I am working on a charging device for a smartphone, it's for a science project, Currently I finished planning all the electronic parts for it and I am getting ready to start building, however I noticed something, after calculations I found out that my new charger will give an output of 5v and 0~0.02 amp, now my question is: will this electricity of 5v and 0.02 amp enough to start charging the phone? I don't really care at this moment about the charging speed, because when presenting it I will just need to show that it starts charging my device, I won't be demonstrating the charging speed, so even if it takes a year to charge that ain't my concern right now. My concern is: will it trigger the phone to charge or not?

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    \$\begingroup\$ Limit a bench power supply to 20mA at 5V and try it out. Probably varies wildly depending on the phone. \$\endgroup\$
    – Wesley Lee
    Nov 20, 2017 at 17:15
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    \$\begingroup\$ In order to charge the battery, your supply needs to deliver enough current to operate the phone AND have some left over to put into the battery. Will 20mA be enough? I'd say it's unlikely. \$\endgroup\$
    – Finbarr
    Nov 20, 2017 at 17:25
  • \$\begingroup\$ @Finbarr Is it always the case? I remember some crappy car charger I had. It allowed the phone to run for a bit longer, while the battery was still drained. And I suspect it killed my battery after all... \$\endgroup\$
    – Eugene Sh.
    Nov 20, 2017 at 17:28
  • \$\begingroup\$ @EugeneSh. Providing a bit of extra power to reduce the drain on the battery is not the same as charging it. The question asks about charging the battery and that can only happen if the phone itself is running from the external supply. \$\endgroup\$
    – Finbarr
    Nov 20, 2017 at 17:31
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    \$\begingroup\$ I doubt that the phone will turn on the charging LED or screen symbol unless it is actually charging the battery. It's not just a power indicator. \$\endgroup\$
    – Finbarr
    Nov 20, 2017 at 17:38

2 Answers 2


Most likely it will cause a problem with certain phones. To get around this output current limitation, some "energy harvesting" devices like yours will instead charge an on-board battery at whatever current your source puts out. Then, when the battery is sufficiently charged, you turn on the charging circuit to the phone.

If you want to learn more about this type of circuit, many manufacturers have developed integrated circuits that take care of a large portion of the difficult part of the circuit. Usually they go over the theory of operation, and you can learn quite a bit just by reading the datasheets.

Here is a link to an application note involving what I described above:


If you want to find more information, google the terms "energy harvesting".


I think you need to plug it into a bench source with the same voltage/current and see.

The power management in modern phones/computers is a bit more complex than you might think. In olden times, if you had a battery connected to a light, and added a photocell across it at the same voltage, really all of the current would go to work. Same for a rechargeable battery.

Nowadays, there is a whole science of the best way to charge and discharge a battery. And it is all digitally controlled. So, for instance, the phone may 'reject' too little current, even though it seems all current would help. Possibly, it determines that more energy gets lost using the extra quantum, or it wants the input to be well/properly conditioned, or the battery to be in a given state, or whatever.


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