Introduction: Cordless Power Tool Conversion 18VDC to 120/240VAC

Picture of Cordless Power Tool Conversion 18VDC to 120/240VAC

Necessity is truly the mother of all inventions....and my case was no different.  About a year ago, I found myself with a dead battery for my Ryobi 18V cordless tools and no means of getting a new one as I was working overseas. So I came up with a setup to run my tools off AC (household current) safely and with no worries of my tools dying halfway into the job.

Now I enjoy all the benefits of battery operated power tools just as much as the next guy....They're convenient, flexible, and you can usually get quite an assortment of tools that run off the same battery.  Then the inevitable get the dreaded blinking set of lights on your charger and the batteries die out almost instantly. Your faced with two options.....Drop $50-$100 on a new set of batteries or try rebuilding them yourself.  Neither one of those two options appealed to me nor did I have the time to order the parts from overseas.  As anyone in my situation might do, I scoured the internet for an alternative solution.  I found guys connecting car batteries to their drill...ummmm no thanks.  Then I came across the idea of using an old laptop power supply in place of the battery.....The voltage seemed right, but alas the wattage was too small.  Even at 180 watts, the biggest pc power supplies couldn't provide enough to overcome the start-up current of my battery operated circular saw or angle grinder. 

The principal of the pc power supply was sound, I just needed something bigger.  After a little more research I found that the common everyday laptop power supply is what they call a "switch power supply".   Turns out, switching power supplies are very common everywhere in the world, affordable and come in a variety of voltage and power ratings... I eventually chose a 350w AC/DC power supply produced by a reputable company called Meanwell with a voltage range of 15-18volts DC.

Wiring of the power supply to a dead battery is very straightforward, but the following instructable goes through the details step-by-step.

Step 1: Electrical Warning

Picture of Electrical Warning

Before starting please understand you have a power supply capable of discharging 20Amps.  Although the setup is fairly simple, if you are not comfortable working with electronics, please seek professional help on this Instructable.

The cable exiting the battery and connector of the DC outlet from the power supply are made from an standard 120VAC wall plug.  The plug and cable was selected for ease of availability and to allow me to use a standard extension cable if needed.  For my case, I will be the only individual using this setup.

Under no circumstances would I plug an AC powered appliance, tool or otherwise, into this power supply.  If you choose to make this setup please think ahead who might be using it.  If there is the remote possibility of someone not trained to using this setup, I would suggest using a different type of connector and cable.  Something more unique but capable of handling the amps. 

One recommended cable is that of a twist lock generator plug. 

Step 2: Tools Needed

Picture of Tools Needed

• (1) dead 18v Ryobi battery
• (1) 15v 350w AC/DC switching power supply
• (1) IEC C14 electrical receptacle (for incoming AC power)
• (1) standard 120vac receptacle to be used as the DC output or a twist-lock generator receptacle
• (1) computer power cable (has the mating connector for the C14 receptacle
• (1) 16-18awg extension cord (male connector is needed or a male twist-lock generator plug)
• (1) small toolbox (big enough for the power supply and to store the battery)....a 50cal ammo can works really well too.

• small Philips screwdriver
• needle nose pliers
• wire cutters
• hot glue gun
• 2 part epoxy
• soldering iron
• razor blade
• hacksaw blade
• drill
• multimeter

Step 3: Gutting the Battery

Picture of Gutting the Battery

1. Using the Philips screwdriver, remove the 6 screws at the base of the battery (special thanks to my lovely 5 year old)
2. Separate the two halves of the housing and pull out the battery pack
a. Save the (2) yellow square buttons and the mating steel spring
3. Cut the leads from the battery to the terminal block
4. Save the plastic terminal block and the positive (+) and negative (-) terminal strips

Step 4: Don't Mind the Wire..."doctor's Orders"

Picture of Don't Mind the Wire..."doctor's Orders"

1. Using heavy gauge power cord (16 awg) cut off the female receptacle end.  The length of the cord is up to you.  Remember this cord is going to be connected up to your power supply, but you can always use an extension cord if your tool will be far from the power supply. I find 3-5 ft of cord is sufficient.
2. Routing the wire....There are 2 options

Option A: Drill a hole in the battery cover the diameter of the outer jacket of the wire.  The location of the hole is your choice.  You can even add a rubber boot over the wire as it exits the battery cover to act as a strain-relief.

