Introduction: Cordless Tool Battery to USB Charger

Picture of Cordless Tool Battery to USB Charger

There are plenty of high capacity battery chargers available to buy and they are certainly a good idea. You don't want to be caught with a dead phone during a power outage. However rather than buy one of these devices why not take advantage of the high capacity batteries that most of us have in our garages already? The battery packs for my Ryobi brand cordless tools are 18V devices. With a few inexpensive parts we can easily turn that into the 5V needed by your cell phone.


1/4 inch plywood

wood glue

1 UBEC (universal battery eliminator circuit) DC/DC step-down converter - 5V@3A outut (available from AdaFruit Industries

1 USB shell connector, type A socket

1 22k Ohm resistor

1 20k Ohm resistor

1 6.2k Ohm resistor

2 picture hanger hooks


perf board

Step 1: Build the Enclosure

Picture of Build the Enclosure

Carefully measure your battery and cut the plywood to the correct dimensions to fit over the battery connector clip. Using a razor blade, cut out slots for the battery's spring clips to fit into. Assemble your enclosure using some wood glue.

Step 2: Make Battery Contacts

Picture of Make Battery Contacts

I had some brass hooks for hanging pictures in my toolbox already. They can be bent and trimmed down to make good battery contacts for our enclosure. We want to make the contacts have some spring so they make good contact and allow for a snug fit onto the battery.

Solder the UBEC onto your contacts paying careful attention to the polarity. Make sure the negative side of the UBEC (black wire) goes to the negative terminal of the battery pack and that the positive side (red wire) goes to the positive terminal.

Cut and glue some pieces of plywood to secure the clips in the proper location.

Step 3: Build Voltage Divider

Picture of Build Voltage Divider

While the UBEC gives us the 5V needed to charge our USB devices, many devices want to see around 3V on the USB data lines in order to charge. To go from 5V down to 3V we need to make a simple voltage divider circuit.

Note that I made two separate voltage dividers (one for each data line) but this shouldn't be necessary.

Cut a hole in your enclosure for the USB connector and then solder the voltage divider to the connector and the UBEC.

Step 4: Finishing Touches

Picture of Finishing Touches

Add some holes for cooling (or just for looks). Throw some finish on the outside of the enclosure to help keep it clean from grubby fingers. Plug in your phone or other USB device and happy charging!


madchenporter (author)2015-10-05

how would i do this with a 20v dewalt battery?

maniacse (author)madchenporter2017-04-07

Same way: recalculate resistors, or use stabiliser with heatsink.

ArtifactOwl (author)2015-11-14

Thank you for this great instructable. I have done some electronics tinkering, but this will be the first "utility" electronics project that I will be using to supply power to expensive things like tablets and phones. For that reason, I want to ask, what UBEC provides the most consistent 5v voltage with the lowest tolerance? And also, how risky is it to supply 5.3v or 5.4v to an ipad or expensive tablet? Can you lower the battery capacity by doing this? Thank you for your help.

madchenporter (author)2015-10-05

how would i do this with a 20v dewalt battery?

LitreOJacob (author)2015-07-11

The website I am looking at offers 22k 20k and 6.2k resistors at 1, 1/2, 1/4, and 1/8 watts each, I'm not sure if the factions are what I need, would it be simple 1 watt resistors all around?

JavaProgrammer (author)2015-04-22

if you had a 12 volt battery you could simply take the guts out of a cigarette lighter to usb adapter.

wavey davey (author)2014-06-16

this is excellent and has spured me on to have a go at one. my drill is 24 volt so i was thinking of getting a 24v truck to 12 car converter and running it to a lighter socket then i could run all the leads i use in my van onsite. i cant see why this wouldnt work ?

WildOne1985 (author)wavey davey2015-02-26

The more voltage conversions you do the less effect you'll end up with. In other words - you loose capacity.

huseyinsafi (author)2014-05-20

you can use LM7505. for 12v to 5v

dbabu (author)huseyinsafi2014-05-20

Linear regulators like LM7505 are very inefficient. If we are using LM7505 to step down 12v to 5v then out of 12watt taken from battery only 5watt will be used by the phone to charge and the rest of the 7watt is lost as heat in LM7505. So use a DC-DC converter to stepdown voltage.

huseyinsafi (author)dbabu2014-09-16

Thank you for information. you're right

Malkaris (author)2014-07-10

I was able to reuse a broken charger for my black and decker

MichaelAtOz (author)2014-05-27

Great idea.

For anyone who has a 3d printer & Ryobi, I modelled the battery inverse (ie the hole it goes into) at

When I get time I'll model one with room for the DC-DC converter USB socket and the clips (great idea to use those wall hangers BTW).

charest58 (author)MichaelAtOz2014-05-28


Kshatrya (author)2014-05-23

This looks well thought out. I don't know what the amp-hour rating on my Ryobi portable batteries, but I'm certain it would keep a phone charged through the 72 hour "you're on your own" post disaster period.

