Need help figuring out how to convert AC powered device to be battery powered - Page 1
Forum section: electronics / beginners

author: Bigstar
author_rank: Newbie
timestamp: October 13, 2020, 08:13:40 pm
content: Hi All,I'm a newbie here; this is my first post.My wife has a nice piece of art that hangs from our wall, and it is illuminated by three short strips of white LEDs, She doesn't like the way the long power cord looks, and she wants to see if I can convert it to use a battery box that could easily fit inside the back of the frame, which is very large and hollow.I figured LEDs don't use much power, so this might be feasible, to convert the lighting to battery power. I would want to be able to use the existing power switch if possible, as it fits nicely in a hole on the bottom of the frame, and is therefore very easy to turn on and off. I have the details that are printed on the power supply plug that is currently in use for this piece of art ->Input: 100-120v~50-60Hz 680 mAOutput: 12v 2ABased on these details, does thisprojectseem achievable by a newbie such as myself? I really have no idea where to start. Any and all help will be greatly appreciated.

author: tunk
author_rank: Super Contributor
timestamp: October 13, 2020, 09:01:46 pm
content: 12V and 2A is 24W which is quite a lot. To get some idea about battery size,consider a 10.8V 1.5Ah battery for a cordless drill. If it really uses 2A, then itwill drain in about 45 minutes.

author: rstofer
author_rank: Super Contributor
timestamp: October 13, 2020, 09:11:57 pm
content: The fact that the power supply is rated for 2A doesn't mean the LEDs actually draw that much. You really need to get an ammeter in the circuit somehow. If the switch terminals are exposed, it's easy. You leave the switch off, set the DMM for Amps, put the test leads in the proper jacks and probe across the switch. The meter will short across the switch and the LEDs will turn on. Record the reading. Put theleads back in a Voltage configuration - VERY IMPORTANT. Never leave a DMM set to read current!I have a lot of motion sensor light fixtures that look like candles and such. They typically have one LED but the battery lasts a long time.

author: Bigstar
author_rank: Newbie
timestamp: October 14, 2020, 08:19:56 pm
content: Thanks very much for the suggestions!I'm not sure how to safely operate a multimeter; I have zero experience. Do you by chance know anyone in the Denver, CO USA area who might be interested in a small sideproject? I'd be happy to cover costs plus a reasonable hourly rate...Quote from: rstofer on October 13, 2020, 09:11:57 pm

author: james_s
author_rank: Super Contributor
timestamp: October 14, 2020, 09:25:50 pm
content: Are there any hackerspace type places there? Maybe a highschool that has an electronics class? This project is trivial for someone with even basic electronics skills, or you could swing by Harbor Freight or whatever with a $10 bill and come home with a suitable multimeter and some change, then spend an hour on youtube learning how to use it and off you go. You could also just get a cord with the appropriate connector on it, connect it to a 12V battery and see how long it lasts.

author: Brumby
author_rank: Unknown
timestamp: October 15, 2020, 02:31:19 am
content: Quote from: rstofer on October 13, 2020, 09:11:57 pmExactly right.What this means is that the LEDs should drawno more than2A - but, in actual fact, they may draw considerably less.This is probably the easiest approach:Quote from: james_s on October 14, 2020, 09:25:50 pm

author: KT88
author_rank: Frequent Contributor
timestamp: October 15, 2020, 09:34:39 am
content: For this application the best solution is probably a batteryholder for three 18650 LiIon cells, a charger and threee cells. Use LG or Samsung cells WITH!!!! protection in the range of 3000mAh. Capacities of > 4000Ah are usually scam. You can solder everything together without risking a firehazard with the empty batteryholder....You may purchase two sets of batteries that you can exchange easily for charging when the are depleted. A cheap soldering iron and a cheap meter will do (Harbor Freight or alike).CheersAndreas

author: ledtester
author_rank: Super Contributor
timestamp: October 15, 2020, 10:08:42 am
content: My two cents: if it isn't immediately obvious how to use the existing LED strips with batteries, just replace them with a strip that you have more information on. You might be only reusing the box and the power switch, but LED strips are pretty inexpensive so it's worth your time to work with known components or to use someone else's tried and true design.

