I accidentally shorted three 18650, 3700 mAh cells connected in series for about 2 seconds. I measured each cell individually with a voltmeter and they read very close to its original rated voltage of 3.7 V. However, when I tried to use the batteries to drive a DC motor, it doesn't work. How do I tell if the 18650 cells are dead? If the voltage between each cell is close to 3.7, can I say that it is still well and alive?
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5\$\begingroup\$ 18650s that say 3700mAh are very likely much smaller capacity. A 2 second short is not likely to have destroyed them although it is likely more than their rated current flowed, aging them somewhat. Did the batteries work to drive the motor before? Do they have short circuit protection? Are you sure the polarity is correct if the motor is unidirectional? \$\endgroup\$– K HCommented Apr 6, 2021 at 7:19
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2\$\begingroup\$ Most probably fine. You need to measure IR, capacity and leakage to tell for sure. \$\endgroup\$– winnyCommented Apr 6, 2021 at 7:22
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\$\begingroup\$ I did the same with 20 Amp drain one for about 6 second. They become warm maybe 70degC, the wire also. I didnt notice almost any change in voltage, still about 4V, works fine. Did you measure the voltage under motor loading? \$\endgroup\$– user208862Commented Apr 6, 2021 at 7:40
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\$\begingroup\$ @KH Yes before the short, the motors were driven. Not sure if they have short circuit protection. Also, forgot to mention that some smoke were present at the wire area i believe. This is probably due to the heat produced at the tip of the wires instead of the battery component burning right? If i charge the battery, it should work fine I believe? \$\endgroup\$– Ong Chee WeiCommented Apr 6, 2021 at 7:50
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\$\begingroup\$ Have you charged the batteries since shorting them? \$\endgroup\$– K HCommented Apr 6, 2021 at 7:52
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2 Answers
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- I suppose its possible that you half damaged the internal fuse (if it has one). A lot of 18650 have an automatic resetting fuse. If its half damaged, it may now trip very easily when pulling even medium current. I have never heard of this happening, just brainstorming.
- Another option is that your motor load is too big. It worked before, but it was just on the edge of tripping the internal 18650 fuse. Now that the batteries are slightly lower voltage, the current is slightly higher and it cant handle it. If this is true, charge the battery and it will work again, though, you are still pulling more current than you should. You can add a fat capacitor, even a super capacitor if the high load is just a spike.
- Something else broke. wiring, circuit board, etc.
PS: I know this question is old, but people still read old questions.
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To answer your questions specifically about testing the batteries:
You should measure the voltage under a reasonable load. Maybe around 1A.
Measure the open circuit voltage. Now connect a power resistor of say 3 ohms and measure the voltage again.
With a little math you can determine the ESR of the battery. It should be very low, certainly less that 500mOhms.
To calculate ESR, simply model the battery as an ideal battery with a resistor in series.
Lets say your open circuit voltage is 4.1V. And with a 3ohm power resistor it's 3.7V.
3.7V / 3ohms = 1.23A
0.4V drop / 1.23A = 0.325ohms (325 mOhms).
As lithium ion batteries get older the ESR tends to go up. I like to use the aged ones for lower draw applications like flashlights.
