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Have you ever wanted to hack your battery-powered LED string lights and make them rechargeable? In this fun DIY project, learn how to replace disposable batteries with lithium cells and transform your LED lights by using a charging board, resistors, and other simple components.

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[0:00] it’s that time of year again and I have
[0:02] noticed that we’ve had a proliferation
[0:03] of these battery powered LED strings
[0:06] they are really convenient there’s no
[0:08] need to overload your sockets with even
[0:10] more plugs but after using up an entire
[0:12] 12-pack of batteries I decided to do a
[0:14] bit of hacking the construction of these
[0:16] lights is pretty simple we can pop off
[0:18] the little plastic cover with a bit of
[0:20] brute force and can see it simply a
[0:22] battery a switch a resistor and a bunch
[0:24] of LEDs in parallel in this particular
[0:27] version the resistor is green red gold
[0:29] which gives us 5.2 ohms measuring the
[0:32] voltage drop across the resistor we get
[0:34] around 670 millivolts so that’s just
[0:37] under 130 milliamps given we have 60
[0:40] LEDs in parallel on the string that’s
[0:43] around 2 milliamps per LED now double a
[0:46] batteries typically have around 2500
[0:48] milliamp hours capacity so we’re looking
[0:50] at around 20 hours total lifetime if we
[0:53] take a look under the microscope we can
[0:55] see just how simple the construction
[0:56] actually is the surface mount LEDs are
[0:59] simply sold it across the two wires and
[1:02] then surrounded by a blob of transparent
[1:03] Goo now a really quick hack would be to
[1:06] just replace this resistor with a higher
[1:08] value the LEDs will light up with just a
[1:11] tiny amount of current and these lights
[1:13] don’t need to be particularly bright so
[1:15] we could easily put a much higher
[1:16] resistor in and increase the battery
[1:18] life considerably an even better hack
[1:20] would be to get rid of the Disposable
[1:22] batteries completely and make the lights
[1:24] rechargeable so over the past year or so
[1:27] I’ve managed to collect a couple of
[1:29] these lithium cells off the street we’ve
[1:31] covered this in a previous video If
[1:33] you’re willing to get your hands dirty
[1:35] you can score some pretty good results
[1:37] I’ve also got a bunch of cells I
[1:39] purchased for some projects they don’t
[1:41] have a huge capacity the ones I
[1:43] collected off the street only have
[1:44] around 500 milliamp hour and that’s
[1:46] similar to the ones I purchased but with
[1:49] the increased resistance we can probably
[1:51] still get a similar amount of run time
[1:52] to the double a batteries I do have a
[1:55] bunch of boards I ordered previously for
[1:57] a project from PCB way who by the way
[1:59] sponsor of the channel they are really
[2:01] nice and I love the red colored soda
[2:03] mask check out the link to PCB way in
[2:05] the description these boards have a
[2:07] charge control chip a low Dropout
[2:09] regulator and a low battery indicator my
[2:12] plan is to use them to replace the
[2:13] batteries in my trackpad but they could
[2:15] be used for this project as well but in
[2:17] the end I have decided that I’ll save
[2:19] these for future as they are quite
[2:21] overkill for this particular hack we
[2:23] don’t need the low battery indicator or
[2:25] the voltage regulator for this
[2:27] particular use case so instead to handle
[2:30] the charging I’ve got these nice cheap
[2:31] little charging boards these are as
[2:34] cheap as chips if you’ll pardon the pun
[2:36] and you can pick these up for next to
[2:38] nothing on Amazon they are really handy
[2:40] as they have both the charge control
[2:42] chip and the battery protection chip
[2:44] built in I’ve added a link in the
[2:46] description it is important to note that
[2:49] not all charging boards do include this
[2:51] protection so if you’re using a
[2:53] different board or you want to use the
[2:54] Salvage cells directly you should add
[2:56] protection it doesn’t take much effort
[2:58] and it’s good to have pieces of of mind
[3:00] I’ve covered this in another previous
[3:02] video the boards I have used the tp4056
[3:05] charge controller and have a 1.2 K
[3:07] programming resistor this means they
[3:10] will charge at a maximum current of one
[3:11] amp the battery protection IC is the
[3:14] standard dw01 connected to a dual mosfet
