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I've got a flashlight with a 3.7 V 18650 cell that charges with a 3.5 mm barrel plug. They provided adapter that says 4.2 V +/- 0.5. Can I use a 5 V USB to barrel plug adapter to charge it?

UPDATE So the adapter clearly says +- .5 v. Is this an obvious misprint. It must be .05 right?

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Check the current rating on the existing power adapter. The amperage it draws should not be greater than the source you are considering to use with the 5v usb to barrel plug adapter. If, for example, your existing adapter lists 300 ma and your available source is 1000 ma, you will not overload that new source. If the existing source lists 1.2 a or 1200 ma and your new source is 1000 ma, it will overheat and likely fail, possibly in a dangerous manner.

It is typical of these small units to have charging times measured in hours, implying low current draw.

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It must be .05 right?

On a 4.2 V PSU that would be 1.2% accuracy. For a cheap power supply that would be unlikely.

Lithium-based cells require careful charging and discharging. I suspect that the power supply designers used the following criteria:

  • Cheap.
  • Cheap.
  • Must charge the battery - even if it takes a long time.
  • Mustn't cause too many to catch fire due to overcharging.

Most devices such as mobile phones use a 5 V power supply (not a "charger") to power the device and the charger or charge management system is built into the phone. It will limit the charge voltage and current to safe values and terminate charging when "full".

I would be very careful with this. Check out Battery University for a wealth of information on all types of batteries.

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I think maybe, definitely NOT do this. Measurement shows the provided adapter/charge controller ranges 4.0 to 4.3 v and current at 200-220 mA.

Plugging into an un charge controlled 5 v 1A power supply immediately drives the current to 1.1 A.

Certainly leading to overheating, possibly venting and leaking, venting leaking and lithium fire, rapid venting, explosion, and then fire. Pick your poison.

UPDATE

Although the adapter is rated at 500 mA; when connected to a 1 A USB load tester it supplies only 250ma.

So, someone tell me, where is the charge controller, in the flashlight or in the wal wart?

FINAL HOPEFUL UPDATE

In a disturbing development after charging at 4 v for 180 mAH the voltage across it dropped to 3.0 and it started getting hot, AND the flashlight is only intermittently shining any light at all.

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  • $\begingroup$ Andy, this seems to be an update to your question rather than an answer. Can you copy the text into your question and delete this to save the mods a job? $\endgroup$ – Transistor Feb 7 '20 at 17:40

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