Option B: Install a swivel adapter I've designed and will be making available on in the near future.  This swivel adapter allows the cable to pivot from the front of the battery pack to the rear anywhere within the 180deg angle.  The remainder of this tutorial will be using the swivel adapter.  I've also uploaded the stl files onto under the title of "18V Battery Swivel Cable Lock" if you have access to a 3D printer.

3. Cut back about 6" of the outer jacket insulation from the cable
4. Strip and tin about 1/4" of the leads.
5. The swivel adapter consists of (3) parts
    (2) clamp halves
    (1) swivel base
6. Make sure the clamp halves can completely encircle the OD of the outer jacket of the cable
7. Sand the ID of the clamp halves until you get a snug fit around the jacket

Step 5: Start Cutting

Picture of Start Cutting

1. Mark out a 1/2" wide path on the bottom of the battery cover to be cut off.
2. Using a 1/8" bit, drill a series of holes along the path long enough to get a hacksaw blade to continue the cut.
3. There will be two standoffs from the original screws on the inside of the bottom cover along the path of where you will be cutting.....cut them off.

Step 6: Break Out the Epoxy

Picture of Break Out the Epoxy

1. Feed the wire through the slot of the bottom cover
2. Using some "Super Glue", attach the clamp halves to the edge of the outer jacket
3. Assemble the swivel adapter to the clamp halves, feeding the stripped wires through the small slot of the swivel adapter
4. Epoxy the swivel adapter to the inside of the bottom cover

Step 7: Fire Up the Soldering Gun

Picture of Fire Up the Soldering Gun

1. Cut off about 4" of the exposed 8" of unsheathed wire
2.  Strip the about 1/4" of insulation from the wires and solder them to the positive and negative terminal strips (see the attached image for the polarity of the cable)
3. Re-assemble the terminal block and install it into the top half of the battery cover
4. Use a hot glue gun to fill the cavity where the terminal block sits
5. Attach wires from the top half cover to the bottom half cover using wire nuts or cable lugs.
6. Reassemble the top and bottom halves of the battery cover using 4 of the 6 screws.  Remember to install the yellow side buttons and springs.

Step 8: Box It Up

Picture of Box It Up

Now its time to make the housing for the power supply. 
1. Arrange the power supply in the box to allow room for the battery adapter and the AC inlet plug and the DC outlet plug
2. Make a template for the four M4x6 mounting screws of the power supply to transfer the location onto the box.  Drill holes in the box to mount the power supply.  But don't mount it just yet.

Step 9: Laying Out the Connectors

Picture of Laying Out the Connectors

1. Locate where you want to mount the incoming AC and outgoing DC receptacles on the box.  I prefer mounting one at each end and dressing the wires under the power supply.
2. Solder (3) 12" 16awg gauge leads to the IEC C14 electrical receptacle for incoming AC power on the positive, neutral and ground leads.
3. Solder (2) 12" 16awg leads to the standard 120vac receptacle for outgoing DC power on the positive and negative leads.
4. Make a couple of templates of the receptacle profile and transfer the outline to the box.  Using a sharp utility knife, make several passes until you cut out the holes for the two receptacles.
5. Attach the receptacles either by screw, glue or snap fit; whichever it was designed for
6. Apply some hot glue over the leads on the receptacle to protect the exposed terminals

