View Full Version : Converting power tool batteries to AC

David L
12-04-2001, 10:35 AM
Does anyone know how to convert an old 18 volt battery to make it plug into an AC outlet. I figure the cells can be replaced with a transformer and cord so as to make an alternative power source for trim and recip saws which seem to have run out of juice in minutes.

The manufacturers Im sure do not advocate this as they make money off the batteries, but hey its worth a shot.

If any one has an idea please post it.

Fried Elliott
12-04-2001, 11:51 AM
Copy of my response to same thread in "Just Talking"...

Can't comment on your specific tool but for battery operated devices in general, this is not a good idea!

From a real-world electrical circuit perspective, the batteries are not pure voltage sources; they also possess some capacitance and some inductance. The rest of the electrical components (e.g., the charger itself and the tool motor) have been designed with this in mind.

So whatever you decide to hook up, the batteries must REMAIN in the "circuit", charged or not.

David L
12-04-2001, 12:54 PM
If the voltage, current and Amps are created to mimick the battery power why must the cells remain in the circuit. I have opend up my DeWalt 18v battery and all it is are a cells strung together, leading to two contacts.

Additionally I beleive Skill makes a device similar to what I am trying to create.

Any more feedback?

12-04-2001, 01:22 PM

Re-read Fried's post.

12-04-2001, 11:32 PM
I don't know if this will help you but....

Most low voltage electric locks are run with 24VDC. Most of the larger systems run on a battery backup. These systems(like your car) convert thier AC input to DC before using the power. In both cases you can start the system and remove the batteries with out a power interuption. I would say that you might be able to do this but you must make sure that the power that is set to the drill, or whatever "cordless" tool you are using, is both rectified and filtered. You would need a low voltage transformer, bridge rectifier, filter(small curcuit board), boxes and conduit to house the equipment.

My guess is that unless you can get electrical supplies for free you would be better of buying the corded equivilent than buying all of this stuff to make a "cord" for your "cordless".

good luck.


12-05-2001, 05:39 AM
To put it simply the system as you purchased it is direct currnet or DC and what you want to do is run it from an alternating current source or AC. This can be done but more trouble than its worth as Chris mentioned..............you will need a power supply


12-05-2001, 09:30 AM
The reason that the batteries need to remain in circuit is that they are the capactive filters for the power supply. The transformer puts out AC and then pulsating DC as it's rectified. This has to be filtered to get anything resembling straight DC and the batteries act as the filter. A battery is, in effect, a very large capacitor. The motor's life will be adversly effected by unfiltered DC causing chatter and buzz and leading to overheating.

A good solid DC power supply will work. The wall transformer types generally don't have filtering internally and won't work well even if the V & A ratings are correct.

If you can scrounge an old battery charger of the right V & A rating this might work well. Like the one used by the tool originally...


Fried Elliott
12-05-2001, 10:30 AM
Thanks guys. In my haste to post and hopefully prevent a potential meltdown, I did not elaborate on the reasoning behind my post.

Well said.

12-05-2001, 12:01 PM
I think somewhere along the line the original problem has been misunderstood. The question was how do you convert a battery so it will plug into an Ac circuit. The answer is you can't a battery has no input power it is only a storage device that will discharge when a load is imposed across its terminals. I think what the answers are tring to do is make it into a power supply that plugs into an AC circuit,can't do that either. For what its worth, remove your battery and chunk it to file 13.Build or buy a power supply that has an output equal to the run terminal requirement, be careful now as I said run circuit requirement and not the oringinal battery pack as the battery pack may have had a Zener diode circuit to maintain a positive voltage magnitude of "X", assuming this is possible to determine then you could hook the output terminals of the power supply directly to the battery mating contacts on the machine and you then would have an AC driven unit from an AC convenience outlet thru the power supply to the machine. The power supply itself contains the capacitive filters to smooth out the rectified AC positive side of the sinesoidal wave form on the secondary of the transformer. The capacitors discharge and charge up as the AC voltage is falling and passing thru Zero potential. when the next cycle occurs it lags the first by a small amount as it happens 60 times per second . The void area that exists between the wave form peaks are filled in and smooths out the gaps with the discharging of the capacitors.

In further explanation rectified AC is not "True DC" as a battery ,it only blocks the negative side of the wave form by routing it thru diodes (which act as a block valve would in a water line) thereby only letting the current flow in one direction ( a characteristic of DC)

Have I got you confused????? test tomorrow

Buy a new one with a line cord or buy a new power pack, you are flirting with danger.


Bob Hobbs
12-05-2001, 09:22 PM

Are you wanting to build an 18v DC power supply capable of running a battery operated tool that is set to run on 18v DC? If so, I don't see where you would have any problem, but I would agree with dicklaxt1 and ChrisM that you would probably want a bridge rectifier with filtered output and I would go further and suggest a voltage divider designed so you could have several voltages available for the different battery operated tools you have.

I'm not sure I would want to convert a cordless tool to a corded tool unless I had some pretty big bucks invested in the tool--my rationale being that if I bought a cordless tool in the first place it was because I wanted the portability and was willing to sacrifice power for convenience.

If you're not comfortable designing the circuitry yourself, your local Amateur Radio club--"hams"--might be able to help. Some Radio Shack stores have knowledgable people--some do not???

bubba blue
12-05-2001, 11:05 PM
>If you can scrounge an old battery charger of the right V &
>A rating this might work well. Like the one used by the tool

Hi All,

Not an electrician/electrical eng. either, but have taken a number of professional and academic courses and made a living (prior life in the Navy) repairing electronic devices (Data Systems Tech).

Unless the charger is designed to power the unit while charging, it may not have enough juice to power the motor directly. All chargers are designed to produce a output amperage (effectively the working energy).

You may find that while the charger may provide enough amperage (source of it's power) to charge the battery, it may not provide a high enough amperage to power the unit directly.

Not sure if there's any harm trying it out though. May want to check with someone more qualified. I imagine there is possibly some kind of wear/stress when running a power tool at lower than normal available power.


bubba blue
12-05-2001, 11:11 PM
>I'm not sure I would want to convert a cordless tool to a
>corded tool unless I had some pretty big bucks invested in
>the tool--my rationale being that if I bought a cordless
>tool in the first place it was because I wanted the
>portability and was willing to sacrifice power for

I could see a possible rationale... when using my cordless tools, there have been many occurances where I used up both batteries and having either a 3rd battery or an AC powered alternative would have been nice.

Of course, that's why I know have 2 cordless drills (12v & 7.2v), a cordless driver (small hand held unit), and a corded drill.

If I had it all to do again, I would have purchased a cordless unit that also offered a corded option instead of my 2nd cordless drill.

Oh well, you live and you learn.