I know this might be a noob question, which I am new to this, but people always told me that you need the correct voltage and amps to power something. In this case, it is a small dc motor I got from radioshack. It can run at 9v,12v, or 18v. It runs at max at 1.9A and the battery supplies only 500mA of power. Am I defying physics here or something? I can't find the answer on google. Here is the dc motor I am using.


How long will the battery last powering this motor?

Thanks for your time and effort explaining to me.

  • 1
    \$\begingroup\$ 1.9V is the maximum, or stall current. It will run slower and/or with less torque than otherwise. \$\endgroup\$ – pjc50 Feb 10 '17 at 22:54
  • \$\begingroup\$ How about a decent data sheet for the motor that states what the no-load current might be? \$\endgroup\$ – Andy aka Feb 10 '17 at 22:56

Batteries actually tend to give much more current, unless it is a very small battery. For your motor you need a battery that can supply 9v or more, to know the time it will work you need to know the current drawn from the motor in load conditions and the mAh rating of your battery, lets say your motor draws 2A and you battery has a capacity of 4000mAh, you divide the capacity with the current 4Ah/2A which gives 2 hours of continuous function.

  • \$\begingroup\$ I looked up the 9v battery amps, it said 500 mAh and yet my motor needs 1.9A. How much current then, does it really supply? \$\endgroup\$ – Sean.D Feb 10 '17 at 23:25
  • \$\begingroup\$ Depends on the type of battery, but for sure it can supply around 2A without damage, try it and see if it doesn't get very hot. If it states 500mAh you should try the calculation above to see how long can it run, test the circuit with your actual load and see cow much current does it require, then divide that current with you 500mAh and you'll get the time it can drive the motor \$\endgroup\$ – Marcelo Espinoza Vargas Feb 11 '17 at 0:03

I recommend you to get a good multimeter and start measuring around - you can learn a lot of stuff by experimenting.

For example, try measuring the short-circuit current if you connect the probes straight to the battery terminals in ammeter (10A) mode. You'll probably see much more than 500 mA, but only briefly. The same goes with other battery types - for a fresh alkaline AA you can briefly get around 10 amps (which is almost 5 times their capacity, around 2000-2200 mAh).

Why is that? First of all, we're comparing apples and oranges; if a cell stores say 2000 mAh, it means it can deliver 2000 mA for one hour, or 200 mA for 10 hours, or 10000 mA for 12 minutes. That's in theory. In practice the chemical reaction inside can't happen that quickly and the current will quickly diminish, while the battery gets very hot and things get quite dangerous. If you're measuring short-circuit currents, please use caution and only do that briefly.

This, combined with what Andy aka said, answers your question. Your motor requires some serious current when it starts, because the rotor needs to have power inserted into it to start turning. Your battery happens to be capable of providing that initial burst. Once the rotor is moving, its inertia keeps it that way and you only need power (much less power, actually) to overcome the friction and other losses, which aren't that large. Again - get a good multimeter and measure the steady-state current, I believe it will be much lower than 1.9A.

  • \$\begingroup\$ I have a multi meter that measures volts, not amps. But ok I understand. \$\endgroup\$ – Sean.D Feb 11 '17 at 0:13
  • \$\begingroup\$ If it is a multi-meter, there must be an "amps" mode :) If it doesn't, do get one that has it - they aren't expensive and you can answer a lot of questions yourself by experimenting. \$\endgroup\$ – anrieff Feb 11 '17 at 0:17
  • \$\begingroup\$ Ya, I just realized it does have an "amps" mode. If I would have known that, I would not have asked this question... Anyways, I measured it and I set it for 10A and it gave 2.7 as an answer. So that is a lot more than 500mAh, lol. \$\endgroup\$ – Sean.D Feb 11 '17 at 0:22
  • \$\begingroup\$ Cool, this is actually more than I expected. It seems for alkalines around 5xC is the instantaneous current you can get. Btw, keep in mind that these 2.7A are only when the battery is fresh. As it drains or ages, that would diminish. \$\endgroup\$ – anrieff Feb 11 '17 at 0:33
  • \$\begingroup\$ Ya, I know it goes down as it gets old. Also, the 9v battery is actually a radio shack battery, not alkaline. I agree, a lot more than I expected too. \$\endgroup\$ – Sean.D Feb 11 '17 at 0:42

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