voltage is around 2.6 to 2.8 volts (red, green or yellow LEDs will internally operate at a much lower voltage).

Also, the lower the operating voltage, the longer the LEDs will last. By default, the  LEDs in the strings are set a little “hot”. The manufacturer doesn’t mind if you are wasting power, or that the lights will burn out too quickly. Instead, the manufacturer wanted to provide a string with the optimum number of LEDs, meaning fewer. The package says that the lights will last 20 years if used three hours a day. For Christmas lights, that is a lot of holiday seasons, and most people probably will never notice if they don’t make it that long.

Now that we understand the science, let’s do the math for what happens to a 25-light string. First, let’s consider the worst case, when the AC line hits 170 volts during the peak. 170 divided by 25 lights gives 6.8 volts. This means that at this worst case condition, the internal resistance is throwing away about two thirds the voltage (and almost all the power) for very little increase in light. On average, the operating voltage is about 120 volts. This means that each light is exposed to an average of 4.8 volts during the active phase of the AC line, still throwing away about half the voltage.

This internal resistor gets in the way of planning, so we really have to look at the current. A quality white lighting LED, which these certainly are not, will easily handle 20 to 60 milliamps, with surges to hundreds. To probe the characteristics of these LEDs, we set up an experiment to drive a few of them directly from our 24 volt battery array. We measured this array at 26.83 volts at the time of the test, a fairly representative full charge, although not as high as would be experienced at the 28.5 to 28.8 volts during a bulk charging operation. See the picture below for our test setup when using six LEDs:


The two LEDs on the left are the seventh and eighth LED we tested, although these are not energized in the picture above. Even in an illuminated room with a flash you can see that these put out a pretty good amount of light. We tested strings of 3, 4, 5, 6, 7 and 8 lights from our battery array. The following table shows the results:

Number of LEDs 8 7 6 5 4 3
Current (mA) 2.66 5.35 10.81 15.32 19.61 Poof!
Volts per LED
at 28.5 volts
3.56 4.07 4.75 5.70 7.13 Poof!

From this data, we were able to estimate that the internal resistor is about 200 ohms, which means that three LEDs should have experienced about 30 milliamps. Clearly, this was too much as all three LEDs immediately failed after a fairly unimpressive flash.

(update: later testing of larger numbers of strings shows a huge variability in individual LEDs. One 4-string used only 10 mA, while another 6-string used 40 mA, and another 6-string burned out. This is undoubtedly why some people experience very poor results with the strings as-built. We’ll post a future article with a way to sort through all this.)

Based on this data, we can recommend three configurations, dim, bright and emergency. These configurations are summarized in the table below:

Battery Array 12 Volts 24 Volts 48 Volts
Dim 4 LEDs 8 LEDs 16 LEDs
Bright 3 LEDs 6 LEDs 12 LEDs
Emergency 2 LEDs 4 LEDs 8 LEDs

The dim configuration, consisting of 8 LEDs for a 24 volt array, would be appropriate for a 24/7 illumination of a non-critical area, simply to avoid a stumbling hazard. The dim configuration would also allow the least power consumption and the longest LED life.

The bright configuration is our default recommendation, consisting of 6 LEDs for a 24 volt array. This configuration allows some amount of work to be accomplished. As such, it might be best as a 24/7 backup for a more critical area, such as fuel storage or the battery array, charger and inverter setup. Power consumption would still be acceptable, as would LED life. This configuration is representative of the average power consumption experienced by the LEDs in their original string.

The emergency configuration, or 4 LEDs for a 24 volt array, would provide the most illumination for short periods, and is representative of the peak power consumption experienced by the LEDs. This might be best used as an emergency signal light if no other lights were available.

Also note that even the bright option uses very little overall power. At 10.81 mA, these lights can operate from our 90 Ahr battery array, and only draw them down to 50% state of charge after about 4000 hours, or around six months, on a single charge. The equivalent life, on a single charge, for the dim, 8-light string is almost two years. This is well under the self-discharge of the batteries themselves.

To wire the strings to your batteries, you will need to know which is the positive end. Each light is held in its socket by a little tab. The wire near this tab is positive, as shown in the picture to the right. This appears to be the case not only with the strings used for this article, but also for other strings at our disposal.

Because we are going to use the three-wire sockets in a future project, save these sockets and cut your strings from the middle of each 25 light segment (3).

We mentioned above that Cat5 cable can be used to drive these lights, making wiring runs cheaper, simpler and safer compared to normal 110 volt extension cords. Cat5/Cat6 cable is usually 24 AWG, although there is some cheaper 26 AWG cable out there. You can also get Cat3 wire, usually 24 AWG, for less than Cat5 if you don’t care about high data rates and just want to use it for power.

For an example, lets assume that you want to drive four strings of 6 LEDs each at a distance of 500 feet. 24 AWG has a resistance of 26 ohms over 1000 feet (500 feet out and 500 feet back). Using the numbers above for our 24 volt array, we know that each string will use 10.81 mA, or a total of 43.24 mA. The resistance of the wire will cause a 1.12 volt drop, which is inconsequential from a 24 volt array (4). At this distance using this inexpensive wire, the LEDs should be nice and bright, and that is only using one pair of the four available. If needed for longer distances, you could use one pair for power and another for ground, and cut the voltage drop in half.

For 26 AWG wire, such as is typically found in modular telephone wire, the resistance jumps to 41 ohms. This causes a 1.77 volt drop 500 feet away, but the LEDs should still work fine. In this case, each LED will still see about 4.4 volts across it (5), which is still better than 7 lights. Or, as above, you could double the wires as before and be better than 24 AWG. No matter which route you take, you can see that driving these from low-voltage wiring is simple and effective.

We’ve seen in this article how Christmas LED lights can be pressed into service as emergency lighting that doesn’t need an inverter or special electronics. We’ve also seen how to optimize selection of the LEDs, and that they can be driven from inexpensive low voltage wiring without difficulty. So, grab a few strings of LED lights today, and make a list for those post-Christmas discounts to get more.

Footnotes:

(1) White LEDs are available in two general options, cool or warm. Cool means that they are bright white, tending toward blue, while warm means that they look a little yellow, like an incandescent bulb.

(2) Although warm white are usually a little more expensive than cool white, Home Depot is advertising a string of 150 warm white lights for $5.88. We’ll check those out also, but many reviews say these don’t last long.

(3) Also, the three-wire sockets are confusingly wired, with every other one socketed backwards, so it is best to avoid these for now. All the sockets in the middle of each 25-light string should be consistent.

(4) Driving the same 24 LEDs from a 12 volt array would require eight strings of three each. This would result in 86.48 milliamps, causing a 2.25 volt drop. A 2.25 volt drop from a 12 volt battery array is much more significant than a 1.12 volt drop from a 24 volt array. As seen in our previous solar articles, such as this one about off-grid lead-acid battery principles, go for the 24 volt array by default. See this article and the recap list at the bottom of it for additional justification.

(5) The math can be made more precise than this, but these estimates are good enough for our purposes here.

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