Step 10: Final Connections

Picture of Final Connections

1. Attached the leads of the connectors to the indicated leads of the power supply.
2. Install the plastic protective cover over the terminals of the power supply
3. Make sure the selector switch on the power supply is set for your country's voltage, either 120 or 240VAC.
4. Plug in your portable power supply using a standard computer cable for the AC receptacle.
5. Plug in the battery adapter into the DC receptacle.
6. Turn on the power supply with the switch located on the IEC C14 electrical receptacle
7. Using a multimeter, check the polarity of the battery terminals and adjust the pot on the power supply (small plastic philips screw to the far left of all the terminal connectors) to above 18VDC.
8. Once confirmed everything is reading correctly, mount the power supply within the box and dress the wires neatly under or behind it.
9. Plug in your battery adapter into your favorite 18V CORDLESS tool and enjoy continuous use.


lousiezhang (author)2017-07-12

Thank you for the instructable. Check this Ryobi ONE+ 18-Volt Replacement Battery if you are intrest. Thank you.

BillS231 (author)2017-01-27

I have just a low amperage RYOBI 18v radio in my shop bathroom that drains batteries quickly and notice it has a USB port to charge cell but will it back charge battery with Male-2-Male input? Tried 1/2 amp cell phone charger first and battery still drained. Trying 2.1amp now...

lousiezhang (author)BillS2312017-07-12

Are you intrest in this Ryobi ONE+ 18-Volt Replacement Battery?

SimonC171 (author)2017-05-24

Thank you for the post. I followed it and made one for my cordless tools. I am not a big fan of battery usage, thinking that all the batteries are environment hazard.

JamieG94 (author)2017-04-01

would this work

abadfart (author)2013-01-12

very nice i thought about building one inside the battery so it would be self contained

ac-dc (author)abadfart2013-01-13

Not reasonably possible. A manufacturer using high density construction might barely be able to do it but a self-made supply of that power density seems very unlikely, not to mention that the power density is too high from an overheating perspective.

wilwrk4tls (author)ac-dc2013-01-14

DeWalt used to have one for their 24V tools that was pretty slick. It snapped in where the battery went and had an integrated fan. I am loving this, though! I think it will absolutely be my next project. I have a decent size collection of 18V Ryobi tools as well and when you start using several at once there don't seem to ever be enough batteries. It saves duplicating tools I only have in cordless and bought that way on purpose!

ac-dc (author)wilwrk4tls2017-01-06

How many tools can you use at once? I'll bet that AC-adapter pack was easily $150 which covers two more batteries, plus it is only enough for ONE tool, unless you swap it from tool to tool, which you can also do with a battery, and it's kind of large and probably every bit as heavy as a Li-Ion battery pack.

Personally I would just buy an AC powered version of my most used tool instead. Even a budget (but not generic junk!) grade AC powered (drill for example) can be had for $50, less than 1/3rd of what I suspect that AC-adapter pack costs.

Plus the tool will probably last longer. Shoehorn a tiny fan into something like that which you use a lot, and it tends to wear out. It's not so much of an issue on things like computers (esp. laptops) because of the upgrade cycle, but most people invested in contractor grade tools enough to buy a $150+ adapter for them, will tend to want to keep using those tools for longer than the adapter lasts too.

In the end I just don't see buying premium cordless tools then not using them cordless. Once one is obviously past the stigma of having a cord, there are a lot of decent quality corded tools out there.

franco40 (author)abadfart2013-01-12

Hey abadfart,

Sounds like a great idea but you might find it hard to fit hard to fit a powersupply big enough into the confined space. Good luck and drop me a line if you can get it done. Loved to see it work.


zappenfusen (author)2013-01-16

Also, huge waste of time.

franco40 (author)zappenfusen2013-01-16

Hey Zapp,
You have a right to your opinion and I respect that, but to each their own. I'm sure there are guys out there that can make use of what I've instructed and that's why I did it. My needs overseas dictated I have a set of cordless tools. You might not be aware of this little fact, but consumers in the U.S. are privileged when it comes to availability and price of products. In southeast Asia where I'm currently working, you can't get a decent set of 18v tools for less than $1000!!! So I when I needed tools I brought them from the U.S. and I didn't want to drop a wad of $$$ on something that might get stolen on the job site. When the battery died, I improvised. Enough said.

