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I am not an electrical engineer but I noticed that many Powerbanks are designed to fail because they allow deep discharging the battery below 3V and then do not charge anymore.

I came up with the idea to build a charging/discharging circuit with a relais and maybe an Arduino. Charging should be done by 5V over USB which seems easy as I just need to scale the voltage down to 4.2V. The deep discharging problem should be solved by measuring the voltage of the battery.

However, I am currently not sure how the manufacturers of such power banks implements simultaneous discharging over the same connector (which is used for charging the battery) e.g. if a smartphone is connected. Because over the same connector the voltage must be also up-regulated to 5V.

Does anyone have an idea how to solve this as a amateur in electronics.

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  • \$\begingroup\$ The simple solution is to just buy better designed power banks. There exist much more clever solutions than using a relay, it could be as simple as a battery protection module. This prevent deep discharging of the battery. \$\endgroup\$ – Bimpelrekkie Oct 19 '17 at 9:32
  • \$\begingroup\$ Probably you are right. Nevertheless, I am still curious how such a simplified circuit may look. \$\endgroup\$ – user52836 Oct 19 '17 at 10:40
  • \$\begingroup\$ The simplest (and probably also cheapest) solution is to use a dedicated chip, like the HT4921: datasheetspdf.com/datasheet/download.php?id=938254 There are also many powerbanks which use a microcontroller and some MOSFETs as switches instead of a dedicated chip. \$\endgroup\$ – Bimpelrekkie Oct 19 '17 at 11:19

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