Well, for me there were a couple of reasons:
1. Dead batteries suck! And, when you're running a circular saw, saw-zall, or pretty much anything else constantly, dead batteries happen very frequently.
2. Batteries are not cheap, and they don't charge fast. Even with 5 batteries and 3 chargers, a good-sized project + my questionable competence at most things one can do with power tools often leads to waiting for dead batteries to recharge.
3. More power:) You've overclocked your computer, so why not overclock your power tools?
4. I *think* this'll be a bit more energy-efficient than recharging and discharging batteries... certainly more earth-friendly if you take into account impact of metal used in extra batteries you won't have to buy...
As usual, this is not the safest or most professional of projects. Please don't hurt yourself any more than you'd like to:-)
Here's a video:
Step 1: Gather Your Supplies
-cordless tool(s) to make work corded, complete with charger. I used these.
-an old ac power cord that you don't mind ruining
-an extension cord to run between charger and tool
-foam or something else that's nonconductive, flexible, and easy to work with. i used a dried-out piece of the nastiness that is known as great stuff.
-your favorite roll of duct tape
-aluminum foil or other conductive, flexible material
And, it'd make life easier to have:
-something to make the corded adaptor easy to remove from the tool. I used these, and they worked well.
-pocket knife, leatherman, or other way of cutting and turning screws
-a pack of spare batteries, like these.
I'm sure you can do this with other tools; I happened to have a set of the cheap-but-usable Ryobi stuff, so that's what I went with.
Step 2: Modify (or Build) the Battery Connector
If your tool set has a carrying accessory that snaps into the battery connector like mine did, this step's easy: just cut a hole through the carrying accessory that's slightly bigger than the cord you want to run through here. If this doesn't exist for your tool, create one out of your favorite nonconductive substance or (carefully) take apart a battery and just use the shell.
I got annoyed by the nylon carrying loop thing, so I cut that off too.
Step 3: Attach the Cord
Next, you'll need to run the end of your AC cord through the hole you just cut in the accessory connector:
1. Push, pull and otherwise force the wire through, with the AC prongs facing the bottom (you'll connect these to the extension cord when you want to run power to the tool). Needlenose pliers were helpful for me on this step.
2. Attach thin strips of duct tape around the cord, right by where the cord enters the accessory connector.
3. Strip the ends of the extension cord's wire.
Step 4: Carve the Connecting Thingie
The tool's leads to connect to the battery are not at the base of the battery connector: they're on the sides, near the top. So, you need something to get your wires up into the tool and touching the tool's battery leads.
I used a dried chunk of Great Stuff, the spray foam that is a pain to remove from pretty much any surface:
1. carve foam chunk such that, when it rests on the accessory connector, it's approximately the same size as the battery pack's lead
2. carve an 'arch' through which the 2 wires can lead to the terminals
3. if the carved foam is too tall, drim down to approximately same height as battery pack
4. run one wire to each side of the arch
5. attach aluminum foil to each wire and wrap the whole thing in duct tape. be sure you leave foil exposed, so that the tools leads can contact the foil on both sides. if (when) the connector won't slide onto your tool, trim off all the extra duct tape you used:)
Step 5: Modify (or Build) the Charger
You'll need something for the other end of the DC-carrying extension cord to attach to, and this something will be an AC outlet connected to the charger.
Easy way to do this: run some aluminum foil or a wire to the point of connection between battery leads and charger. On most modern chargers, you'll need to put a discharged battery in the charger before you can provide any current to the tool. I tried this, and my corded adaptor would stop working whenever the battery became fully charged.
The way I did this: connect to the battery charging terminals, but from inside the charger. This lets me use the charger to recharge a battery while also powering a tool via the corded adaptor, and I don't have anything interfering with the charger connection to the battery. I meaured >10 amps output when a battery was not inserted using this method, so be careful: hooking a tool up to this might turn it / the charger / you into a charred chunk
Yes, both of the aforementioned methods probably increase the likelihood of battery or tool explosions or other nastiness occurring.
Smartest way to do this: measure the current and voltage provided by your battery and create an ac-dc adaptor that'll output this.
Step 6: Rock And/or Roll, Without Recharging!
Check the voltage/amperage output with your multimeter to make sure it's within a range of the intended input to the tool for you to feel safe (note: for me this required keeping a batter recharging while using the corded adaptor. if the battery finishes recharging while i have this connected, that *might* be a problem...), connect an extension cord between dc-carrying ac outlet attached to charger, and plug the other end into your corded cordless adaptor. turn your newly-corded tool on, and rock and roll! if it doesn't work, i'd bet you w/ 10 to 1 odds that your aluminum foil attached to the carved connector thingie isn't making contact with the leads on the tool. Duct tape and foil away until you've got it right, and happy tooling:)