I'm building my own portable power station, using a 12v lead acid battery as a power source. Currently I've got 1 12v socket and 2 usb ports on it. The idea being to use it to power something like a coolbox with the 12v socket and allow phone charging from the USB sockets.

I'm using a LM2596 step down converter to bring 12v to 5v for the usb ports. My question is how do I modify my circuit to ensure that the phones aren't overcharged if left plugged in.

I've tried looking it up on Google but everything I've found so far only covers the issue theoretically I.e. explaining about the different stages a charger goes through such as constant current charging initially then switching constant voltage charging when the battery hits a certain voltage followed by either shutting off or a trickle charge when full rather than explaining how to do it using components.

I want to understand what it is that makes the charger stop charging when the batteries full and how I can build that into my circuit either with individual components or a part like the LM2956.

Below is an article I found that almost answers the question but only goes as far as saying that they've used an LM338 to achieve it.


I don't understand what's happening on/in the LM338 that's making it work.


1 Answer 1


My question is how do I modify my circuit to ensure that the phones aren't overcharged if left plugged in.

You don't have to.

With modern phones the charger is in the phone. The USB cable simply provides a power input for the phone, and circuitry inside the phone handles the charging of the battery from this power source.

Some USB power banks turn off if the load drops down below a set level, but this is to preserve the batteries in the powerbank from discharging through the voltage converter.

  • \$\begingroup\$ There are still these "smart" "turbo" chargers, that have some kind of "communication" between the charger and the phone. \$\endgroup\$
    – Eugene Sh.
    Apr 8, 2022 at 19:49
  • 3
    \$\begingroup\$ True, that's part of the USB PD spec (or similar) where the devices negotiate current and voltage capabilities. This is not what OP is building as he aims for a simple 5V device; and even with these power supplies, the charger is in the phone. My phone jumps to a 9V supply and stays there if the charger offers it. It doesn't negotiate back down. Nor does my laptop. \$\endgroup\$
    – vidarlo
    Apr 8, 2022 at 19:54

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