I value and appreciate good well made tools just as much as the next guy, but my Dewalts, Porter Cables and yes....Milwakees stay home.

ac-dc (author)franco402017-01-06

It should be mentioned that what you wrote 4 years ago is not necessarily true today. Today you can get 3rd party battery packs, inexpensively shipped from Chinese merchants to (almost) anywhere in the world, for the vast majority of major brand cordless tools.

zappenfusen (author)franco402013-01-17

Sorry for the long winded reply & if I offended. I have been wiped out of tools several times in a 30 year career but if I can't use my tools for the purpose I purchased them they are waste of money. I have worked for contractors using my tools since the 1st theft and refusal of my employer at the time to replace them. I continue to supply my own tools with my employer supplying consumables. On the occasion I was relieved of a tool the cost of consumables increases accordingly. Maybe underhanded but have had no complaints from employers. Their insurance may not replace lost personal tools but they haven't seemed to mind replacing said tools through unexplained increases in consumable's cost even when my drill bit, blade purchases, etc. briefly rise to match cost of stolen tools. I guess I'm fortunate to be employed by persons appreciative of the fact I always have the required tool available without searching an entire job-site. I still believe purchase of cheap (Ryobi) tools beats the adaptation of PWM power supplies of adequate output to power a cordless tool. Please accept my apology,

zappenfusen (author)franco402013-01-17

I appreciate your reasoning and well know the sinking feeling when quality tools grow feet and are never seen again. $1000.00 for a rechargeable Lithium set-up would reduce me to corded tools also. The corded tools do have an appreciable increase in torque and speed over battery operated though and in your situation I believe, given the cost, I would stick with corded exclusively and invest in heavy duty extension cords. If job site theft is a main concern when purchasing, choosing, and comparing available tools I believe I would locate workmates a tad more respectful of others tools. There's nothing I hate more than a derelict attempting to sell stolen tools on a jobsite for 1/10 what they actually sell for. I'm adverse to the cost of the required XFMR to supply needed power for cordless tools. I have been fortunate that the majority of jobs I've worked on were inhabited by person's aware that my tools are my living and have respected that fact . Sorry If I offended yet I still feel converting cordless to corded kind of defeats the original purpose, convenience, and increase in utility of the new cordless offerings. In the States availability of High end cordless tool's is evidently 1/2 what you are paying in your locale giving great credence to your solution. I must insert I've a Milwaukee Magnum 1/2" Holeshooter which is 20 years old and will outlive every cordless I've ever purchased as will the Sawzall, Circular saw, etc., Eliminating the umbilical cord though will always increase productivity while decreasing frustrations to an extent making cordless indefensible when power isn't available. You would be amazed the situations absent of mains power making cordless necessary. Good things to be said for both but in my mind if theft dictates selection of needed tool I once again thank my lucky star's for the market I labor in. I will forever have my original corded Milwaukee's purchased when I realized the offerings of local Electrical contractors consisted of the cheapest tools they could find due to the conscienceless, thieving, tool less electricians (?) they insisted on hiring. When the manufacturer's developed cordless my 1st purchase of a Dewalt 14.4 volt drill, though Ni-cad, became my constant companion and now you would have a fight if you attempted confiscation of my newest Li-ion cordless tools. I'll never relinquish my corded tools as there's still jo

zappenfusen (author)zappenfusen2013-01-17

jobs they excel at (18" ships augers!), things only the outrageously priced and over the top 36 volt cordless are evidently meant for as well a stroking the Male ego. Sorry if I offended and good luck with those "footed" tools.

privatier (author)2013-01-17

Here is a cheaper source for your transformer: go to a place for recyling electronics and pick up a UPS. They contain transformers which are powerful enough. Connect the primary side to AC, and the secondary side to a bridge rectifier (35A, <$3), no capactor required. For long heavy duty work mount the rectifier on a heat sink. A CPU cooler from an old computer works just fine. Note that this solution has no inherent current limiter, so do not short the output!

ac-dc (author)privatier2017-01-06

You don't seem to realize how UPS work. The primary side AC transformer is a low wattage model that just slow charges the battery, is not capable of enough current.

