First I am sorry if it is a duplicate of https://electronics.stackexchange.com/a/151157/207677

I have this two cards:

I would like to know if I can plug the output of the battery to USB charging output and also the input of the step-up.


simulate this circuit – Schematic created using CircuitLab

This is for a small mp3 player for my daughter. Thanks in advance for your help from me and my daughter.


Yes but not ideal as it could age the battery much faster. If the charger stays on because the MP3 player draws near or more than the 100mA level where the charger is supposed to shut off then the automatic charger stays on. If the charger had more "smarts" with a timer as a safeguard to limit the duration at 4.2V, that would be best.

- details -

Another possibility is if the charger was modified to reach only 3.9V rather than 4.2V this might reduce the battery capacity but then avoid running out of "juice" and the rapid aging of the battery by sustaining the time the battery stays warm at 4.2V.

This is what the battery should do alone with the charger.

enter image description here

Consider if the MP3 player draws slightly more than 100 mA (green) or 10% which is the cutoff threshold. Then the battery stays at 4.2V and ages much faster, and when near the end of life it means that it only works while the charger is connected. enter image description here

The Magenta color I estimated the reduced battery charge current available but stretched for a longer time. If the MP3 uses less current than 100mA, the charger should shut off OK and the battery returns to a normal 3.9V towards 3.7V.

But if the MP3 player uses more than 100mA as I showed above then the battery stays at 4.2V and more rapidly ages the battery with overvoltage.

Is it safe? It should be, but there can be exceptions.

A failing Li-ion begins to hiss, bulge and leak electrolyte. For your education read here.

  • \$\begingroup\$ I will probably never charge and use the mp3 at the same time. So it should be ok \$\endgroup\$ – Mio Dec 20 '18 at 10:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.