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I have a little bluetooth to aux in my car which is battery powered. I want to hook it up directly without the usb cable hanging when charging. The problem is that I can't leave it always connected because when fully charged, it makes an annoying sound. I am pretty new so correct me if I'm wrong but, could I make a battery simulator/emulator circuit so that the device thinks it is always around 50%? That way it should not make any sound right? If there is a simpler solution, I'm open to ideas. Thank you in advance!

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  • \$\begingroup\$ What kind of annoying sound? \$\endgroup\$ – user253751 Dec 6 '17 at 22:09
  • \$\begingroup\$ It's like an "I'm charged" sound. In order for you to know and unplug the device. \$\endgroup\$ – Tomas Piaggio Dec 6 '17 at 22:49
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    \$\begingroup\$ Will the device not run with the battery disconnected? \$\endgroup\$ – ThreePhaseEel Dec 6 '17 at 23:40
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Inside the device it likely has a small lithium ion battery that has a nominal voltage of 3.7V. It goes down to 2.8V and charges up to 4.2V. The board likely has a charging IC with some cut off voltage to alert, over charge protection. There is likely only two wires to the battery.

You could trick it by connecting a regulated 3.5 to 3.7 volt supply instead of the battery And making sure you never connect the usb power supply. Using the usb 5v supply with an adjustable linear regulator like the lm7805 should work.

This is not guaranteed though.

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