The large transformer is a 12V to 24VDC primary side, 110VAC or 220VAC secondary side, and more often switching transformer on modern UPS.

You should want a capacitor, and a fairly high uF value at that. There will be voltage peaks that could exceed the rating of the transistor used for variable speed throttle, and forward looking, voltage limits for a brushless tool motor controller circuit too, except I would not use an UPS transformer at all.

Remember that if someone needs told how to do anything like this, they will not know the minor details to keep it safe either. Therefore, I could not recommend advising someone to use anything other than a PSU already engineered for the right voltage and output, treated as a sealed, non-user-serviceable unit where they aren't dealing with exposed electricity beyond the low VDC 12V to 18V output.

danchanman (author)2013-08-24

I was wondering if a desktop power supply can be converted? But it only has 12v... Can it still work?

ac-dc (author)danchanman2017-01-06

Can it be converted? Yes, "usually", but if you don't know/have to ask, it is probably beyond your technical ability and probably not safe to try to advise how to do it with a short web forum post.

Briefly what you'd need to do is reduce the threshold of the over current protection circuit (just a feedback loop tweak on the older PC SMPS designs), put a load on 5V rail or disable its OCP entirely, and while you're tweaking feedback loops you can usually tweak it to get another 10%-15% higher voltage.

It's all a hassle to do though, you're better off using a PSU spec'd to handle that job or an unregulated PSU instead even if it is lower efficiency.

Thorn-Boy (author)2014-05-17

Just a thought, but has anyone wrote to Ryobi and asked them to make one?

What kind if price would you pay for a 'Ryobi' made transformer?

I can think of several benfits, but can you think of any more?

I'll list the ones I can think of, but please list any more you can think of:

* Abilty to use mains power when lack of mains power is NOT an issue.
* Longer use times for those with limited number of batteries
* During 'Mains Use' the tool would be lighter (less user fatigue).
* Extended battery lifespan (if batteries only charged when 'Going Remote').
* Less Battery Changes' if working near mains power.
* Less batteries required by user.
* Less batteries produced by manufacturer (better for the environment).
* More environmentally friendly (less dead batteries going to landfill).

My cynical mind says that the reason something like this has not been produced by Ryobi (or other manufactures for that matter) is that it would impact on the future sales of the consumables, namely the expensive batteries.

So how many batteries do you 'Ryobi' users actually own?

Personally, I have four (albeit NiCad) and several chargers, but find I need to swap around a lot when using a few tools at the same time.

These NiCad batteries will die soon though as I've had them a few years now.

Personally, I jump at the chance of a properly made transformer.

No offence intended OP regarding the 'properly made transformer' phrase. I just don't fancy the idea of building one myself.

I would make the jump to 4 x Lithium, but the cost is ridiculously high to get decent batteries when they are only for home use and I would rather have just a couple and a tranformer for when working near mains power.

Nice work by the way.

ac-dc (author)Thorn-Boy2017-01-06

They would not make one because the market for it is too small. People buy lighter weight, lower durability cordless tools because they are cordless, and a suitable well engineered supply that is robust enough for job sites would cost a lot more than a couple battery packs.

Remember, if you have two batteries and a charger than ability to use mains power is what you're already doing with batteries, just a temporary storage method.

Longer use times is not much of an issue. By the time you drag a tethered box around you could have just swapped batteries and have carried a 2nd pack with you, and have a 3rd pack charging, or a multi-port charger capable of 2 or more packs simultaneous.

