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This question is the closest thing that I found similar to my needs, but it is still quite far from the answer I need.

I have 2 cordless power drills, a 12V green Bosch and an 18V DeWalt. Their battery packs (accumulators) pretty much died by being not used. I want to transform the cordless drills into corded drills, but in a "smarter" way. I saw hacks on the net, but they look too "dirty" (mostly, they have a thin cable between the power supply and the tool).

My definition of smart:

  • remove the actual cells from the battery pack;
  • replace the cells with a proper power supply;
  • connect the end of the power supply to the connector of the battery pack;
  • connect the input of the power supply to the wall socket with a cable (through a newly made hole in the plastic wall).

I am not afraid to undergo the transformation except that I do not know how to choose the power supply. The input information that I have is what I can read on the labels: volts and amps on the charger, volts and mAh on the battery pack.

How do I correctly transform these into usable parameters for choosing the power supply (volts, amps, peak amps)?

Of course, I want the power supply to:

  • fit inside the battery pack;
  • be as efficient as possible;
  • optionally / ideally, the power supply should weigh approximately as much as the cells (for comfort while using the tool);
  • deliver enough power to the tool.
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    \$\begingroup\$ Cheaper, better, easier to use, less dangerous and more reliable to buy new batteries. \$\endgroup\$
    – Andy aka
    Commented Feb 15, 2023 at 11:17
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    \$\begingroup\$ You need same voltage and peak amps that read on the label. If you don't have peak amps then you don't know what supply you need. Capacity is irrelevant. The supply must be able to provide huge startup and stall current without shutting down due to overcurrent. I'd estimate tens of amps. The required mains power supply that can deliver the required power is likely larger in size than the original batteries, so, there's likely no way to do it the way you would like to do it. The original plastic case for batteries may not be classified/rated for mains voltage either. \$\endgroup\$
    – Justme
    Commented Feb 15, 2023 at 11:59
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    \$\begingroup\$ @Andyaka: I would not mind doing that, but I am afraid that any batteries I would buy would be 1-time-use batteries, simply because I use the tool rarely. Having a weekly / monthly alarm in my calendar to recharge the said batteries is not very high on my list of priorities. That is why I thought it could be more comfortable to use a power supply instead. But your point is a good one too. \$\endgroup\$
    – virolino
    Commented Feb 15, 2023 at 13:44
  • \$\begingroup\$ @Justme: that is what i was afraid of too - that I would actually need to find a statement about peak current :( Good point about the rating of the plastic case. I fear more about heat than voltage, in this particular case - but they are both relevant. \$\endgroup\$
    – virolino
    Commented Feb 15, 2023 at 13:46
  • \$\begingroup\$ I run a cordless screwdriver off of my bench supply but I had to put a halogen lamp in series to deal with the current surges. As for why not just replace the batteries, the problem I had was I rarely use this screwdriver, so the batteries kept going bad from sitting so long and it wasn't worth replacing them for the little use it gets. Took batteries out of a pack, attached a cable, connect it up with the lamp, works great. \$\endgroup\$
    – GodJihyo
    Commented Feb 15, 2023 at 15:01

4 Answers 4

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the voltage is easy, you should be able to use the voltage of a fully charged battery, for example around 14.4V for the 12V tool. The problem is, that battery tools can draw a lot of current from their batteries. Batteries are well suited to these short current spikes, however a power supply that can handle these currents will be very expensive and bulky. The price of a new tool will likely be cheaper.

I would recommend to look into replacing the cells on these tools. This is a much easier task and a nice skill to have.

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This is possible, but you'll find that a 18V battery pack can deliver pretty high peak current and power, around 20-30A and 400-600W.

Therefore you'd need a rather beefy power supply. For the 12V tool you could use a recycled PC ATX power supply which can be found in a dumpster, but for the 18V tool this is not the case.

If you use a power supply that can't deliver the required current, it will shut down when you need torque.

It will cost more than new batteries, and it will be bulky and impractical.

Basically, if you want a corded driver, there's the Skil Energy which is cheap. Unlike a corded drill, it has proper trigger control so it's possible to drive screws with it. It's not super high quality though, the EMC filter caps on mine had a tendency to fall off the PCB when the drill was dropped, so in the end I got some Makitas.

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  • \$\begingroup\$ You could always put a boost converter from ATX 12V to 18V. However, I highly doubt that ATX power supplies can handle a load such as 12V power tool directly. ATX supplies are extremely poor as generic power supplies, as witnessed by several questions here asking why their supply keeps shutting down when used as a lab supply. \$\endgroup\$
    – Justme
    Commented Feb 15, 2023 at 11:53
  • \$\begingroup\$ @Justme: I also have in mind some downsides regarding the ATX power supply. They would not be my first choice for this particular application. \$\endgroup\$
    – virolino
    Commented Feb 15, 2023 at 12:38
  • \$\begingroup\$ Indeed ATX is not the ideal form factor to attach to one's belt to use a power tool... \$\endgroup\$
    – bobflux
    Commented Feb 15, 2023 at 16:40
  • \$\begingroup\$ @Justme it's very easy to bypass the ATX safety system that shut down when it draws too much current (without any problem on my side, it just require to short a pin of a chip (3 minute solder job), there is a video showing this, I also put a switch on the wire I soldered to only bypasses safety when needed ― eg. inflate tires with it draws >30A) \$\endgroup\$
    – JinSnow
    Commented Dec 8 at 19:28
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    \$\begingroup\$ @JinSnow I am not talking about overcurrent safety. The supplies are supposed to be used with loads that adhere to ATX specifications, and they are not required to operate with loads that do not adhere to the specifications. This includes the load step size, slew rate of how quickly the load is applied or removed, etc. \$\endgroup\$
    – Justme
    Commented Dec 8 at 20:59
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An answer you won't like: sell your cordless power drills and buy corded ones.

Pros:

  • They'll work much better, typically being 2x~4x the power of your cordless ones.
  • You don't need to build anything
  • You will save money

Cons:

  • You lose the opportunity to learn and to say I did it.
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For the 12V drill the most useful cheap solution is to run a long fairly thick lead, and fit a couple of chunky crocodile clips. Then attach to a car battery and use for automotive work.

I did that to a (formerly NiCd) 12V drill and it was quite handy for car jobs when it wasn't easy to run mains to where I was parked.

The fully charged voltages of different 12V-nominal chemistries are close enough given the tolerance of motors and their controllers.

Otherwise, it's only worth doing if you can get a scrap power supply for free. For "18V" the very biggest 19V or 20V laptop supplies (from 10-20 years ago when laptops had bigger batteries and higher power consumption - I've seen some capable of 8A from high-end machines) would be worth a shot, though they still probably wouldn't last forever especially if you make a habit of stalling the drill. Some 18V drills are too beefy even for those. For the "12V" drill I'd try to find a 15V power supply. But I keep power supplies from things I scrap, for odd use cases like this. An ammeter in series would be nice for testing; for "12V" you could try my car battery suggestion with an ammeter to size your power supply.

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