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Power a cordless tool with a wall socket? Answered

I have been experimenting with multiple scrounged mostly computer transformers and wiring them into the battery packs. Most of them work for a while then they quit. They can add a quick charge to bad batteries and I can use the tool directly while plugged in but once the transformer goes the battery soon runs down. I am using 18 and 19.2 volt tools and am wondering what voltage and amperage transformer would work or is this just a fatally flawed set up? I have no electrical background and my main motivation for this project is to give new life and function to otherwise wasted equipment. I would also like to see these unused tools possibly made available to those in less fortunate areas who could use the tools either from an AC conversion or to run from a 12 volt car battery which is common world wide. Thanks for any input

8 Replies

Jack A Lopez (author)2012-04-01

This is a question that gets asked here pretty often.

Also 'ibles have been written on this topic, and possibly a small number of those are worth reading.  A search can find them, but the authors use different words to describe them:  "tool", "cordless", "corded", "convert", "battery", "wall power", "mains power"

By the way, regarding words, you seem to be using the word "transformer" as a substitute for "power supply", and those words are not synonyms .  I'm hoping the links to Wikipedia can explain how these are very different.   I don't mean to be critical, but using the right word can help when pitching your ideas to others.

I think the trick to building a mains powered power supply for your formerly battery powered tools, is understanding what kind of power your tool wants, and what kind various DC power supplies are out there, like old computer power supplies, laptop chargers, automotive battery chargers, "wall warts", "power bricks", etc, and whether or not these will actually be capable of running your, formerly battery powered, power tool.

Mostly, the answer has to do with current, in amperes, that the tool draws while cutting, or drilling, or whatever, and the question of whether or not a particular DC power supply can actually deliver that much current, and deliver it at some significant fraction of its open circuit voltage.

I am guessing that your tools will want something in the range of 3 to 10 amperes, at a voltage around 10 volts or so, and this corresponds to power (P=I*V) in the range of 30 to 100 watts.

In the past I have powered, formerly battery powered,  tools using a modified automotive battery charger.  Also just connecting these tools directly to a car battery, often works pretty well too.

For everything I have described here, I am assuming the actual old battery pack, for the cordless drill or saw or whatever, has been removed from the circuit.  Maybe the plastic shell of the old battery pack is kept, but its only function is to serve as a connector.

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footzapper (author)Jack A Lopez2012-04-02

Thanks for the terminology heads up. As you can probably tell, my sophistication level for electronics is low. I think what I have been using mostly is the AC to DC wall warts I find at the swap meet. I have also been keeping the batteries in the battery pack and connecting wires from the terminals on the battery to the wall wart. I think from what you have said that my choice of power supply is too low in the amperage area. I have been using 200- 800 mA. That is probably burning out the power supply when I use the tool while connected.
The car battery idea is workable and I may try that as well. Also, is it hard to modify a car battery charger?

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Jack A Lopez (author)footzapper2012-04-03

I decided to take some of this junk off the shelf, and take some pictures of it, and see if it worked as well as I remember. 

I have two of these modified-battery-charger-power-supplies.  The smaller of these, I have been using to power a small Peltier-based, beer chiller.  The larger one has been mostly just sitting on a shelf collecting dust.

Since you asked, the only modification consists of adding a large electrolytic filter capacitor.  Well, that and adding some connectors and stuff.

Regarding the kind of battery charger I started with.  It is the dumb kind. Well, I admit that's not very specific, but I use the word "dumb" to differentiate it from the newer "smart" battery chargers, which are smart enough to turn themselves off, or to use feedback, or a charging algorithm of some kind. 

Actually, defining this battery charger in terms of what it is not, will be difficult.  In terms of what it is, this kind of battery charger consists of a transformer, and a simple rectifier circuit consisting of just two diodes.  The actual topology of this circuit is shown in the attached drawing, the first picture.  Often this kind of battery charger has a switch on it for a couple of different charging modes, e.g. for 6V or 12V battery, or slow charge and fast charge, or something like that. The way that switch works is by switching some windings on the primary side of the transformer,and thus changing the turns ratio of the transformer.

The 2nd picture is a battery charger of this kind.  I'm not sure what happened to this charger. It might have been too broken to do anything with, or some of its parts might have saved, or then again the only thing left of it might be that picture. But it is a good picture of an instance of the kind of dumb battery charger I am talking about.

The 3rd picture shows the modified-battery-charger-power-supply I use to run my beer chiller.  The little voltage selector switch has three settings, and when it's on the middle one, the beer chiller draws about 3 amperes. There's a needle on that ammeter, but somehow I took a picture of it with a wire obscuring that needle, pointing at about 3A.

4th picture shows a close-up of the filter cap for the beer chiller supply.  23000 microfarads ( or 0.023 farads).

Regarding the filter capacitor, the purpose of this capacitor is to smooth out the large voltage ripple.  Without a capacitor of any kind, the voltage bounces up and down, as high as Vpeak=(1.41..)*Vrms, and as low as zero.