Besides, on these prosumer grade tools, they really should have a couple minutes to cool down, this is designed into the usable battery capacity, all things budgetized which is why they are so affordable compared to a $300 contractor grade tool combo.

I disagree with the ligher, less fatigue aspect because you can get smaller lighter tools and relative to doing work for that long, the few ounces of the lower cap. battery packs is very slight. Your arm itself weighs lbs relative to this minor weight difference.

It's just not practical, people who have tried various alternative settle for cordless or regular corded tools, not so much a light duty homeowner grade tool rigged to a mains PSU.

If you really just want to use a tool for light duty at home, get an old school car battery charger capable of at least 10A. It'll necessarily output higher than 12V, around 14.4V, and being the "old school" type, they are unregulated so they don't shut down under load, will still keep on going to power the tool at around 144W.

Granted, one rated even higher would be better. I'm just telling you the cheapest way to get the job done with something that can also be useful for its intended purpose, charging vehicle batteries too.

jeff82 (author)2014-05-07

Thanks for this. I plan on doing something similar soon. I was wondering about what power supply to use. I noticed that the one you got was a 15V and the device you are powering is 18V. Is there a reason why the 15V is sufficient? Would I be able to use a power supply as the following:

Thanks for your help.


whitcwa (author)jeff822014-05-23

Batteries have internal resistance. When heavily loaded, their voltage drops. A regulated power supply has very little internal resistance. So a 15 or even 12 volt regulated power supply is plenty of voltage for an 18 volt drill. A couple of years ago, I connected a 34 amp 12 volt supply to an 18 volt drill and it has plenty of torque.

The Jameco supply has nowhere near enough wattage for a drill. Dewalt sells drills rated from 130 to 750 Unit Watt Output

ac-dc (author)whitcwa2017-01-06

Will a 15V or even 12V PSU with sufficient current operate an 18V tool? Yes, almost always it will. however, that does not make it "plenty" of voltage. On the contrary you will tend to have less than 50% power at 12V.

While you can connect something lower and have "plenty" of torque, that is only relative to the particular work you're doing. It could even have more torque than using a worn out 18V NiCd battery, but will definitely not have as much torque as using a modern high capacity Li-Ion battery pack.

exposedwires (author)jeff822014-07-10

That power supply is only 1.7amps, which means you can only supply 30watts to a tool. Most of the ryobi tools are designed to run at around 48 watts, so you will suffer power drops with anything less than 3.0amps

ac-dc (author)exposedwires2017-01-06

lol, no 48W is WAY too low. Anything Ryobi power tool (not just their little vac or work light) is going to need at least 150W to have some margin, and some closer to 350W.

ElectroFrank (author)2013-01-13

My personal favourite trick:
The cheapest source of suitable batteries is those sold for (serious) radio control models.

Take a cheap RC model battery pack with compatible voltage and capacity, and a matching plug.   Then make a five foot lead from the plug to the power tool terminals.

Then just drop the battery pack in your pocket, and conveniently use the power tool with much less weight in your hand.

And if it is a reasonably compatible voltage, and with an appropriate socket fitted, run it from a car battery charger when in the workshop.

If the car battery charger is a slightly lower voltage, the tool will run a bit slow.

If it's a higher voltage, the tool runs faster, but give it regular breaks to cool down or it will overheat !

Ian01 (author)ElectroFrank2013-01-13

RC battery packs are available with a wide range of discharge rates, indicated by a number followed by the letter C (e.g. 10C, 20C), where the number is the fraction of an hour in which the battery can be safely drained. For example, a 3000 mAh 20C pack can be drained in 1/20 h (3 min) and therefore can supply 1 A for 3 min.

Somebody please tell me if anything in italics was wrong, because I suspect that it was.

DOTAU (author)Ian012013-01-17

phase 90 doesn't quite have it correct

The "C" rating tells you how much current relative to capacity, "mil Amp hour", that is safe to draw or charge your battery.