author: pwlps
author_rank: Frequent Contributor
timestamp: October 15, 2020, 03:42:55 pm
content: The voltage, current and current-limiting resistors will depend on the circuit topology used. Led strips can be all parallel or mixed series-parallel, see e.g.https://learn.adafruit.com/rgb-led-strips/schematicThe voltage drop depends on the color, quoting from en.wikipedia.org/wiki/LED_circuit :"Typically, the forward voltage of an LED is between 1.8 and 3.3 volts. It varies by the color of the LED. A red LED typically drops around 1.7 to 2.0 volts, but since both voltage drop and light frequency increase with band gap, a blue LED may drop around 3 to 3.3 volts."For a white led it should be 3-3.5V I think, you can measure it exactly setting your multimeter in the "diode" position.For a series configuration voltage drops are summed.A small led typically needs 10-20mA. The total current for parallel configuration is this current multiplied by the number of leds.Finally the resistor calculation for a given current and voltage drop is explained here:https://ohmslawcalculator.com/led-resistor-calculatorTo minimize wasting power this resistor should be as small as possible, choosing battery voltage as close as possible to the led voltage drop.PS. I think this is a nice little project to start with if you want to learn the basics.

author: james_s
author_rank: Super Contributor
timestamp: October 15, 2020, 05:28:38 pm
content: Quote from: KT88 on October 15, 2020, 09:34:39 amFor a complete novice I would probably suggest 8-10 AA NiMH rechargeable cells. They're robust and readily available, one doesn't have to worry about protected vs bare cells, and chargers are inexpensive and can be found anywhere.

author: kjr18
author_rank: Regular Contributor
timestamp: October 16, 2020, 01:21:24 pm
content: Another easy way to roughly check how much current your led backlight takes, is to use a kill'a'wat type of meter. It will tell you how much power something draws (obviously with losses, so your power measurement will be higher than real led power, but you still will get some information about this).

author: Rick Law
author_rank: Super Contributor
timestamp: October 17, 2020, 03:39:16 am
content: Quote from: Bigstar on October 13, 2020, 08:13:40 pmHi, BigStar,Since you are a total newbie, this is going to take some learning.From what you have described, I take the current supply (the one that saids 12V 2A) is external - a "wall wart" (oversized plug) that plugs into the wall, with a wire that goes from this wall wart to a round plug that plugs into the artwork's frame?If what I described above is right, thegeneral ideais: cut the power cord midway between the wall wart and the round plug. The half with the round plug will be used as the new battery power cable. Connect the cut end to a newly-made power pack. That is the general idea.First, we need to confirm if I am right in paragraphs above, plus a few photos to see how it is first.(1) Take a photo of the wall-wart, particularly the prints on itThis one I am interestedall the lettersprinted on this wall wart, to see if anything missing - I want to see if it is AC or DC. Typically, I expect it to be DC, but can't be sure unless all prints are visible. There may be something like anopen circle symbol, see my attach photo. It could be + or - on yours. Seeing that is critical: this will tell us which is positive and which is negative. Make it an additional photo if need be.(2) a reasonably close up photo of the cord plug that goes into the art workI want to see the shape of the plug, and the shape of the wire. I need to discern is this two separate conductors or a round coax without having to cut it open first.(3) a picture of the hole (socket) where plug goes into the art work .This is necessary in order to start in a way that later (after everything works) is possible to convert the working patch-together solution to a less/least intrusive way can have, rather than starting down a path that it will look intrusive no matter what we doexpectexcept if we start over from step 1.(4) If you can, count the number of LED lights you see. I think the number of LEDs is divisible by 3, that would make things easier.There will be more questions once I can see the cord, plug, and the wall wart. And... if you are going to go down this path of converting it, be prepare to spend a little $ on (a) perhaps a multi-meter (b) battery holder and batteries, and (c) perhaps soldering iron and solder.EDIT: Corrected with strike-out, the word "expect" should be "except"EDIT2: Added (4)

