household batteries
Battery Christmas lights
Most Christmas lights have to be plugged into the wall to get power. But if you could run them off batteries you could put them anywhere an outlet isn’t available — such as on a bicycle.
The kind of electricity you get from the wall is called AC, and the kind you get from batteries is called DC. You have three options for running Christmas lights off DC (batteries):
* Buy a set already wired for DC.
* Use a device called an inverter which converts DC to AC.
* Rewire your Christmas lights to run off DC.
Whichever option you choose, it’s important that you use LED Xmas lights because they use 90-98% less electricity than standard lights, so your batteries will last longer. A lot longer, like 16 times longer. Instead of 15 minutes with incandescents, you can get four hours with LED’s. I just ran a test with a big battery and got only two hours on a 20-bulb strand of old-school lights, and a whopping 31 hours with a 20-bulb strand of LED’s. Dude/dudette, use LED’s.
When I first wrote this page some years ago, LED Xmas lights were rare, and there certainly weren’t any wired to run off batteries, so I was sharing two big secrets: one, that you want LED’s for the longer battery life, and two, how to rewire them to run off batteries. But now you can buy an off-the-shelf solution without having to rewire anything, as listed in the yellow box on the left: lights, battery, and charger. The only thing extra I added was some connectors on the ends of the wires to allow me to easily “plug” the battery to the lights or to the charger. This is a 2200mAh battery and I get 31 hours of runtime (!) with this combo.
So those three off-the-shelf products are the easiest solution. There are others, though. Here are all your options:
- Buy a set already wired for DC. This is far and away the easiest option. You can get them from Inirgee (which will give you a lifetime warranty), or Backwoods Solar, which has an 18-bulb strand that uses 1/2 watt, and a 36-bulb strand that uses one watt. If you know of another online source for DC LED Xmas lights I’ll be happy to list it here too. I did once see a small incandescent (NON-LED) set at a Lowe’s home improvement store for $1.97, but I estimate the two C batteries it takes would only last about 75 minutes, since the lights are standard (incandescent). You really have to go with LED’s if you want any appreciable amount of battery life.
- Buy a set wired for AC and use an inverter to power it with batteries. An inverter changes DC current into AC. You connect a 12V battery to the inverter, and then just plug your Christmas lights into the standard AC outlet on the inverter. It’s like a mini-power plant.
When I first wrote this page and off-the-shelf, pre-wired 12V LED Christmas lights didn’t exist. Now that they do exist, there’s not much reason to go with an inverter. The main reason would be if you already have an inverter or AC LED Xmas lights, so you only need the other piece to get up & running. - Rewire a AC set to run off batteries. It’s really easy to take a set wired for AC and rewire it run off batteries. I’ll show you how to do that further down the page, with nice pretty pictures.
No matter what method you use, you’ll want to know what kind of batteries to use and how long they’ll last. So let’s cover that now.
How much runtime you’ll get from your batteries
Calculating electrical use is easy. The formula is very simple:
Volts x Amps = Watts
Usually we’ll abbreviate. (e.g., 2.5V = 2.5 volts, 25W = 25 watts).
You don’t even have to know what volts, amps, or watts are, as long as you know the formula.
When you use a watt of electricity for an hour, that’s a watt-hour, or Wh. An amp of electricity for an hour is an amp-hour, or Ah. Batteries store such a tiny amount of electricity that they’re usually rated in milliamp-hours instead of amp-hours (mAh). 1800mAh is the same as 1.8Ah.
A typical 50-bulb strand of (NON-LED) Christmas lights uses 25 watts. So each bulb uses about half a watt. (Remember that, we’ll use it later.) Now we need to see how much electricity is stored in a battery.
A typical rechargeable AA battery (NiMH) puts out 1.2V and is rated at 2200mAh. Remembering that V x A = W, we see that a single battery has a capacity of 1.2V x 2.2Ah = 2.64Wh. But the lights use 25 watts. So you’d need ten batteries to power your lights for just one hour. Ouch.
You have four options for getting more runtime out of your batteries:
- Use LED Christmas lights instead, like those from Inirgee. LED’s use 80-90% less electricity than regular lights. So your batteries will last around 16 times longer.
- Use fewer bulbs. Who says you have to use 50 lights? Use only 25 and then your batteries last twice as long. Use even fewer lights and get even more battery time.
- Use more batteries. The more batteries you use, the more total power you’ll have.
- Use higher-capacity batteries. NiMH D-cells store up to 11,000mAh. You could also use a small lead-acid battery or a rechargable pack used for camcorders or remote-controlled toy cars.
