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 ?

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    \$\begingroup\$ It depends on the drill. What are the specifications for the drill? \$\endgroup\$ Oct 18, 2018 at 20:08
  • \$\begingroup\$ Why don’t you measure it? \$\endgroup\$
    – winny
    Oct 18, 2018 at 20:26
  • \$\begingroup\$ @winny Presumably because he doesn't have an 18V supply yet? \$\endgroup\$
    – Finbarr
    Oct 18, 2018 at 20:37
  • \$\begingroup\$ Well, if your input power can't go higher than 12V/20A, that means your output can't be more than 240 watts, so the 400W boost converter would be a waste unless you get a higher-power power supply. \$\endgroup\$
    – Hearth
    Oct 18, 2018 at 20:44
  • \$\begingroup\$ Wouldn't it be just as much money (or less) to get an 18V supply than to get a converter? Why the converter? \$\endgroup\$
    – Reinderien
    Oct 18, 2018 at 21:07

2 Answers 2


how many amps does an 18v cordless drill draw

Current drawn by a brushed DC motor depends on torque and rpm.

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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.

  • \$\begingroup\$ What do you think if i feed my drill from 12v, although I will get a little less power , \$\endgroup\$
    – aymane
    Oct 18, 2018 at 23:56
  • \$\begingroup\$ @aymane, very likely it will not work. As peufeu said, the starting condition is "stall", and the stall impedance is very low. I just tested a no-name 14.4V drill, the impedance is about 0.5 Ohms. Which will take 24 A to start, and your PSU will likely shut itself down, or will be intermittent at least. You have the drill, you have the PSU, why don't you try and tell us the result? \$\endgroup\$ Oct 19, 2018 at 0:09
  • \$\begingroup\$ @Ale..chenski indeed, you are correct. These motors require very high current. \$\endgroup\$
    – bobflux
    Oct 19, 2018 at 0:19
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    \$\begingroup\$ Can you please provide attribution for the graphic you used? \$\endgroup\$ Oct 19, 2018 at 1:11
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    \$\begingroup\$ @ElliotAlderson sure, I took it from another question which doesn't provide attribution ;) \$\endgroup\$
    – bobflux
    Oct 19, 2018 at 14:18

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.

  • \$\begingroup\$ Also try to follow "related" links on the right side of SE EE page for your question, like electronics.stackexchange.com/q/12360/117785 and electronics.stackexchange.com/q/209213/117785. \$\endgroup\$ Oct 18, 2018 at 21:39
  • \$\begingroup\$ I found this boost converter link , what will be the output current? , because I haven't understood this sentence in the description: Max. Output Current: 12A (related to the input and output voltage difference, the greater voltage difference is, the smaller output current will be \$\endgroup\$
    – aymane
    Oct 18, 2018 at 21:47
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    \$\begingroup\$ @aymane, it means that you can't get more than 12 A no matter what. To get 12 A output current you need to have close ratio of input:output (12V into 19V fits this category). More, you will need a strong air fan to cool this device. My feeling is that it won't work well: the drill will start to spin (even this is "maybe", due to start current), but under any load the whole power will collapse, with some good probability into smoke. You might get better results just by feeding your drill from 12V PSU directly, and finding a way to tune it up for higher voltage. \$\endgroup\$ Oct 18, 2018 at 22:27
  • \$\begingroup\$ Ok, i will feed my drill from 12v, although I will get a little less power \$\endgroup\$
    – aymane
    Oct 18, 2018 at 22:45

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