[3:17] looking things up is really
[3:19] straightforward we just connect the
[3:21] lithium battery to the battery terminals
[3:23] the easiest way to do this is to flow
[3:26] some solder onto the terminals and then
[3:28] Reflow the tinned leads onto the pads
[3:30] the two output terminals can be
[3:32] connected directly to the original
[3:34] battery terminals in the Box it’s pretty
[3:36] easy at this point to add in an extra
[3:38] resistor I’m using a 20 ohm resistor
[3:41] which will take our total value up to
[3:43] just over 25 ohms using our previous
[3:45] voltage drop we’ll now just be drawing
[3:48] 26 milliamps which means each LED will
[3:51] be getting about 0.44 milliamps our
[3:54] battery life if we have a 500 milliamp
[3:56] hour capacity should still be around 20
[3:58] hours or so so we’re not really losing
[4:00] anything for charging we could just open
[4:02] the box up and plug in a USB cable but
[4:05] I’ve got a bunch of these wireless
[4:06] charging coils from a previous project
[4:08] so I thought it might be fun to try and
[4:10] make these charge wirelessly we have
[4:12] covered wireless charging in depth in
[4:14] the various Wireless LED videos so check
[4:17] those out if you’re interested it’s a
[4:19] pretty fun technology and it’s
[4:20] surprisingly easy to build your own
[4:22] version initially I was looking at these
[4:24] very small PCB based wireless charging
[4:27] modules but then I realized the maximum
[4:29] current these can output was 40
[4:30] milliamps this would take way too long
[4:32] to charge we’d be there for hours so
[4:35] I’ve opted for these slightly beefier
[4:37] versions so it should give us a quicker
[4:39] charge time the stated Max current
[4:41] output from these should be around 1 amp
[4:43] So in theory we should get quite a
[4:45] decent charge time however after a bit
[4:47] of checking on the reviews of the coils
[4:49] I have started to have some doubts about
[4:51] how well these would actually work there
[4:53] are a few people saying that at 5 volts
[4:55] the output current is actually quite low
[4:57] so I thought I’d better check I’ve
[4:59] hooked up my electronic load and let’s
[5:01] see what we get powering the transmitter
[5:03] at 5 volts is indeed pretty
[5:05] disappointing we do start off nicely
[5:07] with 5 volts being received but as we
[5:10] increase the load current it quickly
[5:12] collapses and we can only get around 160
[5:14] milliamps before the voltage completely
[5:16] disappears the listing does say the
[5:19] transmit board can be powered from 12
[5:21] volts so let’s give that a go now this
[5:23] does seem to perform a lot better we can
[5:25] get up to 0.8 amps before the voltage
[5:27] drops below 4.2 amps the pcbs do start
[5:31] to get slightly warm though so I’m not
[5:33] sure I really want to be using this in
[5:34] anger I also have these two amp
[5:37] receivers that are very beefy so let’s
[5:39] see if these work the clue will see much
[5:41] better insulated and the circuitry is a
[5:43] bit different with these I did manage to
[5:46] get around 300 milliamps on 5 volts
[5:48] before the voltage collapsed trying them
[5:50] on a 12 volt Supply worked a lot better
[5:52] and I was able to get up to 1.8 amps but
[5:55] then my power supply current limited and
[5:58] shut down unfortunately at this point
[6:00] the wireless charging PCB also died and
[6:03] it no longer works there’s a dead short
[6:05] across the empress so something is
[6:07] really broken what’s even worse is
[6:09] during my experiments I managed to blow
[6:11] up one of the charging boards as well
[6:13] sometimes you just have to accept that
[6:15] the gods are against you so I think for
[6:17] now we’ll stick to charging over USB and
[6:20] leave the wireless charging for future
[6:21] investigation everything does fit nicely
[6:24] in the original box I will probably cut
[6:26] a hole for the USB socket and the box is
[6:28] transparent so we can see the charging
[6:30] LED quite easily all in all it’s a nice
[6:33] little hack

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Chris Greening

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A collection of slightly mad projects, instructive/educational videos, and generally interesting stuff. Building projects around the Arduino and ESP32 platforms - we'll be exploring AI, Computer Vision, Audio, 3D Printing - it may get a bit eclectic...

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