author: atmfjstc
author_rank: Regular Contributor
timestamp: October 17, 2020, 12:36:49 pm
content: Hate to rain on your parade, but some quick back-of-the-envelope calculations do not paint an encouraging picture.A single white LED has a power usage of around 70mW (3.5V x 20mA), so keeping it lit for 1 hour requires 70mWH (miliWatt-hours) of energy. A single AA-size rechargeable NiMH battery has a capacity of around 3 Watt-hours, and can thus keep the LED lit for about 3000/70 = 42 hours, so 2 days tops. I would imagine those LED strips have a lot more than just 1 LED in them, probably a minimum of 6 LEDs a piece, so we're already looking at 3x6 = 18 LEDs and thus at least 18 AA batteries to keep them lit for just 2 days.That's a lot of batteries. And having to recharge them every 2 days seems very inconvenient. I for one would demand a runtime of at least 1 week, thus we'd need about 3 times the batteries, like 60 or so. You can get better efficiency with Li-Ion but not really orders of magnitude more. It's not impossible to work with 60 batteries, but just seems like a lot of hassle and complexity when you could instead think of a way to make the power cord blend in with the surroundings better....

author: james_s
author_rank: Super Contributor
timestamp: October 17, 2020, 04:57:47 pm
content: I assumed we were talking about running it for a few hours now and then, not keeping it going continuously. If the goal is to keep it lit all the time then batteries are going to be impractical. There are other options though, use some enameled "magnet wire" or white Kynar wire wrap wire in place of the original cord running up the wall. If you want to get fancy you could paint the thin wires with the same paint used on the wall and they ought to be nearly invisible. Just put the original power jack on the tail end of the new wires and plug the adapter into it down behind the furniture.

author: Rick Law
author_rank: Super Contributor
timestamp: October 17, 2020, 06:14:40 pm
content: Quote from: atmfjstc on October 17, 2020, 12:36:49 pmQuote from: james_s on October 17, 2020, 04:57:47 pmYeah, for long running, lead-acid marine batteries would be the best choice. You can get it near 100AH on just one, but... trying to tape that to the back of the artwork frame would take a miracle, and it would look a hell of a lot worst than a wall-wart.Given he has an on/off switch that he wants to keep, may be it isn't on all day.Frankly, I was going to take him through the step, next would be to power it at a lower voltage (dimmer). I am betting it is n*(3xLED in serial with just a resister) in parallel. Something like 6xAA NiMH - running that at 7V-ish will draw a lot lower current. That would make it longer lasting.But getting pass step 1 (DMM for continuity, V & I measurement) and step 2 (solder some wire) would be an accomplishment. If he can solder a new pair of wires to the V-in, then the world is his to play with.Too bad the OP is rather inexperience yet. An ambiance light sensor switching the LED on/off would be just perfect. But, since he isn't even DMM savvy yet, getting to light sensor switching would be down at step 101+...

author: SpecialK
author_rank: Regular Contributor
timestamp: October 17, 2020, 06:41:51 pm
content: Keep in mind that it would be pretty easy to just have an outlet installed behind the painting. Provided you are comitted to keeping the painting and in that location.

author: james_s
author_rank: Super Contributor
timestamp: October 18, 2020, 05:24:37 am
content: Quote from: SpecialK on October 17, 2020, 06:41:51 pmThat's a bit extreme I think, and it would require a recessed outlet like the kind they used to sell for wall clocks. An easier option is drill a small hole behind the painting and another down low and run the low voltage wire inside the wall. Depending on what's on the other side of the wall it may be practical to poke a hole straight through and plug in the wall wart in the adjacent room where it might be behind something.

author: Rick Law
author_rank: Super Contributor
timestamp: October 18, 2020, 07:31:12 pm
content: At home depot, they have some very nice wall-sticking wire covers. They are rather like a half of tube - sticks to your wall designed to cover wires. A mini-conduit if you will.Looking at the end of either side, they are like a closed 1/2 circle at the end (more oval-ish). The flat side with a thin sticky soft surface to stick to the wall, the circular side faces out.I have 4 feet long on my wall for the wall-lamp, in gold color to match with the "peach-ish" color original wall paint. They were also available in white, silver, black, tan if I recalled. But that was then (over two decades ago).You may want to see if you can find something similar. That do make it look nicer than a bare-wire dangling down to the plug.

author: Bigstar
author_rank: Newbie
timestamp: October 22, 2020, 07:58:01 pm
content: I wanted to thank everyone for all the great ideas! I will post some pics of the existing plug as well as the actual piece of art and the hollow backside of the frame. When I was doing some looking around on the internet, I found a fewfreelancersitesthat look promising for a turnkey project like the one I have in front of me. I'm going to give this a shot, but will get direct assistance if I fail.