I'm trying to charge my laptop through a 12V battery and 12-19V adapter over a long wire but I've noticed that the adapter makes some noise and doesn't work. On the other hand when I connect it close to the battery it works just fine. Is something wrong with long distance transmitting of 12V DC?

The wire is around .5 mm in diameter and 10 meters long. I believe the current is around 3.5 amps.

  • 1
    \$\begingroup\$ What gauge is the wire, and what is the current you are transferring? \$\endgroup\$
    – Majenko
    Jun 14, 2014 at 17:13
  • \$\begingroup\$ it's .5 mm, 10 meters long and the current is 12V DC \$\endgroup\$ Jun 14, 2014 at 17:28
  • 2
    \$\begingroup\$ No, the VOLTAGE is 12V. What is the CURRENT (AMPS) ? \$\endgroup\$
    – Majenko
    Jun 14, 2014 at 17:30
  • \$\begingroup\$ 3.5 AMPs I guess \$\endgroup\$ Jun 14, 2014 at 17:54

1 Answer 1


The wire is probably 24AWG (0.51mm diameter). At a length of 10m you actually have 20m of wire (there and back again) (20m is 65.6 feet).

24AWG wire has a resistance of 84.1976Ω per 1000m, so 20m should be a resistance of around 1.684Ω.

As you can't (or don't know how to) give a current value I can only give some examples.

At 1A the voltage drop across the wire will be (V=R×I) 1.68V, so the voltage at the far end would be (12-1.68=) 10.32.

At 2A the voltage drop would be double that, so the resultant voltage would be 8.64V.

At your guessed 3.5A the voltage drop would be 5.894V, leaving the 12V at a paltry 6.106V.

And so on.

The maximum recommended current for power transmission on that gauge wire is 0.577A. Any more than that and the self-heating of the wire could overwhelm the integrity of the insulation, and even cause the wire itself to melt.

At 0.577A the voltage drop would be (0.577 / 1.684 =) 0.343V, so the voltage at the remote end would be 11.657V.

The answer? Use thicker wire. The thicker the wire the lower the resistance, and the lower the losses it causes.

For 3.5A the recommended minimum gauge wire is 16AWG (~1.3mm copper diameter).

  • \$\begingroup\$ Thumbs up Dude,, you impressed me with your explanation... big fat thanks! \$\endgroup\$ Jun 14, 2014 at 17:55

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