I intended to make a simple electromagnet, then I saw this video where the user is connecting wires together with no resistance at all. I was wondering why it doesn't short out and make the battery dead.

Does this 9v Hi-Watt battery have internal resistance that will prevent shorting out?

If so what will be the current flowing through the battery and how long the battery can last powering the electromagnet, since capacity (like 1000mAh) is not mentioned on it?

  • 4
    \$\begingroup\$ You can probably short circuit that battery for a while. Alkaline and carbon-zinc batteries can typically be shorted for a while without problems. Do not short rechargeable batteries. If the battery gets hot, disconnect it. \$\endgroup\$
    – user57037
    Commented Aug 4, 2017 at 5:57
  • \$\begingroup\$ Those 9V batteries tend to have particularly high internal resistance - they're usually 6x 1.5V cells in series. \$\endgroup\$
    – Bob
    Commented Aug 4, 2017 at 9:45
  • 2
    \$\begingroup\$ Only small batteries huh? I was about to come in here with an anecdote about dropping a spanner across two battery terminals and how the spanner get welded to the posts. \$\endgroup\$
    – Sidney
    Commented Aug 4, 2017 at 14:46
  • \$\begingroup\$ A more efficient design would probably involve a finer wire with more turns. More turns gives you more magnetic flux and thus a stronger magnetic field for less current. \$\endgroup\$
    – Phil Frost
    Commented Aug 4, 2017 at 15:00
  • 3
    \$\begingroup\$ @JimmyJames: 9V + Jeans pocket + Pennies = Ants in your pants dance. \$\endgroup\$ Commented Aug 4, 2017 at 15:38

5 Answers 5


WARNING: DON'T USE LITHIUM BATTERIES, or any type of laptop battery or phone battery. When connected to a small coil of wire, they may catch fire or explode. Even an alkaline battery will heat up dangerously after one or two minutes.

Yes, you can briefly short out a small alkaline battery or carbon-zinc battery without destroying it or causing a fire. But the battery runs down very fast, and heats up quickly. Hint: use a push-button rather than a toggle switch. That way the battery is turned off when you're not holding down the button.

In his video, he really should be using an AA-cell or perhaps a C-cell. He'd still get about the same current, and perhaps a bit longer lifetime for each battery. It's much cheaper to run down a bunch of C-cells, instead of using expensive 9V batteries.

The coil in his video is a short circuit. A few meters of thin wire will have a resistance of less than an ohm. One ohm of #30 gauge wire gives three meters, one ohm of #26 gauge wire gives 7.5 meters (His wire appears to be thicker than #30 gauge.)

The Amazon.com link, those batteries would work fine. Also try AA cell or C cell, plus a plastic battery-holder for those size.

Also see: small coils projects, motor and generator

This one: high speed generator, powered by bow-drill


Here is a simple way to look at it. Batteries have internal resistance. When you short circuit a battery externally, the current will be limited by the internal resistance. If the internal resistance is high, the short circuit current will not be too large.

Older, non-rechargeable batteries tend to have high internal resistance. The carbon-zinc battery you linked to will probably have high internal resistance.

Even small lead, NiMH and lithium ion/polymer batteries have very low internal resistance. If you short one out you may get a giant spark and have lots of heat. So don't short any of these types of batteries!

One other thing. The wire of the electromagnet does have some resistance. So the short circuit current will be somewhat limited.

  • 2
    \$\begingroup\$ Don't forget lead-acid in the low internal resistance list (I've seen the little 10Ah burglar alarm/UPS types used as bench sources when care is needed) \$\endgroup\$
    – Chris H
    Commented Aug 4, 2017 at 9:44
  • 4
    \$\begingroup\$ Whatever you do, DON'T TRY THIS WITH A CAR BATTERY!!!! A fully charged 12V car battery can easily supply 1000 amps or more when short circuited. If you accidentally drop a screwdriver or a wrench across the terminals, you will have a VERY HOT wrench within a few seconds, and apart from getting severely burned, the sparks you create while removing the short will probably ignite the hydrogen gas being produced by the excessive discharge current - which will create an impressively loud bang, and possibly cause some much more serious damage or injury. \$\endgroup\$
    – alephzero
    Commented Aug 4, 2017 at 14:21
  • 5
    \$\begingroup\$ Also, NEVER take any risks with shorting a battery while wearing a watch with a metal strap, or a metal bracelet etc. If the short-circuit current goes through that metal, you may need surgery to remove the metal from your severely burned wrist, followed by a long stay in the hospital's burns treatment unit. \$\endgroup\$
    – alephzero
    Commented Aug 4, 2017 at 14:27

This very much depends on battery type and rating. Also, keep in mind that a coil of wire still has resistance, and is not a real short-circuit. Could very well be some batteries are able to deliver that much current.

Every battery has internal resistance - how much that resistance is depends on a lot of things (type of battery, age of battery, construction, how much charge is still in the battery, temperature, etc). This internal resistance can be more than sufficient to protect smaller batteries like (certain) coincells and certain AA-style cells from damage when shorted.

However, other types of batteries like Lithium-based ones (especially the rechargable types) can have internal resistances in the few milli-ohm range - shorting them out can (and usually will) damage or even destroy the battery.

You can try and calculate the resistance of the coil you will use (or measure it, though that is not straigh forward given the usually quite low impedances involved) and see if your battery is rated to deliver that much current.


how long the battery can last powering the electromagnet?

First, you need to determine the current. If you know the internal resistance, the current will be I=Vbatt/(Rint+Rcoil), otherwise you can just measure it.

Knowing the battery capacity, you can get an estimation of the time it will last as t=Q/I, where Q is the capacity of the battery. Understand that this will be just a rough estimation, because (a) capacity depends on discharge current in most chemistries and (b) the current will not be constant as the battery discharges. If you don't know the capacity, the only way to know is to measure the time.


"No resistance at all" ??? As far as I know, that only happens when everything is chilled in liquid helium, and batteries might not work at such low temperatures. So you have to start the current at room temperature and chill only the electromagnet. If the core of the electromagnet is the right stuff, you don't care if the battery goes dead; the magnetism will remain as long as the liquid helium bath does.

As for the battery going dead, I am sure that it will eventually go dead. If it does not, please contact me and I will buy that battery from you.


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