How to Build an Emergency Phone Charger

If things haven't changed much and I remember correctly, a lot of portable drills use universal motors. These are motors that are called universal because they can run off of both AC and DC power, but are still used in DC only setups like portable tools because they are cheap to make and have some other nice properties. The unfortunate problem is that the field coils require power for it to function as an efficient motor/generator.

A generator is basically a spinning coil in a magnetic field, so that the amount of the magnetic field going through the coil changes with time, ultimately driving a current. A motor does the opposite, using the a current to force the loop to change its position in the magnetic field. Hence, in either case an external magnetic field is needed. For motors, you have the added option of using the incoming electric current to make an electromagnet to create that field. So when the current goes away, there is no field. If you try turning the rotor, without a field, it won't do anything and can't make a generator out of it...

However, in the real world, usually turning the current off leaves a residual magnetic field because of the iron used. This is enough to jump start the process in some generators (others use a battery to help provide an initial field, or use permanent magnets). So you can get it to act like a generator a bit, but depending on how it is wired, and what is connected to, it may produce far less power than a purpose built generator. Universal motors for example, could be particular bad depending on what you hook it up to, as field coils are in series connection, so whatever current is flowing out would be the current to run that needed electromagnet. If the output draws not much current, then the generator would be rather inefficient compared so something that uses the same amount of current as the drill does when running.

tl; dr: That kind of motor makes a really inefficient generator. I would almost be curious if it would be easier to assemble a more efficient one yourself from wire, scraps of metal and permanent magnets for the same amount of effort used here.

That will likely last for only a few minutes. Running simple DC motors in reverse will generate electricity. Modern chargers and DC motors have circuitry that prevents bad things from happening and that circuitry fails if the current flow is too high or gets too hot.

I have tried this sort of thing in the past and have ruined a few items with DC motors in the process.

Should have attached the drill to a stationery bike instead.