Lead acid batteries being charged do not seem to mind this rapidly pulsing voltage, which when charging a battery really ends up looking like a rapidly pulsing current, because the battery voltage tends to stay constant, as the battery sort of is a voltage source.

I found that the Peltier fridge seemed to work a lot better with a filter cap, and the associated small ripple voltage.  I am guessing that power tools like small ripple too, although it may be the case that powertools do not care about big ripple voltage.

The formula that predicts how big ripple voltage will be is ΔV = (I*T)/(2*C), where I is current drawn by the load in amperes, T is the period of the input AC waveform ((1/60) second in my home country), and C is capacitance of the filter cap in farads. This is why I have chosen filter capacitances in the range of 10s of 1000s of microfarads, because I wanted ripple voltage in the range of just a few volts. 

You can read more about the function of this filter capacitor, here:
and here:
and probably other places too.

The 5th picture shows the inside of my larger modified-battery-charger-power-supply.  I wish I had some information on what this thing looked like in its former life, but I think I threw away the case it used to be in.  Its new white plastic case used to be the case for what I think is called a "nubulizer" i.e cloud-maker. It was some sort air pumping device for delivering drugs into a person's lungs. Not sure what I did with the air pump, but this box was just the right size to contain the parts of my new modified-battery-charger-power-supply.  The size of the filter cap for this supply: 66000 microfarads (or 0.066 farads)

The remaining pictures (6 through 11) show the voltage and current while driving some formerly-battery-powered tools.  The markings on the drill said 18V.  Those on the jigsaw say 14.4V.  That piece of material I have clamped to the table there is a piece of OSB (oriental strand board?) that is about 3/8 of an inch thick. In these pictures, when the ammeter is pointing a zero, that means the tool is completly off.  When the ammeter indicates current flowing, then that is a picture of me cutting through the OSB, or drilling some screws into it. 

Guessing that the accuracy of the voltmeter is about +or- 0.4 volts, and the accuracy of that ammeter is +or- 2 amperes or so.  Really the needle on that ammeter was not all steady during the load tests. It was bouncing around all over the place, but of course the camera makes it look like it is standing still.

The long and short of this is that these tools seem to be drawing about 6-8 A, at a voltage of around 10-12 V, and this supply seems to be capable of making them go.  I mean honestly, I have not used this thing enough to really tell if it is reliable or not.  

I mean except for the beer fridge supply.  I have let that one run continuously for weeks, supplying around 3A *12V = 36W electrical, and it has run without problems, at that power level.

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footzapper (author)Jack A Lopez2012-04-04

I thought my prayers had been answered today when I found a 24volt 8 amp charger for an electric wheelchair at the swap meet. I figured this would give me the extra horsepower, that the 12 volt car battery charger I tried couldn't provide, for my 19.2 volt tools. It turns out it is a smart type charger and won't send the current through if the leads are attached to the tool directly. If I connect it to the battery pack the tool runs well and the charger light comes on. I wonder if there is a component in the charger box that can be taken out of the circuit to allow current (24volts) to flow directly to the tool. Any ideas?

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Jack A Lopez (author)footzapper2012-04-04

I do not know what your charger looks like on the inside.  Actually, I don't know what it looks like on the outside either.


You know, if you've got it working, when both the tool and the tool's battery pack are connected, then this might be the way to use it.  I.e. if tool and battery pack together can sort of fool the charger into thinking it is charging a 24V battery, then this might be easiest way for you to use it.

I mean there probably is a way to hack this charger, and permanently force it into particular charging mode, but I don't just know how to do this.  I would have try to figure it out, and that would require more specific information about the internals of the charger, then I would have to explain to you how to do it, and have you do the hack, and all in all, this sounds like a very complicated process in which something could go wrong.

So, if you've got something that works, then this might be victory.  Or at least good enough for now.

Although I would not recommend just leaving the old 19.2V battery attached to this charger, without the tool running, for hours at a time.  This is not because I am worried that the wheelchair charger will actually succeed in charging your old battery, but it might hurt the old battery, or melt it, or set it on fire, because the charger thinks its charging a 24V lead-acid battery. 

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Jack A Lopez (author)2012-04-03

Regarding the idea of just connecting your battery-powered tools to a large (or small) 12V lead-acid battery, here are two instructables that show that being done:



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frollard (author)2012-04-01

A computer psu generally won't give more than 12v. If you use +12 and -12 to make 24 volts, it can only supply a quarter amp; not much power before it burns out the psu.

If you are using 12v tools then a 500w psu should EASILY be able to handle the load on the 12v rail. Remember modern psu's need a dummy load on a 5v line or they will self-combust, releasing the magic smoke.

I would recommend looking up replacement CELLS for inside the battery that is failing -- its a cheap alternative to replacing the whole battery.

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