Amps = C * m A h / 1000 ( the /1000 is to go from mil amps to amps)

So a 1200mAh 10C battery is good for 12A (10 x 1200mA/1000)
A 1200mAh 15C battery would be good for 18A (15 x 1200mA/1000)

Exceed this rating for to long and excessive heat and catastrophic failure will surly follow.

ac-dc (author)DOTAU2017-01-06

You might find some R/C hobby batt packs rated like that, but usually that is not how they're rated. Usually mAh rating is valid for a lower discharge rate than the maximum, while max does not have this direct relationship with mAh... again, unless explicitly stated as such for R/C hobbyist packs which are rated for flight time/safety factors.

phase90 (author)DOTAU2013-01-21

That is basically what I said. Rating * C for max continuous current draw. However, the C rate does not apply to charging at all.

phase90 (author)Ian012013-01-13

Actually, the C rate is the drain rate that is a multiple of the capacity. The 3000 mAh pack that has a 20C rate means that it can provide 60 Amps continuous drain (3 Ah * 20). So, it can provide 3 Amps for 1 hour (3000 mAh) or 60 Amps for 3 minutes (60 minutes / 20 = 3 minutes).

Ian01 (author)phase902013-01-14

That makes a lot more sense. Thanks. :)

pc-fan (author)Ian012014-11-11

@DOTAU, @phase90, @ElectroFrank et al

OK, I have an 18V battery drill; the battery pack contains 15 pieces of cylindrical batteries with the text: "SC1.2V 1200mAh NiCd" – these look like the attached image. Their metal casing is rolled into paper.

How to properly calculate the parameters for a suitable power supply? amps, watts – I guess voltage would be 18V :)

What do the 'SC' letters mean?

I like this article, thanks for the instructable and the info in comments!

ElectroFrank (author)pc-fan2014-11-11

Hi pc-fan, NiCd means Nickel-Cadmium cells which are 1.2V, so 15 in series makes 18V. 1200mAh is the individual cell capacity (1200 mAh = 1.2 Ampere-hour). The battery pack is therefore 18V, 1.2Ah, when new it should provide (about) 1.2A for one hour. But drills can take more like 10A, if so you might (theoretically) get full power continuously for about seven minutes.

To discover the maximum current that could be taken by the drill, remove the battery and measure the motor's resistance with a multimeter. The maximum current taken will be the voltage divided by the resistance. So if the motor shows 3 Ohms resistance, 18V / 3 Ohms = 6 Amps, so you would need a DC power supply providing 18V at (at least) 6 Amps.

(If the drill has a speed control, you would need to open it up and disconnect the motor to measure it's resistance.)

An ordinary cheap car battery charger giving 12V at 5A would do some work, but at much reduced power (the motor could then only take 12V / 3 Ohms = 4 Amps).

Power rating in Watts = Volts x Amps. A power supply unit must be rated at or above the maximum current drawn by the appliance.

(SC is probably just the manufacturer's code name for the type of part.)

ac-dc (author)ElectroFrank2017-01-06

"SC" = sub-C sized cells, by far the most common size for NiCd based cordless tool battery packs.

ac-dc (author)ElectroFrank2017-01-06

In practice it's not very reliable to expect that you can measure motor resistance to determine current. The reading can easily be lower than actual startup and stall current requirements. Generally on a tool using sub-C (what "SC" stands for) cells, you should target at least 10A and usually closer to 20A if not more, keeping in mind that just as with any electric device, you should engineer for some margin for continuous use.

For example if your tool draws 20A so 360W from PSU, you'd be better off getting a 500W PSU for continuous use, not 360W PSU. Granted tools seldom operate continuously at their peak, in cordless hand held form factor plastic encased tools, they would tend to melt or at least get brittle and crack apart.

pc-fan (author)ElectroFrank2014-11-12

Hi ElectroFrank, thanks for your precise and practical answer, this helps a lot! I will do this.

ac-dc (author)phase902017-01-06

You're half right phase90. The rated mAh capacity of the pack is at a specific current they should list. Below that current the realized capacity will rise, but raising current as high as 60A, it'll suffer drastic voltage droop and be practically unusable in under 1 minute, not 3.

