I have bought an old 18v black and decker cordless drill but with broken batteries, for that, I want to buy a boost converter to convert 12v (max 20A ) from a computer power supply to 18v and use this drill as corded drill, but I need to know, how many amps does an 18v cordless drill draw to choose which boost converter to buy 200w 300w 400w ?
how many amps does an 18v cordless drill draw
Current drawn by a brushed DC motor depends on torque and rpm.
Maximum current and maximum torque occur at 0 rpm, when the motor is stalled. Stall current depends only on motor resistance, which you can measure with a multimeter on ohm-meter mode. Measure the resistance of your motor, and you will know its maximum stall current.
Most likely this stall current will be very high, maybe like 30-50 Amps. It will be alright for a battery, but it will be trouble for a mains-powered power supply.
If the power supply cannot deliver this current, then the drill will not work well, it will stall instead of powering through the job.
Besides, boost converters suck at high currents. A 12V to 18V converter for 50A current would be an interesting design. Probably multiphase. Please do not believe the ebay specs. I've talked with a guy who bought some Chinese boost cnverters on ebay, and of course the capacitors on them exploded when he tried to use them at rated power. So, when you say:
A boost converter cost about 6$ and the power supply 25$
The converter will cost $6 and it will explode. Resistive losses are proportional to I^2 which means 50 Amps is 100x more trouble than 5 Amps.
Your problem is that a power supply which can deliver this kind of current without blowing up costs more than a brand new drill with brand new battery.
You can rebuild your power tool battery with new cells, or you can buy a new power tool. These are the only options that make sense.
Cordless power tools operate in a variety of load conditions, so the current consumption varies with load and tool's original quality.
According to this exchange, "Typical current draw on 18V cordless tools?", a good 18-20V cordless tool consumes about 500W (~ 30 A) of power under peak load. To compare, corded power tools are rated with typical "8A at 110V", or 1000 W, and since cordless tools are typically half of power of corded ones, these numbers sound about right. A cheap B&D drill with old-style NiCd battery pack might have just a 300W DC motor.
So the bigger converter is better, and I am afraid that the computer-grade PSU might be a little weak for the job. And you will need to use some flexible AWG10 or AWG12 "silicon" wires to get this current down to the drill.