usb - Android charging behavior when in use -

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i'm working on hardware/firmware/android app project android (in case nexus 7 tablet) connects custom hardware platform via usb. android in accessory mode, means other end of wire (the host) supplies power android. hardware has dedicated 5v 2a switching supply usb connector there plenty of current available tablet.

when android plugged in hardware, reports battery charging , requests 500ma usb connection (for don't know, usb protocol requires device inform host of current requirements). hardware provides current , 5v stays rock-solid.

despite this, charge level never changes long device remains in use. application uses tablet in kiosk mode - display stays on @ full brightness continuously. sleep tablet , battery charges, leave on , there no reported change in battery level.

the explanation can think of android's power supply circuitry cannot simultaneously handle current requirements of both full operation , battery charging. wonder if conscious decision based on expected power availability, , if there may sort of configuration option in os inform os more current is, indeed, available. perhaps os request more current host , have enough both charge battery , run device @ same time.

anyone have data on this? thanks!

when started company designed battery chargers cell phones , like, didn't realize took more providing voltage source usb connector. @ first encountered having provide different voltages on data pins tell connected device how current expect draw. there several manufacturers of usb interface ics dirty work of handshaking , act current limiters well. smartphones today draw average of 1a 5v usb bus. can, , (especially apple products) draw 2.1a if allowed. tablets run in range of 1.5a 2.1a. of above run in mode drawing lesser current, if device finds charger unable supply full current. sometimes, in case, allow device either run or charge, not both. in cases, available current isn't enough maintain charge level , , device's battery discharges, though it's connected charger. if google "usb charger data pin voltages" can see how tell device can draw more current. proper method though use controller ic ti's tps2511 "usb dedicated charging port controller , current limiting power switch". hope helps , isn't too late.


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