Why does it spark at one of the terminals of a rechargeable battery when the 3 V motor is connected as shown below?
It's burning hot to the touch, with the motor connected to the battery only for a few seconds.
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2\$\begingroup\$ For the same reason you get molten metal and sparks at the end of a welding electrode - there's a high resistance path (poor connection) between one of the circuit elements and another.. Also, if that's an 18650 (4.2v) connected to a motor intended for just 3v, you're passing waaay too much current through it and making your job of ensuring adequate connections even harder. \$\endgroup\$– enhzflepCommented Feb 26, 2022 at 14:30
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\$\begingroup\$ @enhzflep I believe that the battery is rated at 3.6 V ( the ampere rating is unknown) because the charger for it can produce 4.2 V and 0.5 A. \$\endgroup\$– ShobehCommented Feb 26, 2022 at 15:46
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\$\begingroup\$ 3.6v is the nominal voltage. Fresh from the charger, that type of cell should be 4.2v. If you can run the same motor from 2x alkaline/carbon-zinc AA cells in series and you no longer find it to be too hot, your lithium battery's voltage is higher than the motor wants. One could reasonably assume the cell to be between 1.0 and 2.6 amp/hours. Running the battery flat then measuring the time required to charge it could give you a rough idea of it's capacity. \$\endgroup\$– enhzflepCommented Feb 26, 2022 at 15:54
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\$\begingroup\$ @enhzflep Ah..I see. My battery must be too powerful for the 3 V toy DC motor's tastes. But when I connect the 3 V toy DC motor to the 4.2 V charger itself, it runs smoothly and had no sparks. \$\endgroup\$– ShobehCommented Feb 26, 2022 at 17:18
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3\$\begingroup\$ That's because the battery is a much stronger current-source. The charger can produce just 0.5A while I'd expect a short-circuit current from an 18650 of about 10A. Because the charger cannot source very much current, if you were to measure the voltage produced while running the motor, it wouldn't be the 4.2v the charger puts out, but would instead sag or drop to a lower level. Doing the same test with the battery, you're not likely to see a change in the voltage when the motor is applied or removed. Consider a car battery VS 10x 1.2v NiCd AA cell. Same voltage, big difference in current. \$\endgroup\$– enhzflepCommented Feb 26, 2022 at 19:28
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