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?
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.
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.