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I'd like to connect output of USB power supply and battery. The latter one by diode. I cannot use diode on the side of power supply due to voltage drop. So my question is whether typical cheap USB power supplies are immune when reverse voltage is applied (with assumption that it will never occur when supply is powered on). And what current lost can it bring?

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closed as unclear what you're asking by Chris Stratton, uint128_t, PeterJ, Neil_UK, Bimpelrekkie Mar 27 '16 at 21:14

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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This question is impossible to answer as there no way of knowing what the circuit used in a USB power adapter looks like. It depends on the model, you would have to open that model and reverse engineer it. Some may have a LED on the 5 V which will drain the battery. The only way you can do this safely is by using a diode. Use a Schottky diode because it will have the lowest forward drop voltage.

Are you planning to connect the battery directly to the 5 V ? If so then that is a bad idea as there are no batteries that have a 5 V typical charging voltage. There needs to be a charging circuit in between. How complex this charging circuit needs to be depends on the type of battery. In general such a charging circuit will take block the flowing of the reverse current.

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  • \$\begingroup\$ I didn't mentioned but actually I'm going to use adjustable step-up converter and li-po accumulator. The only problem is on the USB charger side. I'll probably connect it and measure reverse current. \$\endgroup\$ – ardabro Mar 26 '16 at 22:19
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    \$\begingroup\$ I didn't mentioned but actually... Geez, how are you going to get a proper answer if you leave us guessing about what you're doing ? ALWAYS describe the complete (planned) setup so we know what we're talking about. \$\endgroup\$ – Bimpelrekkie Mar 26 '16 at 22:32
  • \$\begingroup\$ But my question was ONLY about immunity of usb power supply to reverse voltage when powered off. Nothing more. \$\endgroup\$ – ardabro Mar 27 '16 at 20:33
  • \$\begingroup\$ And that I answered. But the question becomes irrelevant if you do not actually connect a battery directly to the 5 V but instead connect it through a step-up converter and possibly a charger circuit. If you want to get a useful and helpful answer then you have to describe the complete system. You make certain assumptions that certain parts are irrelevant while they are not. \$\endgroup\$ – Bimpelrekkie Mar 27 '16 at 21:12
  • \$\begingroup\$ Had I know you were going to use an setp-up converter I would have answered that you should add a diode after the step-up converter and just set the converter to a slightly higher voltage to compensate for the diode drop voltage. That would eliminate all concerns regarding the USB power adapter and reverse current. \$\endgroup\$ – Bimpelrekkie Mar 27 '16 at 21:14

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