It may not be so relevant for cordless tools since they typically stay under 25A and 25A vs 60A is a big difference using 18650 battery packs.

DOTAU (author)phase902013-01-17

3 Ah * 20 C = 3 Amps , not 3 Amp hours

phase90 (author)DOTAU2013-01-18

3 Ah (3000 mAh) is the rating of the cell or pack. If it is one string with nothing in parallel, they are the same. That means that the item can provide 3 amp hours of energy. In an ideal world, it is 3 Amps for 1 hour, 1 Amp for 3 hours, etc.
The C rate is the highest rate at which the cell (or pack) can be drained per the manufacturer's spec. In this case, 20 C. This means that the max drain is 20 times the capacity rating of the battery. 3A * 20C = 60 Amps. It only has a capacity of 3 Ah, so its theoretical capacity limits it to 3 minutes (1 hour = 60 minutes, 60 minutes/20C = 3 minutes). The reality of cell chemistries is that you cannot get the full useable capacity of the cell/pack at discharge rates higher than 1C due to internal series resistance and other factors.

ElectroFrank (author)phase902013-01-14

In practice, the effective capacity of a battery is less at higher discharge rates. At higher currents, more energy is wasted as heat due to the battery's internal resistance. That's why batteries get hot when charging or discharging.

Please can anyone provide a link to a table of those RC pack codes ?  That would be extremely useful.

phase90 (author)ElectroFrank2013-01-14

That is true. Some cell technologies are better than others in this respect, but yes it is harder to get all of the gas out of the tank at higher rates. It is the price we pay for being able to use it quick.
What are RC pack codes?

ElectroFrank (author)phase902013-01-15

The pack codes you mentioned about C rate, 10C, 20C etc.   Or was what you already said all the info there was on that ?   I just wondered if there was any more data that might not be obvious to those less familiar with the RC "scene".

phase90 (author)ElectroFrank2013-01-15

The C rate pertains to the cell itself.
The only pack codes that I know of are the ones that describe the construction (# of cells, negative electrode, positive electrode, shape of cell, size (diameter, width, height)) as given in IEC 61960. This is a standard for secondary (rechargeable) cells for portable applications.

rijack (author)2013-01-12

Awsome idea! I've thougt about this for my DeWalt 18V machines.
How did you figure out that 350W power supply was enough?
Did you measure, or looked up via specifications, or...?

franco40 (author)rijack2013-01-12

Hi rijack,

As I mentioned in my instructable, I originally tried a 180W power supply from a high end laptop but it just bogged down with any load. I looked around the internet and found the 350W which gave double the amps. I figured 20Amps should do it and if it was overkill, it wouldn't be by much. The motors of the tools will only draw as much current as they need. My cordless grinder has the biggest motor....if it didn't stall under load then I'd be safe.

I plan on putting a Youtube video showing the setup in action. With that I'll put an amp meter showing the current draw at startup.

Hope this helps.

pc-fan (author)franco402014-11-11

I like this article, thanks for the instructable and the info in comments!

A friend gave me an 18V battery drill with exhausted cells for free and I would like to modify it to run from the wall outlet without spending much on an expensive power supply.

I have found some examples of utilizing spare components, could a pc power supply be modded for the task? Or maybe something similar that is easy to find? I have a dozen of these PC PSUs rating from around 200W to 500W though they provide 12 volts output.

Cordless Drill to Corded - Modify Cordless Drill to Run from Wall

Power supply to my old Skil 2972

Any ideas or recommendations? THX

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More by franco40:3D Printed Customized 91mm Swiss Army Knife (SAK) ScalesRepair The Leg Locks on Your 728B Manfrotto Tripod w/ 3D Printed PartsCordless Power Tool Conversion 18VDC to 120/240VAC
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