My preferred solution is to use LED lights. That way I don’t have to limit the number of lights I use or deal with buying and recharging a gazillion batteries. I get 31 hours of runtime with the setup listed in the yellow box on the top of this page. LED’s offer other advantages: They don’t burn out (not for about ten years, anyway), and they’re rugged — they don’t break easily like regular flimsy Christmas lights. Most of the lights sold by Inirgee have permanently-mounted bulbs that can’t fall out of their sockets, and they carry a lifetime warranty.
Now that we know that we need to either use LED’s, few lights, or lots of battery power, let’s pick a power source for our lights.
Choosing batteries
You can use standard household batteries (AA/AAA, etc.), or you can buy a battery pack, like the one listed on the top of this page. The advantage of a big battery pack is that there’s only one thing to charge. A single 12V pack takes the place of ten individual household batteries. It’s a lot easier to charge the one pack than ten separate batteries.
On the other hand, if your lightset requires only 2 or 4 batteries, then it’s cheaper to just use those batteries than to buy a big pack. Each rechargeable battery (AA, AAA, etc.) puts out 1.2V. If you need 6.0V, then that’s five batteries (5 x 1.2V = 6.0V). NON-rechargeable batteries put out more voltage (1.5V), but I don’t recommend them because it’s wasteful to use disposable batteries. It’s certainly possible to do, but if you choose to do so then you’re on your own. The numbers that follow assume you’re using rechargeables. To wire up batteries to your lightset, you just need a battery holder from Radio Shack, which costs like $1 or something. If you need an odd number of batteries, like 5, then get a 4-battery holder and a 1-battery holder and tie the wires together. Connect opposite colors together (red +black).
Rewiring Christmas lights to run off batteries
Here’s where you get to play mad scientist. Many devices run off only AC or DC, but lights aren’t picky and will run off either. The trick is just to rewire the strand so the bulbs get the proper voltage. Remember, you really really really should use LED lights or the batteries just won’t last very long. Even so, we’ll use standard (non-LED) bulbs in our example below because they’re the most common.
A wall outlet supplies about 120V, so if there are 50 lights in the strand, each bulb gets 2.4V. The bulbs actually want 2.5V, so supplying them only 2.4V makes them just a tiny bit dimmer, but not much, and the reduced voltage makes them last longer anyway. In our example below we’ll supply 9.6V to our strand with batteries, which will power four bulbs (4 x 2.5V = 10V). Before you start screaming that four bulbs isn’t enough, don’t worry, in a minute I’ll show you how to wire several sets of four together.
Most Xmas bulbs are 2.5V but some are different, and they could be more or less. Usually the voltage is printed on the box the lights came in or on a label on the strand. If they’re LED lights then every color has a different voltage; white LED’s are generally ~3.5V. If you can’t find the voltage, check with the manufacturer. Also, don’t assume that if a 50-bulb strand has 2.5V bulbs that a 100-bulb strand must have 1.25V bulbs; it’s more likely that the 100-bulb strand is just two 50-bulb strands wired together.
Anyway, here’s how to wire four 2.5V lights together:
But what if you want to power more than four measley lights? Easy, just create several sets of four lights and hook them all together. You can have as many sets of four as you like, though the more lights you have the quicker the batteries will run out. Here’s how to wire three sets of four bulbs together.
Here’s what it looks like with actual lights:
STEP 1: Make three sets of four bulbs each
I tied up the slack so the lights would be more compact.
There’s nothing special between each bulb, just a continuous wire.
STEP 2: Connect all the “heads” together.
That is, make sure the beginning of each set is connected by a wire. Normally you’d wrap the power wire around the other wires, but I made the wire separate here so it’s easier to see how the wiring works.
STEP 3: Connect all the “tails” together.
That is, make sure the end of each set is connected by a wire. Normally the return wire is also wrapped around the other wires. I made it separate so it’s easy to see how it’s wired. Note the battery pack is actually eight batteries even though only four are visible (the other four are on the bottom; it’s two batteries deep).
This set would last almost two hours on a set of eight AA 1.2V NiMH batteries with 1800mAH capacity each (like the kind Radio Shack sells). Remember that each bulb uses half a watt-hour per hour. So 12 bulbs use 6wH per hour. Our batteries store (8 batteries x 1.2V x 1800mAh = ) 17,280mWh, or 17 wH. Therefore our 17 wH battery pack will power this 6-watt strand for almost three hours.
So there you have it, three ways to get Christmas lights to run off batteries!
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