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I made a simple electromagnet with a screw and a wire.

When I supply power to it via 1 AA battery (1.5v) and change + and - contacts it acts differently with other magnets (attracts and repels), but when I supply power via my 250mAh, 3.7V Li-pol accumulator, it only attracts other magnets, even if I change the contacts' polarity.

Can I make my electromagnet repel other magnets with Li-pol accumulators supply?

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    \$\begingroup\$ DC is DC; if you reverse the polarity, the rotational sense of the magnetic field reverses. That's a fundamental truth of physics; it very literally is what causes light to exist. I trust that truth 100%. I don't trust you to not have mixed up your magnets, or your contacts, sorry :( \$\endgroup\$
    – Marcus Müller
    Commented Nov 11, 2021 at 23:32
  • \$\begingroup\$ Ok, thanks for the comment, I will make another test tomorrow. Maybe I'm really messed up with contacts. \$\endgroup\$
    – DSP_engineer
    Commented Nov 12, 2021 at 0:03
  • \$\begingroup\$ Sorry, what is a li-pol accumulator? \$\endgroup\$
    – Hearth
    Commented Nov 12, 2021 at 1:07
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    \$\begingroup\$ In some countries across the pond, they translate a Li-PO battery into Li-pol and translate a rechargeable battery into accumulator. \$\endgroup\$
    – Audioguru
    Commented Nov 12, 2021 at 1:16
  • \$\begingroup\$ it only attracts other magnets ... remove power ... it will still attract magnets \$\endgroup\$
    – jsotola
    Commented Nov 12, 2021 at 1:32

1 Answer 1

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A few turns of wire around a screw would short out a battery like this. What's happening is that the Li-po is shutting off using its internal protection circuit.

Let's compare the two batteries when we connect a nearly dead-short load, like your 'few turns':

  • AA battery: battery internal resistance limits the current, energizes the magnet
  • Li-po battery: protection circuit detects a short, cuts off the battery, magnet does not energize

Non-energized electro magnet will still attract magnets, but won’t repel them.

Why does the Li-po have a protection circuit? Li-po and Li-ion batteries have very low internal resistance. Without protection, when shorted they will heat up very quickly and even catch fire.

Fortunately for you the protection circuit did its job, and you only had a non-functional magnet vs. a fiery YouTube-worthy disaster.

In future, never connect a dead short across Li-po or Li-ion battery. Add a series resistance to limit the current.

Related: Why is there so much fear surrounding LiPo batteries?

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  • \$\begingroup\$ Thank you for the comment. Here is a video of what I finally succed to do with AA battery. youtu.be/ScmpNUSmIjw First time I thought that the reason was that the coil wiring was uneven on a screw driver and it messed up the electromagnetic fields, but your explanation looks more reasonable. \$\endgroup\$
    – DSP_engineer
    Commented Nov 13, 2021 at 7:57

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