As an alternative to the Adafruit BEC, I've soldered up a RECOM switching regulator. ( ) It's rated for 500mA, but I've tested them up to 1500mA with almost no voltage drop. The only indication is that they get slightly warm. (Probably reduces the MTBF though.)

Good information about the 3V data lines. I've had several items that needed their own proprietary charger. I bet that was why. (As an experiment, I've added a 3.3V module to see if that will work.)

I'll post the results.

waltbosz (author)2014-05-21

I made something similar. My project was for a portable adjustable DC power supply. I used one of these to lower the voltage, and then I sacrificed a Ryobi One flashlight as the enclosure (it gave me a nice on-off switch). I never tried to use it as a USB charger, but that was on my todo list with it. Good to know about the 3v trick.

Azayles (author)2014-05-20

Instead of a resistor divider, or even a linear regulator, use a car
cigarette lighter charging adaptor. They typically have a wide input
voltage range and are much more efficient.

All in all, a very handy addition to anyone's tool box, especially for long hours on the road!

LiamOSM (author)2014-05-20

Great idea to use the picture frame hooks, I already have an "every-possible-input-to-USB" converter, but I might use the frame hooks to tap into my drill batteries. It's better than using an elastic band to hold wires on the terminals.

macdja38 (author)2014-05-20

I made one of these a long time ago, but instead of using a custom circuit in (i tried that at first) i just used a car charger, its within tolerences for allot of them.

SherpaDoug (author)2014-05-20

The instructable resistors are fine:

5V/(22k + 26.2k) * 26.2k = 2.7V on the data lines.

AxisOverlord (author)2014-05-20

But dividing 5V with 22k and 26.2k would result in a voltage of 4.2V..? Please explain how you expect 3.3V on the data lines...

n8hfi (author)AxisOverlord2014-05-20

The divider gives a nominal 5*26.2/(22+26.2) = 2.7V. With typical resistor tolerances, it's probably 2.2-3.3V.

AxisOverlord (author)n8hfi2014-05-20

Woopsadaisy, You are totally right!

bremus (author)2014-05-20

May be easier to use a 18v to 12v converter then solder on one of those cheap car usb chargers.

David97 (author)2014-05-18

Awesome instructable. Can get becs from just about any place that sells radio control gear.

If you want something with heaps of power, (what you have there is plenty for any USB charged device) i have had great luck with this. I use it to power my robot.

David97 (author)David972014-05-18

bremus (author)David972014-05-20

I love HK.

agis68 (author)2014-05-20

nice!!!! clever using the copper from the frames....(i don't know the word for these "handles"

wilgubeast (author)2014-05-19

So so useful. Thanks for sharing!

Akin Yildiz (author)2014-05-18


andytt11 (author)2014-05-18

Have you tested the output for a range of voltage?

charest58 (author)andytt112014-05-18

The UBEC circuit puts out a steady 5.3 V as long as the input voltage stays above 6V.

nqtronix (author)2014-05-18

A certain on the data lines is only nessesary for Apple devices, various voltages have an effect on the charging speed. How it works in detail is very well described in the documentation of the "Minty Boost" (

Other devices such as my Galaxy S2 don't require any voltage on the data lines, however if they're shorted it will charge with 650mA instead of 400mA (a resistor of up to 300ohm will be fine, too). Shorting the data lines is actually specified in the USB standard as a "dedicated charging port" to supply more current than the 500mA of a normal USB port.

If somebody needs a unsiversal solution you might want to try the TPS2511 by texas instruments. It's as much as I know (I've spend a good few hours on research in the past) the only hand-solderable chip (MSOP-8 SMD package) which has this function. It also features current limiting, short protection, over-temperature protection and on/off switching with a digital input. For 1.87$ @ digikey or 2.09$ @ mouser it's fairly cheap as well.

Using cordlss tool batteries to chareg handheld devices is in general a pretty clever idea, as those batteries store lots of power and can help you out when there's no mains power for any reason. Nice!

mwarning (author)2014-05-17

Great idea! Had one question tho.. cud you combine the 20k and 6.2k res' s into one 26.2k res instead? Not sure if that's a standard value tho. I hate soldering axials. Hey this is great regardless. Gonna make this mod this weekend :)

charest58 (author)mwarning2014-05-17

yep. You could combine them if you can find a resistor of that value. Anything in the range of 26k to 39k will give you a voltage reasonably close.

M3G (author)2014-05-17

Wow, this is a fantastic idea!

Honus (author)2014-05-17

That's great! Could you provide a quick sketch/diagram of your voltage divider circuit? Also, where did you obtain your UBEC? Thanks!

charest58 (author)Honus2014-05-17

I've update my write-up to include this info. Thanks for your interest!

Honus (author)charest582014-05-17


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