I bought one of these low voltage halogen replacement LED bulbs. They can run on AC or DC and at different voltages. It can even run off a 9 volt battery, but drawing 1 ampere.
Connected to a 20 volt DeWalt battery, it draws just half an ampere. So there appears to be some sort of regulator in the bulb that compensates for the different voltages. So I'm pretty sure it's safe to run off the 20 volt tool battery.
I wanted to make something to hook this bulb up to the battery, and for that I needed something that clips onto the battery. These red guards that the batteries ship with are just right for that. They even have holes right above the contacts for the power.
I stuck some nails in the holes, but measuring the voltage, I just get a voltage that slowly drops. Examining it, I realized the red clip has two tabs that push into the battery, and that probably disconnects the cells from the rest of the circuitry.
So I just cut the tabs off on the bandsaw.
I then put two thin nails through a piece of plywood at the right spacing, so the plywood sits on top of the clip.
I wanted to attach the bulb to the plywood. I drew the outline of the bulb on the plywood, then cut that out on the bandsaw.
I also made another block of wood to fit around the base of the bulb. This block screws onto the plywood.
But before screwing it on, I rounded all the edges using my belt sander and strip sander.
Then wiring the bulb to the nails with some telephone wire, just twisted on.
With the block that holds the bulb in place, I could call it done.
But I'm afraid of dropping some tool onto it and accidentally shorting out the battery.
So I made this extra block of wood that fits over the nails and the wires and is held in place with a screw.
Of course, you could save all the trouble and just buy one of these DeWalt work lights that clips onto the battery, but these work lights are kind of expensive for what they are, and the LED bulbs cost less than $10.
It would be cool if DeWalt (or someone else) made a similar small light that is small and cheap and just slips onto the battery, just to always keep in a tool box for when it's needed.
And here's comparing the light output. The LED bulb is an 8 watt bulb, illuminating the spot on the left. The DeWalt work light is illuminating the spot on the right. The DeWalt work light has a much wider beam, so I put it much closer to the wall to get a better comparison. Overall, I think the LED bulb puts out a bit more light, though I'm sure it also uses more power.
The reason I built this thing is that I use the DeWalt work lights to fill in some dark areas here and there while filming, and having another similar light could be handy. The more focused beam will be better some of the time, but less useful at other times.
A note of caution When I built this I assumed the battery pack would have under-voltage protection built in, because the cheap 12-volt Ryobi battery packs I have will automatically shut off on under-voltage. But DeWalt tools have under-voltage cut-off in the tool itself, so the batteries don't need to. I imagine this is also the case for most larger cordless tools. So don't plug one of these in and leave it on overnight.